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Algal biochar: effects and applications

Algal biochar: effects and applications Introduction Macroalgae are ecologically and economically important, providing essential ecosystem services and biomass for foods, phycocolloids, soil additives, animal feeds and neutraceuticals (reviewed in Chopin & Sawhney, 2009 ). Owing to their rapid growth rates ( Mata ., 2010 ), ability to assimilate the nutrients nitrogen (N) and phosphorous ( Neori ., 2003 ), and sequester other elements such as heavy metals ( Mehta & Gaur, 2005 ), macroalgae are also used for the bioremediation of eutrophic and waste waters. Waste streams from aquaculture‐agriculture production can be used as nutrient sources for macroalgal biomass, providing reuse options for further production of animals and crops, or additional value adding products. Algal biomass is increasingly seen as a tool for reducing environmental impacts, and increasing aggregate production, from both agriculture ( Craggs ., 1996; Prein, 2002 ) and aquaculture ( Barrington ., 2009; Troell, 2009 ). A further potential use for macroalgae is in industries that require the mitigation of carbon (C) through biological carbon capture and storage (CCS) such as coal fired power stations, or the productive use of large cogenerated waste water streams such as those associated with coal seam gas production (e.g. Israel ., 2005 ). One of the major limitations in implementing macroalgal‐based remediation and CCS systems is a viable market for the end‐product of the significant quantities of macroalgal biomass that can be produced. The use of macroalgal biomass for biochar (charcoal) production, with energy cogeneration potential, provides a value‐driven model to sequester C and recycle nutrients ( Ross ., 2008; Bird ., 2011 ). Biochar has demonstrated applications as a tool for C sequestration and as a soil ameliorant capable of improving water holding capacity, nutrient status and microbial ecology of many soils ( Lehmann ., 2006; Lehmann & Joseph, 2009; Thies & Rillig, 2009 ). Bird . (2011) presented baseline data on the physio‐chemical properties and potential uses of eight species of macroalgae, sourced from fresh, brackish and marine environments, both before and after pyrolysis to biochar. These biochars were comparatively low in C content, surface area and cation exchange capacity (CEC), but high in pH, ash, N and extractable inorganic nutrients including P, K, Ca and Mg. The study concluded that algal biochar may be an attractive C sequestration and soil amelioration option, providing a high value end‐product for macroalgal biomass with properties likely to provide direct nutrient benefits to soils, and thereby to crop productivity. In this study we extend the study of Bird . (2011) by producing larger quantities of biochar from one freshwater (FW) alga and one saltwater (SW) alga using commercial pyrolysis equipment, and assessing the impact of biochar amendment on plant biomass production in pot‐scale experiments using two soils. The first soil is nutrient‐ and C‐ poor, simulating very low fertility (LF) spoil materials from mining or excavation. The second represents a higher fertility tropical agricultural soil. The paper contributes to developing a value model for the production of macroalgal biomass in a range of production systems for C sequestration, enhanced soil fertility, the bioremediation of waste water, and the remediation of anthropogenic impacts on natural environments. Materials and Methods Algal biomass collection and species composition The FW algal biomass was collected from FW ponds at the Townsville Barramundi Fish Farm in Kelso, Townsville, Australia (S19°21′52″, E146°42′13″). Algal biomass was collected from drained ponds by hand. An area of ∼700 m 2 was sampled providing 700 kg (wet weight) of green tide algal biomass. The biomass was a monoculture of the green alga Cladophora vagabunda (Linnaeus) when 40 samples were examined as described below. The SW algal biomass for biochar production was collected from the bioremediation pond at Good Fortune Bay Fisheries Ltd., Bowen, Australia (S19°56′25″, E147°55′45″) (described in de Paula Silva ., 2008 ). Algal biomass (approximately 6 t, wet weight) was collected from extensive floating mats within the pond using a drag net. Three species co‐exist in floating mats within the bioremediation pond, Cladophora coelothrix Kützing, Chaetomorpha indica (Kützing), and Ulva flexuosa (Wulfen). The species composition and relative abundance of the collected biomass was assessed using 40 random samples (∼10 g). Four random subsamples (∼1 g each) were extracted from each sample (total of 160 data points), prepared on a slide and images captured under a stereo microscope (Leica MZ125, Wetzlar, Germany). Species were identified based on their distinct morphology, and the relative abundance of species quantified by the projected (2‐D) area in the image using leica im 50 (v. 1.20) software. C. coelothrix was the dominant species composing 89% of the total biomass collected. C. indica and U. flexuosa were also present, but in significantly lower percentages (9% and 2%, respectively). The biomass of FW and SW algae was unwashed, pressed and sun‐dried. The FW alga was stored as unprocessed dry‐pressed biomass. The larger amount of SW biomass was hammer‐milled to facilitate storage. Approximately 140 kg of FW dried biomass and 1200 kg of SW dried biomass were obtained. All biomass was stored at ambient temperature until processed into biochar. Biochar production Both FW and SW algae were separately batch pyrolysed with gas flame support using a BigChar ™ 1000 pyrolysis unit by JJA Process Engineers Ltd., Mackay, Queensland. Pyrolysis temperature was monitored during each run and cycled between 250 °C and 400 °C as the gas flame was turned on and off. The algal biochar thus produced was immediately weighed to calculate pyrolysis efficiency (yield). For comparative purposes 20 kg of ligno‐cellulosic (LC) biochar was obtained from commercial charcoal pits operated by Renewable Carbon Resources Australia, near Charleville in southwest Queensland, Australia. The sample was produced 2 days before sampling from a feedstock of ‘Gidgee’ ( Acacia cambagei ), a dense hardwood native to the area. The FW and LC biochars were further crushed to pass a 2 mm sieve in order to make them comparable in particle size to the SW biochar. Soil preparation We used two course‐textured soils of differing fertility for the pot trial experiments. The first soil (LF) was made from washed dry sand mixed 80 : 20 in a cement mixer with dried clay subsoil from a lateritic mottled zone, composed dominantly of kaolinite and iron oxides. This mix was selected to represent very LF soil/spoil material common on brownfield sites. The second soil (high fertility, HF) was collected from the 0 to 25 cm interval of a red kandosol from Mourilyan, north Queensland. The soil was air‐dried and sieved at 5 mm to remove large plant fragments. This soil was selected to represent a natural, higher fertility agricultural soil typical of the humid tropics of Australia. Biochar and soil characterization Cations. All soil samples (LF and HF) and biochar (LC, FW and SW) were dried at 60 °C and aliquots of these materials (crushed where appropriate) were subjected to a range of analyses in order to characterize the material before beginning growth trials. CEC and extractable cations were determined on 1 g aliquots extracted for 15 h with shaking in a 0.01 m silver thiourea solution with analysis by atomic absorption spectrometric determination of elemental abundance (methods 15F1 and 15F3 of Rayment & Higginson, 1992 ). Colwell extractable inorganic phosphorus was determined on 1 g aliquots extracted overnight in 100 mL of 0.5 M NaHCO 3 adjusted to pH 8.5 with NaOH, with spectrophotometric determination of P (method 9B1 of Rayment & Higginson, 1992 ). Electrical conductivity and pH (H 2 O and CaCl 2 ) were measured on 1 : 5 mixes of sample and solution. Elemental composition. C, N and hydrogen (H) abundance was determined by elemental analysis and ash content by loss on ignition at 550 °C for 2 h. Oxygen abundance was estimated as the difference between the sum of C, H, N, ash content and 100% ( Cheng ., 2006 ). Surface area and volume parameters. The surface area and pore volume parameters of single point surface area, BET surface area, Langmuir surface area, single point adsorption total pore volume and BJH adsorption/desorption cumulative pore volume were determined on the three raw biochar samples (LC, FW and SW) by N adsorption using standard techniques by Particle and Surface Sciences Pty Ltd., Gosford, New South Wales, Australia. Elemental composition and trace element leaching The potential for leaching of some toxic trace elements from biochar in situ could negatively