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Biomass production and energy balance of a 12‐year‐old short‐rotation coppice poplar stand under different cutting cycles

Biomass production and energy balance of a 12‐year‐old short‐rotation coppice poplar stand under... <h1>Introduction</h1> The use of biomass for energy purposes is an important European Union policy, which aims to mitigate the effects of climate change by reducing greenhouse gas emissions from fossil fuel combustion, and securing the energy supply through the diversification of energy sources ( Gasol et al. , 2008 ). Biomass exploitation could increase employment in rural areas and provide an additional income for farmers. In addition, the European biomass market is increasing and in Italy, woody chip biomass at 40% of moisture can sell for up to €47 t −1 ( Gasol et al. , 2009a ). Poplar ( Populus spp.) is one of the most promising woody energy crops due to its high yield and high ecological interest in terms of low input requirements and biodiversity maintenance ( Börjesson, 1999 ). This species is thus grown as a short-rotation coppice (SRC), which consists of fast-growing plants being harvested (coppiced) at very short intervals (1–5 years). The largest dataset concerning SRC crop management is for the most part limited to northern and central Europe and North America ( Heilman & Norby, 1998; Labrecque & Teodorescu, 2005; Spinelli & Hartsough, 2006; Pei et al. , 2008 ). For Mediterranean countries (e.g. Italy), information on SRC is also available, but it focuses mainly on a few agronomic aspects such as productivity, fertilization ( Moscatelli et al. , 2008 ), water requirements ( Guidi et al. , 2008a ) and some qualitative harvestable biomass traits ( Guidi et al. , 2008b ). For those sites where water is not a limiting factor, plant spacing and cutting cycles are the most important agronomic aspects on which the establishment of an SRC plantation and its biomass production depend considerably. Nevertheless, currently, little information is available on these aspects. Many studies report that spacing and SRC cutting cycles are closely related. In general, maximum yields are achieved early in dense plantations, but wider-spaced plantations ensure the highest long-term biomass yield ( Mitchell, 1995; Proe et al. , 2002 ). On the other hand, under short cutting cycles, SRC stands have a shorter duration as they are more exposed to pathogens ( Sennerby-Forsse et al. , 1992 ). As these interactions are also affected by other factors such as location, soil type etc. ( Liesebach et al. , 1999 ), data from long-term trials carried out under a Mediterranean climate may offer additional information to better understand SRC dynamics in this environment. In addition, a sustainable energy crop should be characterized not only by high production but also by low input requirements, which subsequently lead to a high energy efficiency and low environmental impact. For this reason, to achieve a sustainable energy production system, the choice of species and crop management is crucial for the development of strategies that allow for the replacement of fossil sources. Although there are several criteria for the selection of suitable energy crops, energy balance is a very important tool to evaluate the energy sustainability of crop systems. There have been a number of energy budget studies on energy crops. Some researchers have reported on the energy balance of poplar SRC and have shown a positive response ( Dubuisson & Sintzoff, 1998; Scholz et al. , 1998; Matthews, 2001 ). Output/input energy ratios varied from 20 to 55 depending on growing conditions, inputs and assumed boundaries to the system ( Zavitkovski, 1979; Matthews, 2001; Hakkila & Parikka, 2002; Nonhebel, 2002; Heller et al. 2003; Gasol et al. , 2009b ). As found in other research on perennial energy crops, management intensity, such as fertilization, plant density and tillage, affects energy efficiency ( Angelini et al. , 2005; Sartori et al. , 2005; Boehmel et al. , 2008 ). The output/input ratio in most annual energy crops ranges from 2 to 18 ( Nassi o Di Nasso et al. , 2009 ), whereas in perennial herbaceous species such as miscanthus and giant reed, the ratio ranges from 30 to 50, respectively ( Angelini et al. , 2005, 2009a, b ). Forest-based biomass from conventional forestry systems have a typical ratio of 10–25 ( Mead & Pimentel, 2006 ). However, little information is available on the variability of energy balance in relation to crop age and to different cutting cycles. Most of the available data on the poplar SRC energy balance are based on predicted values and do not necessarily refer to real field conditions and, in any case, never refer to crop management. Thus, the aim of this paper was to carry out a long-term study (12 years) based on poplar SRC in order to evaluate dry matter, energy-use efficiency and net energy yield from this crop in relation to different harvest frequencies (1-, 2- and 3-year cutting cycles). <h1>Materials and methods</h1> <h2>Site description and crop management</h2> A long-term crop management system field experiment was conducted in Pisa (43°40′N, 10°19′E), Italy, from 1996 to 2008. The soil was a Xerofluvent (clay 20.1%, silt 40.5%, sand 39.4%), typical of the lower River Arno, which is an alluvial plain characterized by a superficial water table (1.8 m deep in the driest