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J. Hydrol. Hydromech., 69, 2021, 4, 369–377 ©2021. This is an open access article distributed DOI: 10.2478/johh-2021-0022 under the Creative Commons Attribution ISSN 1338-4333 NonCommercial-NoDerivatives 4.0 License Water repellency decreases with increasing carbonate content and pH for different biocrust types on sand dunes 1, 2* 3, 4 5 2 Sylvie Laureen Drahorad , Vincent J. M. N. L. Felde , Ruth H. Ellerbrock , Anja Henss Institute for Soil Science and Soil Conservation, Research Centre for BioSystems, Land Use and Nutrition (iFZ), Justus Liebig University Giessen, Giessen, Germany. Institute for Physical Chemistry, Justus Liebig University Giessen, Giessen, Germany. Department of Soil Science, University of Kassel, Witzenhausen, Germany. Institute of Soil Science, Leibniz University Hanover, Hanover, Germany. Working Group: Hydropedology, Research Area 1 "Landscape Functioning", Leibniz Centre for Agricultural Landscape Research (ZALF), Müncheberg, Germany. Corresponding author. Tel.: +49(0)641-9934522. E-mail: Sylvie.Drahorad@phys.chemie.uni-giessen.de Abstract: Biocrusts are biological communities that occupy the soil surface, accumulate organic matter and mineral particles and hence strongly affect the properties of the soils they cover. Moreover, by affecting water repellency, biocrusts may cause a preferential infiltration of rainwater, with a high impact on the formation of local water pathways, especially for sand dunes. The aim of this study is to shed light on the connections between water repellency and pH, carbonate and organic matter content in two dune ecosystems with different biocrust types. For this, we used contact angle measurements, gas volumetric carbonate determination and organic matter characterization via FT-IR and TOF- SIMS. In both ecosystems, moss-dominated biocrusts showed higher water repellency and higher amounts of organic matter compared to algal or cyanobacterial biocrusts. Surprisingly, the biocrusts of the two dune systems did not show differences in organic matter composition or organic coatings of the mineral grains. Biocrusts on the more acidic dunes showed a significantly higher level of water repellency as compared to higher carbonate containing dunes. We conclude that the driving factor for the increase in water repellency between cyanobacterial and moss-dominated biocrusts within one study site is the content of organic matter. However, when comparing the different study sites, we found that higher amounts of carbonate reduced biocrust water repellency. Keywords: Organic matter composition; Surface characteristics; TOF-SIMS; Biocrust; Carbonate content; Water repellency. INTRODUCTION (Leelamanie and Karube, 2009; Vogelmann et al., 2013; Woche et al., 2005). OM consists of fresh plant tissues, plant waxes and Water repellency (WR) is an important factor for surface and a high number of amphiphilic compounds like fatty acids. These subsurface water redistribution, plant growth and aggregate compounds can form OM coatings (Graber et al., 2009; Morley stability, as well as soil erosion (Doerr et al., 2000; Zheng et al., et al., 2005). If OM components are mixed with mineral parti- 2016). While WR can occur on a variety of soil types and tex- cles, the WR increases only slightly while a coating of particles tures, it affects soils with a high content of sand particles (like with OM results in more intense WR (Bisdom et al., 1993). dune soils) to a higher degree than soils with a fine texture However, WR is a highly dynamic soil property. For exam- (González-Peñaloza et al., 2013; Woche et al., 2005). Especial- ple, WR increases with decreasing water content (Dekker and ly the amount and composition of mineral particles and organic Ritsema, 1994) while under laboratory conditions WR decreas- matter (OM) affect the extent and persistence of WR. The ef- es with increasing soil pH (Diehl et al., 2010). For Mediterra- fect of texture on WR can be explained by the specific surface nean soils, the persistence of WR in the field was found to area (SSA) of the mineral particles, which increases with de- decrease with increasing pH value (Mataix-Solera et al., 2007; creasing particle size. Hence, for the coating of fine-grained Zavala et al., 2009). soil particles, a higher amount of hydrophobic OM is needed as Biocrusts cover soils as part of early ecological succession compared to coarser particles (González-Peñaloza et al., 2013). or as permanent soil cover in semiarid and in humid climates Assuming the same amount of hydrophobic OM, this relation including sand dunes around the world (Belnap, 2006; Nierop causes a decrease in WR with decreasing particle size (Woche et al., 2001; Tighe et al., 2012). Biocrusts consist of cyanobac- et al., 2005; Zheng et al., 2016). Consequently, the addition of teria, algae, lichens, bacteria, fungi and mosses in different smaller particles like clay and silt decreased WR and texture is ratios depending on climate and successional stage. During the most predictive factor influencing WR (McKissock et