affect plant growth and end‐use (e.g. Hossain ., 2010 ). The application of biochar, regardless of biomass source from which it is derived, must meet legislative and/or voluntary code of conduct guidelines for biosolids and fertilizer additions to agricultural land (e.g. NSW EPA, 1997; US EPA, 1999; FIFA, 2010 ). To quantify the loss of trace elements through leaching, and compliance of algal derived biochars with legislative guidelines, the trace element composition of both the raw algal biochar, and algal biochar leached through a 0.45 μm filter over a period 30 min with 250 mL of deionized water, was determined by the Advanced Analytical Centre at James Cook University. About 0.1 g sample was weighed into a digestion vessel (Milestone START‐D), 2.5 mL SupraPure (Merk Germany) double distilled HNO 3 , 0.5 mL AR Grade H 2 O 2 was added, left for 2 h to let the reaction complete then heated by microwave to 180 °C and kept at this temperature for 10 min. After cooling, the digested sample was quantitatively transferred into a PTFE beaker and 0.5 mL SupraPure HF and HClO 4 was added and heated on a hotplate to incipient dryness. Thereafter 5 mL 10% HNO 3 was used to redissolve the sample and the resultant solution diluted to 100 mL for analysis. Sample analysis was carried out by a Varian Liberty Series II Inductively Coupled Plasma Atomic Emission Spectrometer (Al, Ca, Fe, K, Na, P and S), and Bruker 820‐MS Inductively Coupled Plasma Mass Spectrometer (remaining elements). The full suite of trace elements analysed are listed in Table 2 (also see results section). 2 Elemental composition of saltwater (SW) and freshwater (FW) algal biochar before and after (SW‐wash; FW‐wash) leaching with water (see text) Element SW SW‐wash FW FW‐wash Biosolid limits C 17.4 29.5 11.6 14.3 H 1.77 3.53 0.7 N 3.27 6.55 1.32 3.13 Al 28400 34700 43000 45300 As 1.77 1.77 † 3.74 3.56 ‡ 10–75 Ca 9350 7190 † 78300 77200 ‡ Cd 0.0573 0.0826 0.257 0.273 † 0.15–85 Co 5.92 7.85 35.2 24.6 † Cr 14.4 23.9 7.52 6.43 ‡ 50–100 Cu 46.6 63.2 37.7 35.8 ‡ 50–4300 Fe 14800 17700 13800 14400 Hg ≤ 0.5 0.5 1.84 0.835 † 1–60 K 44000 10600 † 20500 16700 † Mg 12500 9330 † 4760 4600 ‡ Mn 906 1080 445 416 ‡ Na 53900 7540 † 6840 5000 † Ni 5.66 7.80 5.75 5.38 ‡ 25–420 P 3730 3790 † 2450 2530 Pb 6.44 8.23 35.3 37.5 1–840 S 20600 6500 † 4260 2150 ‡ V 31.4 42.0 38.0 40.1 Zn 49.1 66.8 132 161 150–7500 Limits for biosolids and fertilizer additions to agricultural land compiled from NSW EPA (1997) , US EPA (1999) , and FIFA (2010) * C, H, and N were measured in wt% while all other elements were measured in mg kg −1 . † >20% leached out of biochar after wash. ‡ >10 % leached out of biochar after wash. Growth trials Growth trials were conducted independently using soil types from two extremes of soil fertility. The first trial with LF soil provides a synopsis of effects of the addition of algal biochar to soil requiring rehabilitation. The second with HF soil provides a synopsis of the effects of algal biochar on already productive soils. The two growth trials were carried out using 25 cm long, 10 cm diameter PVC tubes, each with five replicates of control and treatment. All trials included each type of biochar added to the soil at a rate of 35 g kg −1 (LC, FW and SW) and a soil control without biochar. A second factor, fertilizer (with and without), was included in each trial. A commercially available fertilizer (Yates lush lawn) containing N, P and K in the ratio of 3.6 : 6.7 : 3.2 was added to each biochar treatment at 50 g m −2 (50% of the recommended application rate), and was added at a higher dose to the control (100 g m −2 ; 100% of the recommended application rate), in order to compensate for the additional nutrients present in the algal biochars. The LF growth trial spanned 47 days in July–August, 2010, while the HF growth trial spanned 27 days in August–September, 2010. For both trials, each pot was wet to water holding capacity and allowed to stabilize for 2 weeks, with periodic rewetting to ∼60% of water holding capacity, monitored by periodic weighing. It is likely that soluble salts were substantially removed during this phase. The trial was done in a shade house to provide uniform diffuse sunlight to all pots and temperature and humidity was logged hourly throughout the experiments using a data logger (Hygrochron ™ iButton). Five seeds of sorghum were planted in each replicate pot. In the LF trial, the total number of plants in each pot was reduced to two after 1–2 weeks by removing the weakest seedlings as determined by height. Similarly, in the HF trial, the total number in each pot was reduced to three. At the end of the growing period, all above ground biomass for each remaining plant was harvested, dried at 60 °C and weighed. Growth rates (mg dry weight day −1 ) were measured as dry biomass divided by the growing period in days (LF trial=47 days, HF trial=27 days). Growth data for each trial was analysed separately using a 3‐factor mixed model analysis of variance ( anova ) with two fixed factors (biochar source and fertilizer) and 1 nested factor, ‘pot’ nested within source × fertilizer ( n =2 plants per pot for trial 1, and, n =3 for trial 2) as the error term for the model. All data were natural log‐transformed to meet the assumptions of anova . Results and Discussion Biochar and soil characterization The algal biochars (SW and FW) exhibit many characteristics similar to those previously reported for eight species by Bird . (2011) in that they are comparatively low in C content, surface area and CEC, and high in ash, N and extractable inorganic nutrients including P, K, Ca and Mg. Table 1 provides a summary of the physical and chemical characteristics of soils and biochars used in the study. The major differences between the algal biochars are that SW biochar has a considerably higher fraction that is soluble than FW biochar (29% and 11%, respectively), hence higher electrical conductivity and higher abundances of extractable cations such as sodium ( Table 1 ). SW biochar also has higher extractable nutrients (P, K and Mg), lower ash content and higher C and N content than FW biochar ( Table 1 ). The LC biochar is similar to the algal biochars in terms of pH, CEC and surface area characteristics, but has a considerably higher C content of 72%, compared with 11.6% for FW biochar, and 17.4% for SW biochar. It also has lower ash and considerably lower nutrient (N, P and K) contents ( Table 1 ). For example, the abundance of N and phosphorous, key elements for plant growth, are more than an order of magnitude lower in LC biochar (0.03% N, 13.6 mg kg −1 P) compared with FW (1.32% N, 761 mg kg −1 P) and SW (3.27% N, 1448 mg kg −1 P) biochar. 1 Physical and chemical characteristics of soils and biochars used in sorghum growth trials Parameter LF † HF LC FW SW Production (%) Pyrolysis yield NA NA ND 67.0 74.5 Fraction soluble NA NA ND 11.0 29.0 Cations pH (H 2 O 1 : 5) 5.90 6.10 8.1 8.3 6.1 pH (CaCl 2 1 : 5) 5.69 4.68 7.9 7.8 6.1 EC (H 2 O 1 : 5) (mS cm −1 ) 0.07 0.04 0.12 3.08 27.90 CEC (AgTU extract) cmol (+)·kg −1 0.81 1.23 14.1 19.3 13.1 Ca (0.1 m BaCl 2 /NH 4 Cl) cmol(+) kg −1 0.41 0.53 21.8 23.5 23.2 Mg (0.1 m BaCl 2 /NH 4 Cl) cmol(+) kg −1 0.17 0.26 0.17 9.4 50.0 K (0.1 m BaCl 2 /NH 4 Cl) cmol(+) kg −1 0.03 0.13 0.12 14.5 83.3 Na (0.1 m BaCl 2 /NH 4 Cl) cmol(+) kg −1 0.13 0.10 1.09 12.6 177.1 Extractable P (Colwell) mg kg −1 7.1 155 13.6 761 1448 Elemental composition C (wt%) 0.06 0.49 72.8 11.6 17.4 H (wt%) 3.22 0.70 1.77 N (wt%) N.D. 0.03 0.03 1.32 3.27 O (wt%) (by difference) 15.3 11.7 18.1 Ash (wt%) 8.6 74.7 59.4 Atomic O/C 0.16 0.76 0.78 Atomic H/C 0.53 0.72 1.22 Surface area (m 2 g −1 ) Single point Surface area 6.69 7.62 2.05 BET surface area 6.81 8.29 2.41 Langmuir surface area 9.36 11.89 3.66 BJH adsorption Cum. surface area 2.69 8.81 2.67 BJH desorption Cum. surface area 0.23 10.67 3.23 Volume (cm 3 g −1 ) Single point Adsorp. total pore Vol. 0.007 0.029 0.008 BJH adsorption Cum. pore volume 0.007 0.035 0.010 BJH desorption Cum. pore volume 0.003 0.033 0.009 NB. * EC, extractable cations; CEC, cation exchange capacity. † LF, low fertility soil; HF, high fertility agricultural soil; LC, ligno‐cellulosic biochar; FW, freshwater algal biochar; SW, saltwater algal biochar; NA, not applicable; ND, not determined. The soils used in this study are similar in terms of their slightly acid pH, comparatively low CEC and extractable cations, with the LF soil being texturally a sandy loam and the HF soil a clay loam. The soils differ in that the HF soil has at least five times, and in some cases more than an order of magnitude, higher C and nutrient (N, P and K) contents compared with the LF soil ( Table 1 ). Overall, the characteristics of the algal biochars produced for this study are broadly similar to those described by Bird . (2011) and confirm that algal biochar is a low surface