conditions) and good nutrient availability (organic matter 1.8%, total nitrogen content 1.3 g kg −1 , available phosphorus 8.8 mg kg −1 and exchangeable potassium 128.3 mg kg −1 ). The average climate conditions of the site during the trial are shown in Fig. 1 . During the experimental period (from 1996 to 2008), a considerable variability in rainfall was observed from year to year with a mean annual rainfall of approximately 750 mm (from 936 to 655). <h2>Management regime and plant measurements</h2> In the winter of 1996, an SRC poplar plantation was established on a former arable land using 20-cm-long unrooted Populus deltoides Bartr. (clone Lux) cuttings. Three powered tractors were used for cropping ( Table 1 ). As the main goal of the trial was to perform an energy balance of a poplar SRC cropping system, large stands were required to implement the effective mechanization of all operations (i.e. site preparation, planting, tending and harvesting). Therefore, three 500 m 2 contiguous plots were set up, and each was assigned to a specific cutting cycle of 1 (T1), 2 (T2) or 3 (T3) years ( Fig. 2 ). Soil was tilled according to the standard practices of the area (e.g. 50 cm deep ploughing as the main tillage in the autumn, and disk and rotary harrowing before planting). Chemical weed control was performed before planting by applying 2.5 kg ha −1 of Glyphosate. In addition, 48 kg ha −1 N, 144 ha −1 P 2 O 5 and 144 kg ha −1 K 2 O were incorporated into the soil. The plant density was 10 000 plants per hectare and the planting design was 2 m × 0.5 m. During year 2, mechanical weedings were performed as poplars are particularly susceptible to weed competition during the first growing season ( Fig. 2 ). The harvest was performed, using a cut and chip harvester (Spapperi RT capable of harvesting with a diameter of up to 180 mm), always at the end of February before the vegetative regrowth. Ten days after each harvest, N fertilization (rate 100 kg ha −1 N) was applied and mechanical weeding was performed twice in the following spring. No operations were performed in any of the nonharvest years ( Fig. 2 ). When the stump survival rate dropped below 35%, the crop system was considered concluded and land clearing was performed ( Fig. 2 ) through either: mulcher and ploughing (50 cm depth) for T1; forestry shredder, disk harrowing and ploughing (50 cm depth) for T2; forestry shredder, disk harrowing, ploughing (50 cm depth) and disk harrowing for T3. The different techniques were chosen according to the root system whose dimensions varied according to the frequency of the harvest. Biometric parameters were assessed at every harvest by measuring (1) the diameter of the main stem (20 cm above the stump); (2) the number of shoots per stump; (3) the survival rate of the stump; (4) the fresh aboveground biomass of the whole plant. Data were sampled in five transects (5 m long) per plot, which were always located within the three central rows of the field. Biomass samples of each plot were oven dried at 105 °C (to constant mass) and the dry matter content was determined. For each parameter, a Student's t -test was used to compare means under different treatments. <h2>Energy analysis</h2> This study considered energy flows relating to the operations required to grow a short-rotation coppice poplar and turn harvested biomass into usable solid biofuel (woody chip). Postprocessing and the conversion of woodchips were excluded from the analysis. The propagation material energy cost was considered negligible as the harvested plant material was both anatomically similar to the organs and tissues used to establish the crop, and because it represented <3% of the total crop yield ( Angelini et al. 2005 ). In accordance with Zenter et al. (1998) , the energy from human labour was not assessed as it was <0.2% of the total energy input. The energy balance was carried out by calculating energy costs for the production and repair of machinery, fertilizers and herbicides, and fuel and lubricant consumption. Inputs and outputs were converted into energy unit measures using coefficients found in the literature ( Table 2 ). Inputs can be classified as follows: (1) direct energy, which consists of the fossil energy consumed on the farm as diesel fuel and lubricant oil to power engines and (2) indirect energy, which is the fossil energy consumed outside the farm for the manufacture of the means of production. The energy costs for construction, depreciation and maintenance were calculated, taking into account the average lifespan and time spent using machines, multiplied by the energy coefficients. Fuel and lubricant oil costs for planting, cutting and chipping were measured onsite during the trial, while for the other operations, the data reported by Bonari et al. (1992) in the same environment were used. To evaluate the calorific value, poplar biomass was milled in a Retsch SM 1 rotor mill (Italy) to <297 μm and the calorific value was determined using a Leco AC 300 calorimeter (USA) according to the standard method proposed by the American Society for Testing and Materials (ASTM D2015). The energy output was determined subsequently by multiplying the dry matter yield by the calorific value of the plant materials. In addition, we determined the efficiency of the crop energy production, calculated as a ratio between the energy output and the energy input, and the net energy