area, comparatively nutrient rich but C poor biochar with properties similar to biochars produced, for example, from chicken manure ( Tagoe ., 2008; Chan & Xu, 2009 ). However, the algal biochars in this study differ from those in Bird . (2011) , despite being produced from the same algal species in that the pyrolysis yield was comparatively high (>67%), ash content was also high (>59%) and, in the case of SW biochar, the pH was acidic at 6.1. These differences are likely the result of moving to pilot scale production with commercial pyrolysis equipment rather than controlled laboratory production, with less control over the ‘purity’ of materials collected in the field and less control over pyrolysis conditions, with temperature within a single run cycling from 250 to 400 °C. It was also found that due to the comparatively high ash content the algal material would not pyrolyse without input of additional gas‐fired heating. It seems likely that commercial production of algal biochar would benefit from co‐firing with a lower ash, higher C feedstock such as wood waste, for example similar to the LC biochar used in this study. Co‐firing with LC material may have other benefits in that, depending on species, C content surface area and CEC capacity could be increased in the product. Elemental composition and trace element leaching The trace element compositions of the algal biochars, before and after leaching in water, are shown in Table 2 . SW biochar is considerably higher than FW biochar in elements associated with seawater (Na, K, Mg and S), while FW biochar contains higher abundances of Ca and some trace metals (As, Co, Pb, Hg, Cd and Zn). All trace metals in both biochars (before leaching) are below, or at the low end, of biosolid limits imposed by a range of countries for use in agriculture (e.g. NSW EPA, 1997; US EPA, 1999; FIFA, 2010 ). Elements exhibit a range of solubilities, with some being readily leached (Na and S), others being immobile (Al, Fe, V, Pb and Zn) and most, including trace metals, exhibiting intermediate behaviour. Trace metals in FW biochar, in general, appear more susceptible to leaching than the same trace metals in SW biochar. Note that the concentrations of some elements rise in the biochars after leaching, due to leaching of the most soluble cations from both biochars. The elemental composition of the algal biochars ( Table 2 ) indicates that both biochars contain a complete suite of macro and micro‐nutrients necessary to support plant growth. The comparatively high levels of N, P and K in particular suggest that algal biochar can act directly as a fertilizer without the addition of other constituents. Some trace metals are toxic and some countries have legislated minimum levels of these elements in biosolids that are to be incorporated into agricultural soils. The initial test for the solubility of the elements in the algal biochar indicates that most of the nutrient stock in the biochar is soluble and hence bioavailable, while trace, potentially toxic metals are partly leachable. Further, more detailed research is required to quantify the degree to which, and conditions under which, trace metals are likely to be mobile. The abundances of As, Cr, Cu, Hg, Ni and Zn are well below the minimum levels required for biosolids use on agricultural land ( NSW EPA, 1997; US EPA, 1999; FIFA, 2010 ). While Cd in FW biochar (0.25 mg kg −1 ) is marginally above the most stringent legislated limit for this element (0.15 mg kg −1 ; European Union) and Pb in both SW (6.4 mg kg −1 ) and FW (35.3 mg kg −1 ) biochar is above the minimum limit for some other nations (1 mg kg −1 ; Belgium), however… all are an order of magnitude below the highest minima imposed by national or state legislation in Australia ( Table 2 ). This suggests that algal biochar (unless derived from algae grown in contaminated water) is suitable for unrestricted application as a soil amendment and fertilizer on agricultural land. Growth trials The most critical data for plant growth is growth rate (mg dry weight day −1 ). In the LF trial, the addition of any type of biochar strongly enhanced growth of sorghum compared with soil without biochar ( anova , F (2,24) =10.75, P <0.001). Furthermore, the addition of fertilizer strongly enhanced growth rates compared with the same treatments without fertilizer ( anova , F (1,24) =15.71, P =0.001), except for the soil control (no addition of biochar; Table 3 , Fig. 1 ). Analysis of growth rates with controls omitted, to conform to assumptions of homogeneity due to very low growth rates for controls, demonstrates a larger effect for the type of biochar than for fertilizer ( Fig. 1 ). Biochar derived from SW algae gave an approximate 90% increase in growth rate compared with biochar from either LC biomass or FW algae. The effect of fertilizer was smaller, but still strongly significant, with a mean increase of 58% in growth rate across treatments with the addition of fertilizer. Notably, plants grown in LF soil with the addition of biochar derived from SW algae was greater than all other treatments, with or without fertilizer. 3 Physical characteristics of low fertility soil and biomass of plants from sorghum growth trials after 42 days Characteristics † Control Soil Ligno‐cellulosic Biochar Freshwater Biochar Saltwater Biochar − ‡ + § − + − + − + Soil Bulk density (g cm −3 ) ± SD 1.68 ± 0.02 1.65 ± 0.02 1.58 ± 0.04 1.57 ± 0.03 1.57 ± 0.02 1.57 ± 0.02 1.55 ± 0.02 1.55 ± 0.02 Soil pH 6.1 5.7 5.4 5.7 5.8 5.9 5.9 5.6 Biomass Plants germinated 16 17 14 18 15 18 17 17 Leaf C/N 17.8 10.8 13.3 16.8 14.3 17.1 16.2 16.0 Growth (mg day −1 ) ± SD 2.56 ± 0.58 1.62 ± 0.5 17.86 ± 8.1 29.69 ± 9.2 16.14 ± 03.4 32.16 ± 8.60 38.33 ± 11.46 52.57 ± 11.27 Mass (mg plant −1 ) ± SD 121 ± 27 76 ± 24 840 ± 404 1395 ± 380 759 ± 160 1512 ± 404 1801 ± 538 2470 ± 529 Height (cm) ± SD 20.30 ± 4.2 16.1 ± 3.2 47.6 ± 11 58.7 ± 6.1 52.0 ± 4.1 60.0 ± 5.2 63.9 ± 7.3 63.6 ± 3.3 * Average T =24.3°C; range=12.8–35.4°C. † Growth, mean daily growth rate; mass, mean dry weight per plant at harvest; height, mean height at harvest. All means calculated with n =5 pots per treatment. ‡ w/o, without fertilizer. § With, with fertilizer. 1 Sorghum growth rates using low fertility soil and biochar with and without fertilizer. Note: DW, dry weight; LC, ligno‐cellulosic biochar; FW, freshwater biochar; SW, saltwater biochar. Data show means (+1 standard error) of n =5 pots per treatment. Summary data from the LF growth trial using soil representing very LF spoil materials from mining or excavation is provided in Table 3 . The pH of the soil at the end of the experiment ranged from 5.4 (LC without fertilizer [LC−]) to 6.1 (control soil without fertilizer [C−]). Germination rates were relatively low across all treatments (66 ± 6%), with no consistent patterns of germination. Leaf C/N ratio ranged from 10.8 (control with fertilizer [C+]) to 17.8 (C−). Measures of plant performance reflect mean growth rates with the best performing treatments with SW biochar providing 14.9 times (SW−) and 20.2 times (SW+) of the control without fertilizer (C−). Similarly, plant heights were the greatest in these SW biochar treatments ( Table 3 ). The growth rates in the HF trial varied between combinations of biochar (including the control) and fertilizer application, indicated by a significant interaction between these two factors ( anova , F (3,32) =3.41, P =0.029). Similar to the LF trial, the addition of biochar derived from FW and SW biomass enhanced growth rates by 76.2% and 39.9%, respectively, compared with the relevant control (C−; Fig. 2 ). However, the addition of LC biochar had no impact on mean growth rates of the comparative control (C−). There was a range of effects with the addition of fertilizer, with strong increases in growth rates for fertilizer additions to the control (84.4%), and LC biochar treatment (65.1%). In contrast, fertilizer addition only had a small, or negative, effect when added to FW (+13.1%) or SW (−4.8%) treatments ( Fig. 2 ). The reduced effect of fertilizer in the algal biochar treatments was the main driver of the interaction term in the statistical model. 2 Sorghum growth rates using high fertility soil and biochar with and without fertilizer. Note: DW, dry weight; LC, ligno‐cellulosic biochar; FW, freshwater biochar; SW, saltwater biochar. Data show means (+1 standard error) of n =5 pots per treatment. The data from the HF growth trial using an agricultural soil from the tropics is provided in Table 4 . The pH of the soil at the end of the experiment ranged from 4.4 (SW+ and SW−) to 5.9 (LC−). Again measures of plant performance reflect mean growth rates with smaller effects of treatments than in the LF growth trial. At harvest the mean height of plants across each amendment ranged more narrowly from 48.4 ± 2.9 cm (C−) to 57 ± 4.4 cm (SW−) and dry biomass from 771 ± 353 (C−) to 1536 ± 413 mg (FW+). Leaf C/N ratio ranged from 9.9 (LC−) to 29.1 (C−). 4 Physical characteristics of highly fertile agricultural soil and biomass of plants from sorghum growth trials after 27 days Characteristics † Control Soil Ligno‐cellulosic Biochar Freshwater Biochar Saltwater Biochar − ‡ + § − + − + − + Soil Bulk density (g cm −3 ) ± SD 1.37 ± 0.01 1.41 ± 0.02 1.41 ± 0.02 1.42 ± 0.01 1.42 ± 0.02 1.42 ± 0.01 1.40 ± 0.02 1.41 ± 0.02 Soil pH 5.5 5.2 5.9 5.4 5.1 4.7 4.4 4.4 Biomass Plants germinated 19 15 14 17 14 16 17 18 Leaf C/N 29.1 12.9 9.9 10.4 15.8 10.7 16.9 13.8 Growth (mg day −1 ) ± SD 28.6 ± 2.2 52.6 ± 6.3 24.7 ± 11.0 40.8 ± 4.2 50.3 ± 5.1 56.9 ± 4.2 39.9 ± 7.4 38.0 ± 12.7 Mass (mg plant −1 ) ± SD 771 ± 59 1420 ± 170 668 ± 297 1102 ± 113 1358 ± 137 1535 ± 119 1078 ± 199 1026 ± 342 Height (cm) ± SD 48.4 ± 2.9 51.8 ± 3.3 51.8 ± 4.7 54.6 ± 2.1 48.6 ± 4.1 50.8 ± 3.6 57.0 ± 4.4 53.4 ± 3.4 * Average T =26.7°C; range=18.9–39.2°C. † Growth, mean daily growth rate; mass, mean dry weight per plant at harvest; height, mean height at harvest. All means calculated with n =5 pots per treatment. ‡ Without fertilizer. § With fertilizer. The main result overall is that all the biochar amendments had a major impact on relative growth rates compared with the controls either with, or without, inorganic fertilizer in very poor (LF) soil. The average growth rate for all biochar‐amended replicates on LF soil (38.1 mg dry weight day −1 ) was 60% higher than the average growth rate for the non‐biochar amendments (18.8 mg dry weight day −1 ), translating into an average of 14 times more biomass accumulated in the biochar‐amended trials over the course of the growing period. The dramatic difference in growth rate cannot be due to nutrient loading alone as the control with fertilizer did not perform better than the control without fertilizer, and the LC amendment had similar or lower fertilizer loadings to the controls. It is likely that the biochars both reduced root penetration resistance in these high bulk density soils and also provided soil ‘habitat’ and a (minor) source of labile C enabling the plants to grow normally in a soil that replicates material that might result from mining or excavation, largely devoid of C, nutrients, and soil structure. The maximum growth rates for any treatment combination between the two trials were very similar for SW biochar (with fertilizer) in LF soil and FW biochar (with fertilizer) in HF soil, measuring 52.6 and 56.9 mg dry weight day −1 , respectively. Notably, irrespective of soil quality, there are clear benefits for biomass production through the addition of algal biochar. These benefits are differentiated from LC biochar which did not enhance growth in HF soil. The lower impact of all biochar amendments in the HF soil suggests that where nutrients are available in sufficient quantity from the soil itself, there is less benefit of biochar addition to plant productivity. The implication is that algal biochar appears to provide an alternative or complimentary source to commercial fertilizers in HF soils. Production of algal biochar by commercial pyrolysis has indicated that due to relatively high ash content, cofiring with more readily combustible feedstocks will be required for unassisted pyrolysis, and that with a suitable choice of high surface area feedstock, the properties of the biochar thus produced may be further enhanced in term of surface area and CEC. Co‐firing with high C feedstocks would also increase the C sequestration potential of the material. As is often the case, the interactions between biochar, soil and plants in this study is complex. Biochar (algal and LC) had a major impact on above ground biomass where the soil was poor in nutrients and C, and had not yet developed ‘normal’ soil processes and habitats. This suggests that biochar will provide a tool to facilitate revegetation of excavated areas and mining spoil, with algal biochar providing the specific added benefit of high nutrient content. There was smaller additional benefit derived from adding biochar to an agricultural soil that already contained a significant C and nutrient stock. However, the ongoing replenishment of this nutrient stock as it becomes depleted by cropping might be assisted by the periodic addition of algal biochar. An additional potential advantage of using macroalgae for biochar production is that this reduces the weight and volume of material that has to be transported from the aquaculture site to an end use. Elemental analysis of biochar from both SW and FW algal species has demonstrated that the macronutrient benefits also extend to significant quantities of micronutrients (trace elements) and that these are largely bio‐available. The content of potentially toxic trace elements is below the legislated biosolids limits for direct application on agricultural lands for both the FW and SW algal biochar used in this study, and this is likely to be generically the case for algae grown in uncontaminated waters. Some macroalgae bioaccumulate toxic elements from wastewater streams ( Mehta & Gaur, 2005; Singh ., 2007 ) hence it may prove possible to concentrate toxic trace elements within or onto algae, and then immobilize at least a fraction of these in algal biochar. Such ‘contaminated’ biochar could, for example, be used as a soil ameliorant under crops grown for biofuels, or during revegetation of contaminated mine spoil. Further research would be required to develop this possibility. Conclusions This study has confirmed the conclusions of Bird . (2011) that biochar derived from macroalgae has properties, including comparatively high nutrient content, that make it suitable for use as a soil ameliorant with some capacity for long‐term C sequestration. Algal biochar‐derived from the remediation of wastewater from aquaculture, agriculture, eutrophied natural waterways, or saline waste water sources, could provide a significant revenue stream as a soil ameliorant and fertilizer, beyond its direct value as a tool for water remediation, and long‐term soil C sequestration. Acknowledgements This study was funded by a Uniquest Trailblazer grant to Bird and de Nys. We thank staff of the Townsville Barramundi Fish Farm and Good Fortune Bay Fisheries Ltd for access to the algae used in this study, staff of BigChar Ltd. and Renewable Carbon Resources Australia Ltd. for biochar production. T. Forsyth for assistance with the pot trials, S. Berthelsen and Y. Hu for some biochar chemical analyses and M. Vucko for editorial assistance. References Barrington K , Chopin T , Robinson S ( 2009 ) Integrated multitrophic aquaculture (IMTA) in marine temperate waters. FAO Fisheries and Aquaculture Techinical Paper No. 529 . In: Integrated Mariculture: a Global Review (ed. Soto D ), pp. 7 – 46 . FAO , Rome. Bird MI , Wurster CW , de Paula Silva PH , Bass A , de Nys R ( 2011 ) Algal biochar – production and properties . Bioresource Technology , 102 , 1886 – 1891 . Chan KY , Xu ZH ( 2009 ) Biochar ‐ Nutrient properties and their enhancement . In: Biochar for Environmental Management. Science and Technology (eds Lehman J , Joseph S ), pp. 67 – 84 . Earthscan , London, UK. Cheng CH , Lehmann J , Thies JE , Burton SD , Engelhard MH ( 2006 ) Oxidation of black carbon by biotic and abiotic processes . Organic Geochemistry , 37 , 1477 – 1488 . Chopin T , Sawhney M ( 2009 ) Seaweeds and their mariculture . In: The Encyclopedia of Ecology. Ecological Engineering , (Vol. 3) (eds Jörgensen SE , Fath BD ), pp. 4477 – 4486 . Elsevier , Oxford. Craggs RJ , Adey WH , Jessup BK , Oswald WJ ( 1996 ) A controlled stream mesocosm for tertiary treatment of sewage . Ecological Engineering , 6 , 149 – 169 . de Paula Silva PH , McBride S , de Nys R , Paul NA ( 2008 ) Integrating filamentous ‘green tide’ algae into tropical pond‐based aquaculture . Aquaculture , 284 , 74 – 80 . FIFA ( 2010 ). Draft Code of Practice for Fertilizer Description and Labelling. Fertilizer Industry Federation of Australia. Available at: (accessed 20 September 2010). Hossain MK , Strezov V , Chan KY , Nelson PF ( 2010 ) Agronomic properties of wastewater sludge biochar and bioavailability of metals in production of cherry tomato ( Lycopersicon esculentum ) . Chemosphere , 78 , 1167 – 1171 . Israel A , Gavrieli J , Glazer A , Friedlander M ( 2005 ) Utilization of flue gas from a power plant for tank cultivation of the red seaweed Gracilaria cornea . Aquaculture , 249 , 311 – 316 . Lehmann J , Gaunt J , Rondon M ( 2006 ) Bio‐char sequestration in terrestrial ecosystems – a review . Mitigation and Adaptation Strategies for Global Change , 11 , 403 – 427 . Lehmann J , Joseph S ( 2009 ) Biochar for Environmental Management . Earthscan , Virginia. Mata L , Schuenhoff A , Santos R ( 2010 ) A direct comparison of the performance of the seaweed biofilters, Asparagopsis armata and Ulva rigida . Journal of Applied Phycology , 22 , 639 – 644 . Mehta SK , Gaur JP ( 2005 ) Use of algae for removing heavy metal ions from wastewater : progress and prospects. Critical Review Biotechnology , 25 , 113 – 152 . Neori A , Msuya FE , Shauli L , Schuenhoff A , Kopel F , Shpigel M ( 2003 ) A novel three‐stage seaweed ( Ulva lactuca ) biofilter design for integrated mariculture . Journal of Applied Phycology , 15 , 543 – 553 . NSW EPA ( 1997 ). Environmental Guidelines: Use and Disposal of Biosolids Products. NSW Environmental Protection agency, Sydney, 92 pp. Prein M ( 2002 ) Integration of aquaculture into crop–animal systems in Asia . Agricultural Systems , 71 , 127 – 146 . Rayment GE , Higginson FR ( 1992 ) Australian Laboratory Handbook of Soil and Water Chemical Methods – Australian Soil and Land Survey Handbook . Intaka Press , Melbourne and Sydney. Ross A , Jones JM , Kubacki ML , Bridgeman TG ( 2008 ) Classification of macroalgae as fuel and its thermochemical behaviour . Bioresource Technology , 99 , 6494 – 6504 . Singh A , Mehta SK , Gaur JP ( 2007 ) Removal of heavy metals from aqueous solution by common freshwater filamentous algae . World Journal Microbial Biotechnlogy , 23 , 1115 – 1120 . Tagoe SO , Horiuchi T , Matsui T ( 2008 ) Effects of carbonized and dried chicken manures on the growth, yield, and N content of soybean . Plant Soil , 306 , 211 – 220 . Thies JE , Rillig MC ( 2009 ) Characteristics of biochar : biological properties. In: Biochar for Environmental Management (eds Lehmann J , Joseph S ), pp. 85 – 106 . Earthscan , Virginia. Troell M ( 2009 ) Integrated marine and brackish water aquaculture in tropical regions . In: Integrated Mariculture: a Global Review. FAO Fisheries and Aquaculture Technical Paper No. 529 (ed. Soto D ), pp. 47 – 131 . FAO , Rome. U.S. EPA ( 1999 ). Background Report on Fertilizer Use, Contaminants, and Regulation. Washington, DC: National Program Chemicals Division, Office of Pollution Prevention and Toxics, U.S. Environmental Protection Agency. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png GCB Bioenergy Wiley

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Wiley
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Copyright © 2012 Blackwell Publishing Ltd
ISSN
1757-1693
eISSN
1757-1707
DOI
10.1111/j.1757-1707.2011.01109.x
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Abstract

Introduction Macroalgae are ecologically and economically important, providing essential ecosystem services and biomass for foods, phycocolloids, soil additives, animal feeds and neutraceuticals (reviewed in Chopin & Sawhney, 2009 ). Owing to their rapid growth rates ( Mata ., 2010 ), ability to assimilate the nutrients nitrogen (N) and phosphorous ( Neori ., 2003 ), and sequester other elements such as heavy metals ( Mehta & Gaur, 2005 ), macroalgae are also used for the bioremediation of eutrophic and waste waters. Waste streams from aquaculture‐agriculture production can be used as nutrient sources for macroalgal biomass, providing reuse options for further production of animals and crops, or additional value adding products. Algal biomass is increasingly seen as a tool for reducing environmental impacts, and increasing aggregate production, from both agriculture ( Craggs ., 1996; Prein, 2002 ) and aquaculture ( Barrington ., 2009; Troell, 2009 ). A further potential use for macroalgae is in industries that require the mitigation of carbon (C) through biological carbon capture and storage (CCS) such as coal fired power stations, or the productive use of large cogenerated waste water streams such as those associated with coal seam gas production (e.g. Israel ., 2005 ). One of the major limitations in implementing macroalgal‐based remediation and CCS systems is a viable market for the end‐product of the significant quantities of macroalgal biomass that can be produced. The use of macroalgal biomass for biochar (charcoal) production, with energy cogeneration potential, provides a value‐driven model to sequester C and recycle nutrients ( Ross ., 2008; Bird ., 2011 ). Biochar has demonstrated applications as a tool for C sequestration and as a soil ameliorant capable of improving water holding capacity, nutrient status and microbial ecology of many soils ( Lehmann ., 2006; Lehmann & Joseph, 2009; Thies & Rillig, 2009 ). Bird . (2011) presented baseline data on the physio‐chemical properties and potential uses of eight species of macroalgae, sourced from fresh, brackish and marine environments, both before and after pyrolysis to biochar. These biochars were comparatively low in C content, surface area and cation exchange capacity (CEC), but high in pH, ash, N and extractable inorganic nutrients including P, K, Ca and Mg. The study concluded that algal biochar may be an attractive C sequestration and soil amelioration option, providing a high value end‐product for macroalgal biomass with properties likely to provide direct nutrient benefits to soils, and thereby to crop productivity. In this study we extend the study of Bird . (2011) by producing larger quantities of biochar from one freshwater (FW) alga and one saltwater (SW) alga using commercial pyrolysis equipment, and assessing the impact of biochar amendment on plant biomass production in pot‐scale experiments using two soils. The first soil is nutrient‐ and C‐ poor, simulating very low fertility (LF) spoil materials from mining or excavation. The second represents a higher fertility tropical agricultural soil. The paper contributes to developing a value model for the production of macroalgal biomass in a range of production systems for C sequestration, enhanced soil fertility, the bioremediation of waste water, and the remediation of anthropogenic impacts on natural environments. Materials and Methods Algal biomass collection and species composition The FW algal biomass was collected from FW ponds at the Townsville Barramundi Fish Farm in Kelso, Townsville, Australia (S19°21′52″, E146°42′13″). Algal biomass was collected from drained ponds by hand. An area of ∼700 m 2 was sampled providing 700 kg (wet weight) of green tide algal biomass. The biomass was a monoculture of the green alga Cladophora vagabunda (Linnaeus) when 40 samples were examined as described below. The SW algal biomass for biochar production was collected from the bioremediation pond at Good Fortune Bay Fisheries Ltd., Bowen, Australia (S19°56′25″, E147°55′45″) (described in de Paula Silva ., 2008 ). Algal biomass (approximately 6 t, wet weight) was collected from extensive floating mats within the pond using a drag net. Three species co‐exist in floating mats within the bioremediation pond, Cladophora coelothrix Kützing, Chaetomorpha indica (Kützing), and Ulva flexuosa (Wulfen). The species composition and relative abundance of the collected biomass was assessed using 40 random samples (∼10 g). Four random subsamples (∼1 g each) were extracted from each sample (total of 160 data points), prepared on a slide and images captured under a stereo microscope (Leica MZ125, Wetzlar, Germany). Species were identified based on their distinct morphology, and the relative abundance of species quantified by the projected (2‐D) area in the image using leica im 50 (v. 1.20) software. C. coelothrix was the dominant species composing 89% of the total biomass collected. C. indica and U. flexuosa were also present, but in significantly lower percentages (9% and 2%, respectively). The biomass of FW and SW algae was unwashed, pressed and sun‐dried. The FW alga was stored as unprocessed dry‐pressed biomass. The larger amount of SW biomass was hammer‐milled to facilitate storage. Approximately 140 kg of FW dried biomass and 1200 kg of SW dried biomass were obtained. All biomass was stored at ambient temperature until processed into biochar. Biochar production Both FW and SW algae were separately batch pyrolysed with gas flame support using a BigChar ™ 1000 pyrolysis unit by JJA Process Engineers Ltd., Mackay, Queensland. Pyrolysis temperature was monitored during each run and cycled between 250 °C and 400 °C as the gas flame was turned on and off. The algal biochar thus produced was immediately weighed to calculate pyrolysis efficiency (yield). For comparative purposes 20 kg of ligno‐cellulosic (LC) biochar was obtained from commercial charcoal pits operated by Renewable Carbon Resources Australia, near Charleville in southwest Queensland, Australia. The sample was produced 2 days before sampling from a feedstock of ‘Gidgee’ ( Acacia cambagei ), a dense hardwood native to the area. The FW and LC biochars were further crushed to pass a 2 mm sieve in order to make them comparable in particle size to the SW biochar. Soil preparation We used two course‐textured soils of differing fertility for the pot trial experiments. The first soil (LF) was made from washed dry sand mixed 80 : 20 in a cement mixer with dried