yield was calculated as the difference between the energy output and the energy input. <h1>Results</h1> <h2>Growth and stand biomass production</h2> Poplar SRC behaviour differed significantly according to the treatment ( Table 3 ). Biomass yield patterns for the three rotation cycles are shown in Fig. 3 . The first difference was the duration of the stand, which was only 7 years under the annual rotation and 12 years under the T2 and T3 harvest regimes. In addition, not only did the extent of biomass yield differ according to the rotation cycle but while under T1, the biomass yield increased within the first three harvests and then decreased, the T2 and especially T3 yields decreased from the first harvest onwards. Thus, while in T1, the maximum yield (16.4 t ha −1 ) was recorded during the third harvest for longer rotations, the highest yield was always obtained at the first harvest (T2=45 t ha −1 , T3=72.9 t ha −1 ). The stump survival rate also varied according to the rotation length ( Fig. 4 ). It showed a rapid decrease under the T1 treatment when it declined to about 5% after 7 years. For longer rotations, the decrease in the stump survival rate was slower than that in the previous case. T2 and T3 survival rates followed almost the same pattern during the early stages. Afterwards, T3 showed a higher survival rate than T2, and by the end of the trial, when both treatments were harvested, the survival rate ranged on average from 15% (T2) to 29% (T3). Other growth parameters are shown in Table 3 . Significant differences were observed in the average stem diameter size, which ranged between 19.1 mm (T1), 34.2 mm (T2) and 46.1 mm (T3). The number of shoots per stump was lower under T2 and T3 treatments compared with T1. Lastly, the biomass yield, which varied significantly according to the rotation length, ranged from a minimum of 9.9 t ha −1 yr −1 under T1 treatment to a maximum of 16.4 t ha −1 yr −1 under the longest rotation (T3). <h2>Energy balance</h2> Total energy input differed according to harvest frequency ( Table 4 ). The higher energy cost in T2 was due to the number of harvests and due to the high energy requirement for cutting and chipping. The establishment phase accounted for 20% of the total energy costs. Furthermore, the input decreased in the subsequent years because there was no soil management and planting. Thus, the annual energy cost ranged from 10.8 GJ ha −1 in T1 to 6.5 and 5.2 GJ ha −1 in T2 and T3, respectively ( Table 4 ). The higher energy input was due to fertilization (42%, 34% and 25% of the total input for T1, T2 and T3, respectively). On the other hand, mechanization required less energy than fertilization and it mainly concerned harvesting and chipping. The total energy output showed different values in relation to harvest intervals and increased from T1 to T3; T1 output increased slightly from the first to the third harvest and decreased in the subsequent harvests. T2 output increased only from the first to the second harvest, while T3 output decreased from the first harvest onwards ( Table 5 ). To evaluate the performance of SRC poplar, we considered the energy efficiency and the net energy yield. The mean energy efficiency (i.e. output/input) was 19.3 (T1), 40.4 (T2) and 60.8 (T3) ( Table 5 ). Similarly, the most favourable net energy yield was achieved for T3, when it decreased from 1322 GJ ha −1 at the first harvest to 368 GJ ha −1 in the fourth with a total value of about 3600 GJ ha −1 ( Table 5 ). <h1>Discussion</h1> This long-term trial revealed that different harvesting cycles can affect most growth parameters including the stem diameter dimension, number of shoots per stump and biomass yield. Our results are consistent with most of the available data, which indicate a strong relationship between the cutting cycle and the productivity of the stand ( Deckyn et al. , 2004 ). This is most likely due to the coexistence of several effects occurring after each coppicing, including the decrease in aboveground growth ( Heilman & Peabody, 1981; Blake, 1983; Bédéneau & Auclair, 1989; Armstrong et al. , 1999 ) and increased stool mortality ( Kopp et al. , 2001; Labrecque & Teodorescu, 2005 ). In our trial, a lower biomass yield was achieved under a 1-year cutting cycle (T1) where severe stool mortality led to a very low annual biomass yield. However, this is comparable to the average data found by other authors. For longer rotations, we found that biomass yields were sometimes consistent with several cases reported in the literature ( Pontailler et al. , 1999; Kopp et al. , 2001 ) but slightly higher compared with others ( Laureysens et al. , 2004, 2005 ). However, this could be partially due to the different climate conditions of the sites and also due to the specific clone used, both of which are key factors affecting SRC establishment and growth ( Ceulemans & Deraedt, 1999; Hofmann-Schielle et al. , 1999; Liesebach et al. , 1999; Al Afas et al. , 2008 ). Our results, in accordance with other studies ( Dubuisson & Sintzoff, 1998; Matthews, 2001; Nonhebel, 2002 ), show that poplar SRC is characterized by a positive energy balance. As expected, net energy returns for these three harvest treatments showed differences in terms of higher net energy yield values in T3 than in T2 and T1. In our study, this was related to the higher biomass production and the lower frequency of fertilization and harvesting during the experimental period, which characterized T3. This represents a good method of reducing fertilizer use and subsequently water pollution caused by nutrient leaching. In addition, all the treatments considered in these studies provide an opportunity to contain pesticide use, thus leading to an improvement in biodiversity and a reduction in environmental pollution. A 3-year harvest frequency could reduce the environmental impact and guarantee a high net energy yield. The data reported in the literature partially confirm our results. However, it is difficult to compare values from different studies, as the methods used for crop management vary from study to study. There are also different approaches to energy balance assessments, and there is sometimes disagreement over which energy input should be included in the budget. Dubuisson & Sintzoff (1998) reported lower values than ours mainly due to their fertilization management because they applied nitrogen, phosphorus and potassium fertilization after each harvest. In addition, our data showed low input requirements for harvest and chipping. This may be due to the improvement in poplar SRC mechanization over the last decade. Scholz et al. (1998) presented lower input costs than ours mainly due to the absence of fertilization, while output/input ratios were lower because storage and transport were included in the energy budget calculation. Moreover, Matthews (2001) showed lower input costs due to the low fertilization, which was only used for crop establishment. Most data in the literature present energy budgets derived from estimated data regarding dry matter yield and coppice lifespan. Our results, on the other hand, came from field trials where yield and SRC lifespan were authentic. Moreover, the use of different values of equivalent coefficients may contribute to the differences observed. Comparing the poplar SRC energy balance with other potential energy crops that are particularly suited to the climate in southern Europe, our results showed that poplar energy efficiency is higher than annual and similar to perennial crops ( Table 6 ). Perennial herbaceous crops such as miscanthus and giant reed are characterized by higher production levels and consequently, higher outputs and net energy yields. However, these perennial herbaceous species presented difficulties in terms of mechanization, particularly in terms of harvesting machines and the lack of propagation means, which is currently only possible through rhizomes or micropropagated plants. Thus, introducing perennial herbaceous species into cropping systems requires more research on cropping techniques, as well as genetic and logistical aspects. One of the advantages of the agricultural production of poplar SRC, on the other hand, is that in the geographical area that we studied, poplar is traditionally used in paper and wood production. Consequently, propagation mechanization and logistical aspects have already been assessed, and thus a bioenergy chain based on poplar biomass may be easier to perform. <h1>Conclusions</h1> The poplar SRC is one of the most promising woody species that can be grown in a temperate climate. In this trial, we showed that with harvesting cycles of more than 1 year, this species can survive up to 12 years and provide considerable annual biomass yields. According to our calculations, the poplar bioenergy system cultivated in southern Europe showed a positive energy balance characterized by a high energy efficiency. Our results indicate that the choice of harvest interval has a considerable bearing on energy yields. In fact, the energy efficiency of poplar SRC improves from T1 to T3 (19 vs. 60). In addition, the total consumption of nonrenewable energy associated with the production of 1 MJ of energy in the form of wood fuel is 0.058, 0.024 and 0.017 MJ MJ −1 , whereas the input requirement for each ton of produced biomass is 1, 0.45 and 0.31 MJ t −1 for T1, T2 and T3, respectively. Although this study was not specifically designed to evaluate factors related to cropping system sustainability, our results showed that poplar SRC with a production cycle of 12 years and a harvest cycle of 3 years can contribute to agronomic and environmental sustainability. This applies not only in terms of its high yield and energy efficiency but also in terms of its positive influence on soil fertility, on limiting soil tillage and on the environment, given the low pesticide and nutrient requirements. Energy ratio coefficients are extremely sensitive to hypotheses concerning energy balance methods (system boundaries and equivalent coefficients) and crop management including cutting cycle, fertilizer treatments, field area, clone choice, harvesting and chipping machinery. More research is needed, however, to investigate whether different poplar SRC crop managements can influence the sustainability of the bio-energy chain from an environmental and financial point of view. In addition, the energy balance also needs to be extended to the fuel-processing phase. This would help to establish whether the production of wood fuel from SRC is truly sustainable and to standardize conventions for estimating the energy budgets that define the system boundaries and the equivalent coefficients. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png GCB Bioenergy Wiley

Biomass production and energy balance of a 12‐year‐old short‐rotation coppice poplar stand under different cutting cycles