clay subsoil from a lateritic mottled zone, composed dominantly of kaolinite and iron oxides. This mix was selected to represent very LF soil/spoil material common on brownfield sites. The second soil (high fertility, HF) was collected from the 0 to 25 cm interval of a red kandosol from Mourilyan, north Queensland. The soil was air‐dried and sieved at 5 mm to remove large plant fragments. This soil was selected to represent a natural, higher fertility agricultural soil typical of the humid tropics of Australia. Biochar and soil characterization Cations. All soil samples (LF and HF) and biochar (LC, FW and SW) were dried at 60 °C and aliquots of these materials (crushed where appropriate) were subjected to a range of analyses in order to characterize the material before beginning growth trials. CEC and extractable cations were determined on 1 g aliquots extracted for 15 h with shaking in a 0.01 m silver thiourea solution with analysis by atomic absorption spectrometric determination of elemental abundance (methods 15F1 and 15F3 of Rayment & Higginson, 1992 ). Colwell extractable inorganic phosphorus was determined on 1 g aliquots extracted overnight in 100 mL of 0.5 M NaHCO 3 adjusted to pH 8.5 with NaOH, with spectrophotometric determination of P (method 9B1 of Rayment & Higginson, 1992 ). Electrical conductivity and pH (H 2 O and CaCl 2 ) were measured on 1 : 5 mixes of sample and solution. Elemental composition. C, N and hydrogen (H) abundance was determined by elemental analysis and ash content by loss on ignition at 550 °C for 2 h. Oxygen abundance was estimated as the difference between the sum of C, H, N, ash content and 100% ( Cheng ., 2006 ). Surface area and volume parameters. The surface area and pore volume parameters of single point surface area, BET surface area, Langmuir surface area, single point adsorption total pore volume and BJH adsorption/desorption cumulative pore volume were determined on the three raw biochar samples (LC, FW and SW) by N adsorption using standard techniques by Particle and Surface Sciences Pty Ltd., Gosford, New South Wales, Australia. Elemental composition and trace element leaching The potential for leaching of some toxic trace elements from biochar in situ could negatively affect plant growth and end‐use (e.g. Hossain ., 2010 ). The application of biochar, regardless of biomass source from which it is derived, must meet legislative and/or voluntary code of conduct guidelines for biosolids and fertilizer additions to agricultural land (e.g. NSW EPA, 1997; US EPA, 1999; FIFA, 2010 ). To quantify the loss of trace elements through leaching, and compliance of algal derived biochars with legislative guidelines, the trace element composition of both the raw algal biochar, and algal biochar leached through a 0.45 μm filter over a period 30 min with 250 mL of deionized water, was determined by the Advanced Analytical Centre at James Cook University. About 0.1 g sample was weighed into a digestion vessel (Milestone START‐D), 2.5 mL SupraPure (Merk Germany) double distilled HNO 3 , 0.5 mL AR Grade H 2 O 2 was added, left for 2 h to let the reaction complete then heated by microwave to 180 °C and kept at this temperature for 10 min. After cooling, the digested sample was quantitatively transferred into a PTFE beaker and 0.5 mL SupraPure HF and HClO 4 was added and heated on a hotplate to incipient dryness. Thereafter 5 mL 10% HNO 3 was used to redissolve the sample and the resultant solution diluted to 100 mL for analysis. Sample analysis was carried out by a Varian Liberty Series II Inductively Coupled Plasma Atomic Emission Spectrometer (Al, Ca, Fe, K, Na, P and S), and Bruker 820‐MS Inductively Coupled Plasma Mass Spectrometer (remaining elements). The full suite of trace elements analysed are listed in Table 2 (also see results section). 2 Elemental composition of saltwater (SW) and freshwater (FW) algal biochar before and after (SW‐wash; FW‐wash) leaching with water (see text) Element SW SW‐wash FW FW‐wash Biosolid limits C 17.4 29.5 11.6 14.3 H 1.77 3.53 0.7 N 3.27 6.55 1.32 3.13 Al 28400 34700 43000 45300 As 1.77 1.77 † 3.74 3.56 ‡ 10–75 Ca 9350 7190 † 78300 77200 ‡ Cd 0.0573 0.0826 0.257 0.273 † 0.15–85 Co 5.92 7.85 35.2 24.6 † Cr 14.4 23.9 7.52 6.43 ‡ 50–100 Cu 46.6 63.2 37.7 35.8 ‡ 50–4300 Fe 14800 17700 13800 14400 Hg ≤ 0.5 0.5 1.84 0.835 † 1–60 K 44000 10600 † 20500 16700 † Mg 12500 9330 † 4760 4600 ‡ Mn 906 1080 445 416 ‡ Na 53900 7540 † 6840 5000 † Ni 5.66 7.80 5.75 5.38 ‡ 25–420 P 3730 3790 † 2450 2530 Pb 6.44 8.23 35.3 37.5 1–840 S 20600 6500 † 4260 2150 ‡ V 31.4 42.0 38.0 40.1 Zn 49.1 66.8 132 161 150–7500 Limits for biosolids and fertilizer additions to agricultural land compiled from NSW EPA (1997) , US EPA (1999) , and FIFA (2010) * C, H, and N were measured in wt% while all other elements were measured in mg kg −1 . † >20% leached out of biochar after wash. ‡ >10 % leached out of biochar after wash. Growth trials Growth trials were conducted independently using soil types from two extremes of soil fertility. The first trial with LF soil provides a synopsis of effects of the addition of algal biochar to soil requiring rehabilitation. The second with HF soil provides a synopsis of the effects of algal biochar on already productive soils. The two growth trials were carried out using 25 cm long, 10 cm diameter PVC tubes, each with five replicates of control and treatment. All trials included each type of biochar added to the soil at a rate of 35 g kg −1 (LC, FW and SW) and a soil control without biochar. A second factor, fertilizer (with and without), was included in each trial. A commercially available fertilizer (Yates lush lawn) containing N, P and K in the ratio of 3.6 : 6.7 : 3.2 was added to each biochar treatment at 50 g m −2 (50% of the recommended application rate), and was added at a higher dose to the control (100 g m −2 ; 100% of the recommended application rate), in order to compensate for the additional nutrients present in the algal biochars. The LF growth trial spanned 47 days in July–August, 2010, while the HF growth trial spanned 27 days in August–September, 2010. For both trials, each pot was wet to water holding capacity and allowed to stabilize for 2 weeks, with periodic rewetting to ∼60% of water holding capacity, monitored by periodic weighing. It is likely that soluble salts were substantially removed during this phase. The trial was done in a shade house to provide uniform diffuse sunlight to all pots and temperature and humidity was logged hourly throughout the experiments using a data logger (Hygrochron ™ iButton). Five seeds of sorghum were planted in each replicate pot. In the LF trial, the total number of plants in each pot was reduced to two after 1–2 weeks by removing the weakest seedlings as determined by height. Similarly, in the HF trial, the total number in each pot was reduced to three. At the end of the growing period, all above ground biomass for each remaining plant was harvested, dried at 60 °C and weighed. Growth rates (mg dry weight day −1 ) were measured as dry biomass divided by the growing period in days (LF trial=47 days, HF trial=27 days). Growth data for each trial was analysed separately using a 3‐factor mixed model analysis of variance ( anova ) with two fixed factors (biochar source and fertilizer) and 1 nested factor, ‘pot’ nested within source × fertilizer ( n =2 plants per pot for trial 1, and, n =3 for trial 2) as the error term for the model. All data were natural log‐transformed to meet the assumptions of anova . Results and Discussion Biochar and soil characterization The algal biochars (SW and FW) exhibit many characteristics similar to those previously reported for eight species by Bird . (2011) in that they are comparatively low in C content, surface area and CEC, and high in ash, N and extractable inorganic nutrients including P, K, Ca and Mg. Table 1 provides a summary of the physical and chemical characteristics of soils and biochars used in the study. The major differences between the algal biochars are that SW biochar has a considerably higher fraction that is soluble than FW biochar (29% and 11%, respectively), hence higher electrical conductivity and higher abundances of extractable cations such as sodium ( Table 1 ). SW biochar also has higher extractable nutrients (P, K and Mg), lower ash content and higher C and N content than FW biochar ( Table 1 ). The LC biochar is similar to the algal biochars in terms of pH, CEC and surface area characteristics, but has a considerably higher C content of 72%, compared with 11.6% for FW biochar, and 17.4% for SW biochar. It also has lower ash and considerably lower nutrient (N, P and K) contents ( Table 1 ). For example, the abundance of N and phosphorous, key elements for plant growth, are more than an order of magnitude lower in LC biochar (0.03% N, 13.6 mg kg −1 P) compared with FW (1.32% N, 761 mg kg −1 P) and SW (3.27% N, 1448 mg kg −1 P) biochar. 