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References (50)

Publisher
Wiley
Copyright
Copyright © 2010 Wiley Subscription Services
ISSN
1757-1693
eISSN
1757-1707
DOI
10.1111/j.1757-1707.2010.01043.x
Publisher site
See Article on Publisher Site

Abstract

<h1>Introduction</h1> The use of biomass for energy purposes is an important European Union policy, which aims to mitigate the effects of climate change by reducing greenhouse gas emissions from fossil fuel combustion, and securing the energy supply through the diversification of energy sources ( Gasol et al. , 2008 ). Biomass exploitation could increase employment in rural areas and provide an additional income for farmers. In addition, the European biomass market is increasing and in Italy, woody chip biomass at 40% of moisture can sell for up to €47 t −1 ( Gasol et al. , 2009a ). Poplar ( Populus spp.) is one of the most promising woody energy crops due to its high yield and high ecological interest in terms of low input requirements and biodiversity maintenance ( Börjesson, 1999 ). This species is thus grown as a short-rotation coppice (SRC), which consists of fast-growing plants being harvested (coppiced) at very short intervals (1–5 years). The largest dataset concerning SRC crop management is for the most part limited to northern and central Europe and North America ( Heilman & Norby, 1998; Labrecque & Teodorescu, 2005; Spinelli & Hartsough, 2006; Pei et al. , 2008 ). For Mediterranean countries (e.g. Italy), information on SRC is also available, but it focuses mainly on a few agronomic aspects such as productivity, fertilization ( Moscatelli et al. , 2008 ), water requirements ( Guidi et al. , 2008a ) and some qualitative harvestable biomass traits ( Guidi et al. , 2008b ). For those sites where water is not a limiting factor, plant spacing and cutting cycles are the most important agronomic aspects on which the establishment of an SRC plantation and its biomass production depend considerably. Nevertheless, currently, little information is available on these aspects. Many studies report that spacing and SRC cutting cycles are closely related. In general, maximum yields are achieved early in dense plantations, but wider-spaced plantations ensure the highest long-term biomass yield ( Mitchell, 1995; Proe et al. , 2002 ). On the other hand, under short cutting cycles, SRC stands have a shorter duration as they are more exposed to pathogens ( Sennerby-Forsse et al. , 1992 ). As these interactions are also affected by other factors such as location, soil type etc. ( Liesebach et al. , 1999 ), data from long-term trials carried out under a Mediterranean climate may offer additional information to better understand SRC dynamics in this environment. In addition, a sustainable energy crop should be characterized not only by high production but also by low input requirements, which subsequently lead to a high energy efficiency and low environmental impact. For this reason, to achieve a sustainable energy production system, the choice of species and crop management is crucial for the development of strategies that allow for the replacement of fossil sources. Although there are several criteria for the selection of suitable energy crops, energy balance is a very important tool to evaluate the energy sustainability of crop systems. There have been a number of energy budget studies on energy crops. Some researchers have reported on the energy balance of poplar SRC and have shown a positive response ( Dubuisson & Sintzoff, 1998; Scholz et al. , 1998; Matthews, 2001 ). Output/input energy ratios varied from 20 to 55 depending on growing conditions, inputs and assumed boundaries to the system ( Zavitkovski, 1979; Matthews, 2001; Hakkila & Parikka, 2002; Nonhebel, 2002; Heller et al. 2003; Gasol et al. , 2009b ). As found in other research on perennial energy crops, management intensity, such as fertilization, plant density and tillage, affects energy efficiency ( Angelini et al. , 2005; Sartori et al. , 2005; Boehmel et al. , 2008 ). The output/input ratio in most annual energy crops ranges from 2 to 18 ( Nassi o Di Nasso et al. , 2009 ), whereas in perennial herbaceous species such as miscanthus and giant reed, the ratio ranges from 30 to 50, respectively ( Angelini et al. , 2005, 2009a, b ). Forest-based biomass from conventional forestry systems have a typical ratio of 10–25 ( Mead & Pimentel, 2006 ). However, little information is available on the variability of energy balance in relation to crop age and to different cutting cycles. Most of the available data on the poplar SRC energy balance are based on predicted values and do not necessarily refer to real field conditions and, in any case, never refer to crop management. Thus, the aim of this paper was to carry out a long-term study (12 years) based on poplar SRC in order to evaluate dry matter, energy-use efficiency and net energy yield from this crop in relation to different harvest frequencies (1-, 2- and 3-year cutting cycles). <h1>Materials and methods</h1> <h2>Site description and crop management</h2> A long-term crop management system field experiment was conducted in Pisa (43°40′N, 10°19′E), Italy, from 1996 to 2008. The soil was a Xerofluvent (clay 20.1%, silt 40.5%, sand 39.4%), typical of the lower River Arno, which is an alluvial plain characterized by a superficial water table (1.8 m deep in the driest conditions) and good nutrient availability (organic matter 1.8%, total nitrogen content 1.3 g kg −1 , available phosphorus 8.8 mg kg −1 and exchangeable potassium 128.3 mg kg −1 ). The average climate conditions of the site during the trial are shown in Fig. 1 . During the experimental period (from 1996 to 2008), a considerable variability in rainfall was observed from year to year with a mean annual rainfall of approximately 750 mm (from 936 to 655). <h2>Management regime and plant measurements</h2> In the winter of 1996, an SRC poplar plantation was established on a former arable land using 20-cm-long unrooted Populus deltoides Bartr. (clone Lux) cuttings. Three powered tractors were used for cropping ( Table 1 ). As the main goal of the trial was to perform an energy balance of a poplar SRC cropping system, large stands were required to implement the effective mechanization of all operations (i.e. site preparation, planting, tending and harvesting). Therefore, three 500 m 2 contiguous plots were set up, and each was assigned to a specific cutting cycle of 1 (T1), 2 (T2) or 3 (T3) years ( Fig. 2 ). Soil was tilled according to the standard practices of the area (e.g. 50 cm deep ploughing as the main tillage in the autumn, and disk and rotary harrowing before planting). Chemical weed control was performed before planting by applying 2.5 kg ha −1 of Glyphosate. In addition, 48 kg ha −1 N, 144 ha −1 P 2 O 5 and 144 kg ha −1 K 2 O were incorporated into the soil. The plant density was 10 000 plants per hectare and the planting design was 2 m × 0.5 m. During year 2, mechanical weedings were performed as poplars are particularly susceptible to weed competition during the first growing season ( Fig. 2 ). The harvest was performed, using a cut and chip harvester (Spapperi RT capable of harvesting with a diameter of up to 180 mm), always at the end of February before the vegetative regrowth. Ten days after each harvest, N fertilization (rate 100 kg ha −1 N) was applied and mechanical weeding was performed twice in the following spring. No operations were performed in any of the nonharvest years ( Fig. 2 ). When the stump survival rate dropped below 35%, the crop system was considered concluded and land clearing was performed ( Fig. 2 ) through either: mulcher and ploughing (50 cm depth) for T1; forestry shredder, disk harrowing and ploughing (50 cm depth) for T2; forestry shredder, disk harrowing, ploughing (50 cm depth) and disk harrowing for T3. The different techniques were chosen according to the root system whose dimensions varied according to the frequency of the harvest. Biometric parameters were assessed at every harvest by measuring (1) the diameter of the main stem (20 cm above the stump); (2) the number of shoots per stump; (3) the survival rate of the stump; (4) the fresh aboveground biomass of the whole plant. Data were sampled in five transects (5 m long) per plot, which were always located within the three central rows of the field. Biomass samples of each plot were oven dried at 105 °C (to constant mass) and the dry matter content was determined. For each parameter, a Student's t -test was used to compare means under different treatments. <h2>Energy analysis</h2> This study considered energy flows relating to the operations required to grow a short-rotation coppice poplar and turn harvested biomass into usable solid biofuel (woody chip). Postprocessing and the conversion of woodchips were excluded from the analysis. The propagation material energy cost was considered negligible as the harvested plant material was both anatomically similar to the organs and tissues used to establish the crop, and because it represented <3% of the total crop yield ( Angelini et al. 2005 ). In accordance with Zenter et al. (1998) , the energy from human labour was not assessed as it was <0.2% of the total energy input. The energy balance was carried out by calculating energy costs for the production and repair of machinery, fertilizers and herbicides, and fuel and lubricant consumption. Inputs and outputs were converted into energy unit measures using coefficients found in the literature ( Table 2 ). Inputs can be classified as follows: (1) direct energy, which consists of the fossil energy consumed on the farm as diesel fuel and lubricant oil to power engines and (2) indirect energy, which is the fossil energy consumed outside the farm for the manufacture of the means of production. The energy costs for construction, depreciation and maintenance were calculated, taking into account the average lifespan and time spent using machines, multiplied by the energy coefficients. Fuel and lubricant oil costs for planting, cutting and chipping were measured onsite during the trial, while for the other operations, the data reported by Bonari et al. (1992) in the same environment were used. To evaluate the calorific value, poplar biomass was milled in a Retsch SM 1 rotor mill (Italy) to <297 μm and the calorific value was determined using a Leco AC 300 calorimeter (USA) according to the standard method proposed by the American Society for Testing and Materials (ASTM D2015). The energy output was determined subsequently by multiplying the dry matter yield by the calorific value of the plant materials. In addition, we determined the efficiency of the crop energy production, calculated as a ratio between the energy output and