1 Physical and chemical characteristics of soils and biochars used in sorghum growth trials Parameter LF † HF LC FW SW Production (%) Pyrolysis yield NA NA ND 67.0 74.5 Fraction soluble NA NA ND 11.0 29.0 Cations pH (H 2 O 1 : 5) 5.90 6.10 8.1 8.3 6.1 pH (CaCl 2 1 : 5) 5.69 4.68 7.9 7.8 6.1 EC (H 2 O 1 : 5) (mS cm −1 ) 0.07 0.04 0.12 3.08 27.90 CEC (AgTU extract) cmol (+)·kg −1 0.81 1.23 14.1 19.3 13.1 Ca (0.1 m BaCl 2 /NH 4 Cl) cmol(+) kg −1 0.41 0.53 21.8 23.5 23.2 Mg (0.1 m BaCl 2 /NH 4 Cl) cmol(+) kg −1 0.17 0.26 0.17 9.4 50.0 K (0.1 m BaCl 2 /NH 4 Cl) cmol(+) kg −1 0.03 0.13 0.12 14.5 83.3 Na (0.1 m BaCl 2 /NH 4 Cl) cmol(+) kg −1 0.13 0.10 1.09 12.6 177.1 Extractable P (Colwell) mg kg −1 7.1 155 13.6 761 1448 Elemental composition C (wt%) 0.06 0.49 72.8 11.6 17.4 H (wt%) 3.22 0.70 1.77 N (wt%) N.D. 0.03 0.03 1.32 3.27 O (wt%) (by difference) 15.3 11.7 18.1 Ash (wt%) 8.6 74.7 59.4 Atomic O/C 0.16 0.76 0.78 Atomic H/C 0.53 0.72 1.22 Surface area (m 2 g −1 ) Single point Surface area 6.69 7.62 2.05 BET surface area 6.81 8.29 2.41 Langmuir surface area 9.36 11.89 3.66 BJH adsorption Cum. surface area 2.69 8.81 2.67 BJH desorption Cum. surface area 0.23 10.67 3.23 Volume (cm 3 g −1 ) Single point Adsorp. total pore Vol. 0.007 0.029 0.008 BJH adsorption Cum. pore volume 0.007 0.035 0.010 BJH desorption Cum. pore volume 0.003 0.033 0.009 NB. * EC, extractable cations; CEC, cation exchange capacity. † LF, low fertility soil; HF, high fertility agricultural soil; LC, ligno‐cellulosic biochar; FW, freshwater algal biochar; SW, saltwater algal biochar; NA, not applicable; ND, not determined. The soils used in this study are similar in terms of their slightly acid pH, comparatively low CEC and extractable cations, with the LF soil being texturally a sandy loam and the HF soil a clay loam. The soils differ in that the HF soil has at least five times, and in some cases more than an order of magnitude, higher C and nutrient (N, P and K) contents compared with the LF soil ( Table 1 ). Overall, the characteristics of the algal biochars produced for this study are broadly similar to those described by Bird . (2011) and confirm that algal biochar is a low surface area, comparatively nutrient rich but C poor biochar with properties similar to biochars produced, for example, from chicken manure ( Tagoe ., 2008; Chan & Xu, 2009 ). However, the algal biochars in this study differ from those in Bird . (2011) , despite being produced from the same algal species in that the pyrolysis yield was comparatively high (>67%), ash content was also high (>59%) and, in the case of SW biochar, the pH was acidic at 6.1. These differences are likely the result of moving to pilot scale production with commercial pyrolysis equipment rather than controlled laboratory production, with less control over the ‘purity’ of materials collected in the field and less control over pyrolysis conditions, with temperature within a single run cycling from 250 to 400 °C. It was also found that due to the comparatively high ash content the algal material would not pyrolyse without input of additional gas‐fired heating. It seems likely that commercial production of algal biochar would benefit from co‐firing with a lower ash, higher C feedstock such as wood waste, for example similar to the LC biochar used in this study. Co‐firing with LC material may have other benefits in that, depending on species, C content surface area and CEC capacity could be increased in the product. Elemental composition and trace element leaching The trace element compositions of the algal biochars, before and after leaching in water, are shown in Table 2 . SW biochar is considerably higher than FW biochar in elements associated with seawater (Na, K, Mg and S), while FW biochar contains higher abundances of Ca and some trace metals (As, Co, Pb, Hg, Cd and Zn). All trace metals in both biochars (before leaching) are below, or at the low end, of biosolid limits imposed by a range of countries for use in agriculture (e.g. NSW EPA, 1997; US EPA, 1999; FIFA, 2010 ). Elements exhibit a range of solubilities, with some being readily leached (Na and S), others being immobile (Al, Fe, V, Pb and Zn) and most, including trace metals, exhibiting intermediate behaviour. Trace metals in FW biochar, in general, appear more susceptible to leaching than the same trace metals in SW biochar. Note that the concentrations of some elements rise in the biochars after leaching, due to leaching of the most soluble cations from both biochars. The elemental composition of the algal biochars ( Table 2 ) indicates that both biochars contain a complete suite of macro and micro‐nutrients necessary to support plant growth. The comparatively high levels of N, P and K in particular suggest that algal biochar can act directly as a fertilizer without the addition of other constituents. Some trace metals are toxic and some countries have legislated minimum levels of these elements in biosolids that are to be incorporated into agricultural soils. The initial test for the solubility of the elements in the algal biochar indicates that most of the nutrient stock in the biochar is soluble and hence bioavailable, while trace, potentially toxic metals are partly leachable. Further, more detailed research is required to quantify the degree to which, and conditions under which, trace metals are likely to be mobile. The abundances of As, Cr, Cu, Hg, Ni and Zn are well below the minimum levels required for biosolids use on agricultural land ( NSW EPA, 1997; US EPA, 1999; FIFA, 2010 ). While Cd in FW biochar (0.25 mg kg −1 ) is marginally above the most stringent legislated limit for this element (0.15 mg kg −1 ; European Union) and Pb in both SW (6.4 mg kg −1 ) and FW (35.3 mg kg −1 ) biochar is above the minimum limit for some other nations (1 mg kg −1 ; Belgium), however… all are an order of magnitude below the highest minima imposed by national or state legislation in Australia ( Table 2 ). This suggests that algal biochar (unless derived from algae grown in contaminated water) is suitable for unrestricted application as a soil amendment and fertilizer on agricultural land. Growth trials The most critical data for plant growth is growth rate (mg dry weight day −1 ). In the LF trial, the addition of any type of biochar strongly enhanced growth of sorghum compared with soil without biochar ( anova , F (2,24) =10.75, P <0.001). Furthermore, the addition of fertilizer strongly enhanced growth rates compared with the same treatments without fertilizer ( anova , F (1,24) =15.71, P =0.001), except for the soil control (no addition of biochar; Table 3 , Fig. 1 ). Analysis of growth rates with controls omitted, to conform to assumptions of homogeneity due to very low growth rates for controls, demonstrates a larger effect for the type of biochar than for fertilizer ( Fig. 1 ). Biochar derived from SW algae gave an approximate 90% increase in growth rate compared with biochar from either LC biomass or FW algae. The effect of fertilizer was smaller, but still strongly significant, with a mean increase of 58% in growth rate across treatments with the addition of fertilizer. Notably, plants grown in LF soil with the addition of biochar derived from SW algae was greater than all other treatments, with or without fertilizer. 3 Physical characteristics of low fertility soil and biomass of plants from sorghum growth trials after 42 days Characteristics † Control Soil Ligno‐cellulosic Biochar Freshwater Biochar Saltwater Biochar − ‡ + § − + − + − + Soil Bulk density (g cm −3 ) ± SD 1.68 ± 0.02 1.65 ± 0.02 1.58 ± 0.04 1.57 ± 0.03 1.57 ± 0.02 1.57 ± 0.02 1.55 ± 0.02 1.55 ± 0.02 Soil pH 6.1 5.7 5.4 5.7 5.8 5.9 5.9 5.6 Biomass Plants germinated 16 17 14 18 15 18 17 17 Leaf C/N 17.8 10.8 13.3 16.8 14.3 17.1 16.2 16.0 Growth (mg day −1 ) ± SD 2.56 ± 0.58 1.62 ± 0.5 17.86 ± 8.1 29.69 ± 9.2 16.14 ± 03.4 32.16 ± 8.60 38.33 ± 11.46 52.57 ± 11.27 Mass (mg plant −1 ) ± SD 121 ± 27 76 ± 24 840 ± 404 1395 ± 380 759 ± 160 1512 ± 404 1801 ± 538 2470 ± 529 Height (cm) ± SD 20.30 ± 4.2 16.1 ± 3.2 47.6 ± 11 58.7 ± 6.1 52.0 ± 4.1 60.0 ± 5.2 63.9 ± 7.3 63.6 ± 3.3 * Average T =24.3°C; range=12.8–35.4°C. † Growth, mean daily growth rate; mass, mean dry weight per plant at harvest; height, mean height at harvest. All means calculated with n =5 pots per treatment. ‡ w/o, without fertilizer. § With, with fertilizer. 