the energy input, and the net energy yield was calculated as the difference between the energy output and the energy input. <h1>Results</h1> <h2>Growth and stand biomass production</h2> Poplar SRC behaviour differed significantly according to the treatment ( Table 3 ). Biomass yield patterns for the three rotation cycles are shown in Fig. 3 . The first difference was the duration of the stand, which was only 7 years under the annual rotation and 12 years under the T2 and T3 harvest regimes. In addition, not only did the extent of biomass yield differ according to the rotation cycle but while under T1, the biomass yield increased within the first three harvests and then decreased, the T2 and especially T3 yields decreased from the first harvest onwards. Thus, while in T1, the maximum yield (16.4 t ha −1 ) was recorded during the third harvest for longer rotations, the highest yield was always obtained at the first harvest (T2=45 t ha −1 , T3=72.9 t ha −1 ). The stump survival rate also varied according to the rotation length ( Fig. 4 ). It showed a rapid decrease under the T1 treatment when it declined to about 5% after 7 years. For longer rotations, the decrease in the stump survival rate was slower than that in the previous case. T2 and T3 survival rates followed almost the same pattern during the early stages. Afterwards, T3 showed a higher survival rate than T2, and by the end of the trial, when both treatments were harvested, the survival rate ranged on average from 15% (T2) to 29% (T3). Other growth parameters are shown in Table 3 . Significant differences were observed in the average stem diameter size, which ranged between 19.1 mm (T1), 34.2 mm (T2) and 46.1 mm (T3). The number of shoots per stump was lower under T2 and T3 treatments compared with T1. Lastly, the biomass yield, which varied significantly according to the rotation length, ranged from a minimum of 9.9 t ha −1 yr −1 under T1 treatment to a maximum of 16.4 t ha −1 yr −1 under the longest rotation (T3). <h2>Energy balance</h2> Total energy input differed according to harvest frequency ( Table 4 ). The higher energy cost in T2 was due to the number of harvests and due to the high energy requirement for cutting and chipping. The establishment phase accounted for 20% of the total energy costs. Furthermore, the input decreased in the subsequent years because there was no soil management and planting. Thus, the annual energy cost ranged from 10.8 GJ ha −1 in T1 to 6.5 and 5.2 GJ ha −1 in T2 and T3, respectively ( Table 4 ). The higher energy input was due to fertilization (42%, 34% and 25% of the total input for T1, T2 and T3, respectively). On the other hand, mechanization required less energy than fertilization and it mainly concerned harvesting and chipping. The total energy output showed different values in relation to harvest intervals and increased from T1 to T3; T1 output increased slightly from the first to the third harvest and decreased in the subsequent harvests. T2 output increased only from the first to the second harvest, while T3 output decreased from the first harvest onwards ( Table 5 ). To evaluate the performance of SRC poplar, we considered the energy efficiency and the net energy yield. The mean energy efficiency (i.e. output/input) was 19.3 (T1), 40.4 (T2) and 60.8 (T3) ( Table 5 ). Similarly, the most favourable net energy yield was achieved for T3, when it decreased from 1322 GJ ha −1 at the first harvest to 368 GJ ha −1 in the fourth with a total value of about 3600 GJ ha −1 ( Table 5 ). <h1>Discussion</h1> This long-term trial revealed that different harvesting cycles can affect most growth parameters including the stem diameter dimension, number of shoots per stump and biomass yield. Our results are consistent with most of the available data, which indicate a strong relationship between the cutting cycle and the productivity of the stand ( Deckyn et al. , 2004 ). This is most likely due to the coexistence of several effects occurring after each coppicing, including the decrease in aboveground growth ( Heilman & Peabody, 1981; Blake, 1983; Bédéneau & Auclair, 1989; Armstrong et al. , 1999 ) and increased stool mortality ( Kopp et al. , 2001; Labrecque & Teodorescu, 2005 ). In our trial, a lower biomass yield was achieved under a 1-year cutting cycle (T1) where severe stool mortality led to a very low annual biomass yield. However, this is comparable to the average data found by other authors. For longer rotations, we found that biomass yields were sometimes consistent with several cases reported in the literature ( Pontailler et al. , 1999; Kopp et al. , 2001 ) but slightly higher compared with others ( Laureysens et al. , 2004, 2005 ). However, this could be partially due to the different climate conditions of the sites and also due to the specific clone used, both of which are key factors affecting SRC establishment and growth ( Ceulemans & Deraedt, 1999; Hofmann-Schielle et al. , 1999; Liesebach et al. , 1999; Al Afas et al. , 2008 ). Our results, in accordance with other studies ( Dubuisson & Sintzoff, 1998; Matthews, 2001; Nonhebel, 2002 ), show that poplar SRC is characterized by a positive energy balance. As expected, net energy returns for these three harvest treatments showed differences in terms of higher net energy yield values in T3 than in T2 and T1. In our study, this was related to the higher biomass production and the lower frequency of fertilization and harvesting during the experimental period, which characterized T3. This represents a good method of reducing fertilizer use and subsequently water pollution caused by nutrient leaching. In addition, all the treatments considered in these studies provide an opportunity to contain pesticide use, thus leading to an improvement in biodiversity and a reduction in environmental pollution. A 3-year harvest frequency could reduce the environmental impact and guarantee a high net energy yield. The data reported in the literature partially confirm our results. However, it is difficult to compare values from different studies, as the methods used for crop management vary from study to study. There are also different approaches to energy balance assessments, and there is sometimes disagreement over which energy input should be included in the budget. Dubuisson & Sintzoff (1998) reported lower values than ours mainly due to their fertilization management because they applied nitrogen, phosphorus and potassium fertilization after each harvest. In addition, our data showed low input requirements for harvest and chipping. This may be due to the improvement in poplar SRC mechanization over the last decade. Scholz et al. (1998) presented lower input costs than ours mainly due to the absence of fertilization, while output/input ratios were lower because storage and transport were included in the energy budget calculation. Moreover, Matthews (2001) showed lower input costs due to the low fertilization, which was only used for crop establishment. Most data in the literature present energy budgets derived from estimated data regarding dry matter yield and coppice lifespan. Our results, on the other hand, came from field trials where yield and SRC lifespan were authentic. Moreover, the use of different values of equivalent coefficients may contribute to the differences observed. Comparing the poplar SRC energy balance with other potential energy crops that are particularly suited to the climate in southern Europe, our results showed that poplar energy efficiency is higher than annual and similar to perennial crops ( Table 6 ). Perennial herbaceous crops such as miscanthus and giant reed are characterized by higher production levels and consequently, higher outputs and net energy yields. However, these perennial herbaceous species presented difficulties in terms of mechanization, particularly in terms of harvesting machines and the lack of propagation means, which is currently only possible through rhizomes or micropropagated plants. Thus, introducing perennial herbaceous species into cropping systems requires more research on cropping techniques, as well as genetic and logistical aspects. One of the advantages of the agricultural production of poplar SRC, on the other hand, is that in the geographical area that we studied, poplar is traditionally used in paper and wood production. Consequently, propagation mechanization and logistical aspects have already been assessed, and thus a bioenergy chain based on poplar biomass may be easier to perform. <h1>Conclusions</h1> The poplar SRC is one of the most promising woody species that can be grown in a temperate climate. In this trial, we showed that with harvesting cycles of more than 1 year, this species can survive up to 12 years and provide considerable annual biomass yields. According to our calculations, the poplar bioenergy system cultivated in southern Europe showed a positive energy balance characterized by a high energy efficiency. Our results indicate that the choice of harvest interval has a considerable bearing on energy yields. In fact, the energy efficiency of poplar SRC improves from T1 to T3 (19 vs. 60). In addition, the total consumption of nonrenewable energy associated with the production of 1 MJ of energy in the form of wood fuel is 0.058, 0.024 and 0.017 MJ MJ −1 , whereas the input requirement for each ton of produced biomass is 1, 0.45 and 0.31 MJ t −1 for T1, T2 and T3, respectively. Although this study was not specifically designed to evaluate factors related to cropping system sustainability, our results showed that poplar SRC with a production cycle of 12 years and a harvest cycle of 3 years can contribute to agronomic and environmental sustainability. This applies not only in terms of its high yield and energy efficiency but also in terms of its positive influence on soil fertility, on limiting soil tillage and on the environment, given the low pesticide and nutrient requirements. Energy ratio coefficients are extremely sensitive to hypotheses concerning energy balance methods (system boundaries and equivalent coefficients) and crop management including cutting cycle, fertilizer treatments, field area, clone choice, harvesting and chipping machinery. More research is needed, however, to investigate whether different poplar SRC crop managements can influence the sustainability of the bio-energy chain from an environmental and financial point of view. In addition, the energy balance also needs to be extended to the fuel-processing phase. This would help to establish whether the production of wood fuel from SRC is truly sustainable and to standardize conventions for estimating the energy budgets that define the system boundaries and the equivalent coefficients.

Journal

GCB BioenergyWiley

Published: Jan 1, 2010

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