1 Sorghum growth rates using low fertility soil and biochar with and without fertilizer. Note: DW, dry weight; LC, ligno‐cellulosic biochar; FW, freshwater biochar; SW, saltwater biochar. Data show means (+1 standard error) of n =5 pots per treatment. Summary data from the LF growth trial using soil representing very LF spoil materials from mining or excavation is provided in Table 3 . The pH of the soil at the end of the experiment ranged from 5.4 (LC without fertilizer [LC−]) to 6.1 (control soil without fertilizer [C−]). Germination rates were relatively low across all treatments (66 ± 6%), with no consistent patterns of germination. Leaf C/N ratio ranged from 10.8 (control with fertilizer [C+]) to 17.8 (C−). Measures of plant performance reflect mean growth rates with the best performing treatments with SW biochar providing 14.9 times (SW−) and 20.2 times (SW+) of the control without fertilizer (C−). Similarly, plant heights were the greatest in these SW biochar treatments ( Table 3 ). The growth rates in the HF trial varied between combinations of biochar (including the control) and fertilizer application, indicated by a significant interaction between these two factors ( anova , F (3,32) =3.41, P =0.029). Similar to the LF trial, the addition of biochar derived from FW and SW biomass enhanced growth rates by 76.2% and 39.9%, respectively, compared with the relevant control (C−; Fig. 2 ). However, the addition of LC biochar had no impact on mean growth rates of the comparative control (C−). There was a range of effects with the addition of fertilizer, with strong increases in growth rates for fertilizer additions to the control (84.4%), and LC biochar treatment (65.1%). In contrast, fertilizer addition only had a small, or negative, effect when added to FW (+13.1%) or SW (−4.8%) treatments ( Fig. 2 ). The reduced effect of fertilizer in the algal biochar treatments was the main driver of the interaction term in the statistical model. 2 Sorghum growth rates using high fertility soil and biochar with and without fertilizer. Note: DW, dry weight; LC, ligno‐cellulosic biochar; FW, freshwater biochar; SW, saltwater biochar. Data show means (+1 standard error) of n =5 pots per treatment. The data from the HF growth trial using an agricultural soil from the tropics is provided in Table 4 . The pH of the soil at the end of the experiment ranged from 4.4 (SW+ and SW−) to 5.9 (LC−). Again measures of plant performance reflect mean growth rates with smaller effects of treatments than in the LF growth trial. At harvest the mean height of plants across each amendment ranged more narrowly from 48.4 ± 2.9 cm (C−) to 57 ± 4.4 cm (SW−) and dry biomass from 771 ± 353 (C−) to 1536 ± 413 mg (FW+). Leaf C/N ratio ranged from 9.9 (LC−) to 29.1 (C−). 4 Physical characteristics of highly fertile agricultural soil and biomass of plants from sorghum growth trials after 27 days Characteristics † Control Soil Ligno‐cellulosic Biochar Freshwater Biochar Saltwater Biochar − ‡ + § − + − + − + Soil Bulk density (g cm −3 ) ± SD 1.37 ± 0.01 1.41 ± 0.02 1.41 ± 0.02 1.42 ± 0.01 1.42 ± 0.02 1.42 ± 0.01 1.40 ± 0.02 1.41 ± 0.02 Soil pH 5.5 5.2 5.9 5.4 5.1 4.7 4.4 4.4 Biomass Plants germinated 19 15 14 17 14 16 17 18 Leaf C/N 29.1 12.9 9.9 10.4 15.8 10.7 16.9 13.8 Growth (mg day −1 ) ± SD 28.6 ± 2.2 52.6 ± 6.3 24.7 ± 11.0 40.8 ± 4.2 50.3 ± 5.1 56.9 ± 4.2 39.9 ± 7.4 38.0 ± 12.7 Mass (mg plant −1 ) ± SD 771 ± 59 1420 ± 170 668 ± 297 1102 ± 113 1358 ± 137 1535 ± 119 1078 ± 199 1026 ± 342 Height (cm) ± SD 48.4 ± 2.9 51.8 ± 3.3 51.8 ± 4.7 54.6 ± 2.1 48.6 ± 4.1 50.8 ± 3.6 57.0 ± 4.4 53.4 ± 3.4 * Average T =26.7°C; range=18.9–39.2°C. † Growth, mean daily growth rate; mass, mean dry weight per plant at harvest; height, mean height at harvest. All means calculated with n =5 pots per treatment. ‡ Without fertilizer. § With fertilizer. The main result overall is that all the biochar amendments had a major impact on relative growth rates compared with the controls either with, or without, inorganic fertilizer in very poor (LF) soil. The average growth rate for all biochar‐amended replicates on LF soil (38.1 mg dry weight day −1 ) was 60% higher than the average growth rate for the non‐biochar amendments (18.8 mg dry weight day −1 ), translating into an average of 14 times more biomass accumulated in the biochar‐amended trials over the course of the growing period. The dramatic difference in growth rate cannot be due to nutrient loading alone as the control with fertilizer did not perform better than the control without fertilizer, and the LC amendment had similar or lower fertilizer loadings to the controls. It is likely that the biochars both reduced root penetration resistance in these high bulk density soils and also provided soil ‘habitat’ and a (minor) source of labile C enabling the plants to grow normally in a soil that replicates material that might result from mining or excavation, largely devoid of C, nutrients, and soil structure. The maximum growth rates for any treatment combination between the two trials were very similar for SW biochar (with fertilizer) in LF soil and FW biochar (with fertilizer) in HF soil, measuring 52.6 and 56.9 mg dry weight day −1 , respectively. Notably, irrespective of soil quality, there are clear benefits for biomass production through the addition of algal biochar. These benefits are differentiated from LC biochar which did not enhance growth in HF soil. The lower impact of all biochar amendments in the HF soil suggests that where nutrients are available in sufficient quantity from the soil itself, there is less benefit of biochar addition to plant productivity. The implication is that algal biochar appears to provide an alternative or complimentary source to commercial fertilizers in HF soils. Production of algal biochar by commercial pyrolysis has indicated that due to relatively high ash content, cofiring with more readily combustible feedstocks will be required for unassisted pyrolysis, and that with a suitable choice of high surface area feedstock, the properties of the biochar thus produced may be further enhanced in term of surface area and CEC. Co‐firing with high C feedstocks would also increase the C sequestration potential of the material. As is often the case, the interactions between biochar, soil and plants in this study is complex. Biochar (algal and LC) had a major impact on above ground biomass where the soil was poor in nutrients and C, and had not yet developed ‘normal’ soil processes and habitats. This suggests that biochar will provide a tool to facilitate revegetation of excavated areas and mining spoil, with algal biochar providing the specific added benefit of high nutrient content. There was smaller additional benefit derived from adding biochar to an agricultural soil that already contained a significant C and nutrient stock. However, the ongoing replenishment of this nutrient stock as it becomes depleted by cropping might be assisted by the periodic addition of algal biochar. An additional potential advantage of using macroalgae for biochar production is that this reduces the weight and volume of material that has to be transported from the aquaculture site to an end use. Elemental analysis of biochar from both SW and FW algal species has demonstrated that the macronutrient benefits also extend to significant quantities of micronutrients (trace elements) and that these are largely bio‐available. The content of potentially toxic trace elements is below the legislated biosolids limits for direct application on agricultural lands for both the FW and SW algal biochar used in this study, and this is likely to be generically the case for algae grown in uncontaminated waters. Some macroalgae bioaccumulate toxic elements from wastewater streams ( Mehta & Gaur, 2005; Singh ., 2007 ) hence it may prove possible to concentrate toxic trace elements within or onto algae, and then immobilize at least a fraction of these in algal biochar. Such ‘contaminated’ biochar could, for example, be used as a soil ameliorant under crops grown for biofuels, or during revegetation of contaminated mine spoil. Further research would be required to develop this possibility. Conclusions This study has confirmed the conclusions of Bird . (2011) that biochar derived from macroalgae has properties, including comparatively high nutrient content, that make it suitable for use as a soil ameliorant with some capacity for long‐term C sequestration. Algal biochar‐derived from the remediation of wastewater from aquaculture, agriculture, eutrophied natural waterways, or saline waste water sources, could provide a significant revenue stream as a soil ameliorant and fertilizer, beyond its direct value as a tool for water remediation, and long‐term soil C sequestration. Acknowledgements This study was funded by a Uniquest Trailblazer grant to Bird and de Nys. We thank staff of the Townsville Barramundi Fish Farm and Good Fortune Bay Fisheries Ltd for access to the algae used in this study, staff of BigChar Ltd. and Renewable Carbon Resources Australia Ltd. for biochar production. T. Forsyth for assistance with the pot trials, S. Berthelsen and Y. 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Journal

GCB BioenergyWiley

Published: Jan 1, 2012

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