al., growth, these organisms influence the soil pH, accumulate 2000). Additionally, the application of lime also affects WR via carbon, nitrogen (Chamizo et al., 2012; Lichner et al., 2018) two mechanis. First, the input of fine particles increases the and other elements (Beraldi-Campesi et al., 2009), stabilize the SSA and secondly because it increases the decomposition of soil surface and change the soil structure (Felde et al., 2014) of hydrophobic compounds by bacteria due to the creation of more the upper soil layer at millimeter scale. Since biocrust for- favorable environmental conditions (Roper, 2005). Most studies mation changes the properties of the very soil surface, it also show a positive correlation of WR and the amount of OM with affects WR and hydraulic conductivity of the soil surface a non-linear increase in WR with increasing OM content (Gypser et al. 2016; Tighe et al., 2012). In most studies, bi- 369 Sylvie Laureen Drahorad, Vincent J. M. N. L. Felde, Ruth H. Ellerbrock, Anja Henss ocrusts show only subcritical WR values, including studies on Negev and moss-dominated biocrusts at less disturbed areas in biocrusts in the Negev (Gypser et al., 2016; Keck et al., 2016; Sekule and northexposed dune slopes in the Negev. Sampling Kidron and Büdel, 2014). The semi-arid northwestern Negev included three depths: i) the topcrust (TC; 0–2 mm) ii) the provides unique growing conditions for biocrust organisms, as underlying subcrust (SC; 2–20 mm) and iii) the topsoil (TS; the sand dunes are part of a nature protection area and the eco- 20–100 mm). We analyzed soil texture, carbonate content, pH system has a high input of dust (Littmann and Schulz, 2008) and CA on these samples. In addition, we used the Water Drop and moisture via dew (Jacobs et al., 2000). The geological Penetration Time (WDPT) test for the description of actual material is enriched in carbonates, has an alkaline pH and bi- repellency on intact in-situ biocrusts (only for the TC). For the ocrust growth is rather fast (Kidron et al., 2020). In contrast to characterization of OM of Negev biocrusts we used the TC and this, biocrusts from the humid Sekule site in Slovakia show SC samples of cyanobacterial- and moss-dominated biocrusts, strong WR on carbonate-free dunes with an acidic pH-value to allow a good comparability with existing results of the same (Lichner et al., 2012). This effect increased with ongoing bi- biocrust types at Sekule site. To test the hypothesis of changes ocrust development and increasing amounts of OM (Drahorad in the organic coatings of mineral biocrust particles, we isolated et al., 2013b; Drahorad et al., 2020). Until now, studies compar- particles from moss-dominated biocrusts of the two study sites. ing the WR and correlated soil properties of biocrusts in con- These TCs are characterized via ToF-SIMS. We concentrated trasting ecosystems are missing. We compared biocrusts of the on these samples as moss-dominated biocrusts showed the Negev dunes (Israel) and at Sekule (Slovakia) to test the hy- highest WR. Therefore, we expect to find the highest differ- pothesis that higher pH values (as induced by higher carbonate ences in coating thickness and composition for mineral particles contents) are correlated with lower levels of WR in biocrusts. between the two study sites. To check this, we correlated contact angle (CA) data with pH values. Moreover, as earlier studies did show that an increase in Sample treatment and analysis WR correlates with an increase in OM but does not relate to OM characteristics at the Sekule site, we hypothesized that the The content of sand particles and finer (i.e. silt and clay) par- same effect will be visible for biocrusts of the Negev. To test ticles (2000–63 µm and < 63 µm, respectively) was classified this, we compared data on OM amounts of two biocrust types at based on wet sieving according to ISO 11277. All samples were each study site and characterized the OM at the Negev site dried at 105°C and sieved (2 mm) and an aliquot was finely using Fourier transform infrared spectrometry (FT-IR). Moreo- ground (0.05 mm) for the measurement of total carbon (C) and ver, organic coatings may play a major role in WR of biocrusts. total nitrogen (N) by dry combustion (Vario EL CNS analyzer). We use Time of Flight Secondary Ion Mass Spectrometry For the Negev samples, the carbonate content was analyzed (TOF-SIMS) as powerful tool for the characterization of envi- gas-volumetrically using a Scheibler apparatus according to ronmental samples to show the surface characteristics of bi- ISO 10963. For the Negev, this includes mainly calcium car- ocrust-associated mineral grains (Arenas-Lago et al., 2016; bonates and to a lesser extent, magnesium carbonates (Rozen- Cliff et al., 2002). As the biocrusts at Sekule developed under stein et al., 2014). The amount of total organic carbon (TOC) humid climate conditions, we hypothesize a higher OM accu- was calculated as the difference between carbonate content and mulation in these biocrusts as compared to biocrusts of the total carbon. The pH value was measured in a 1:5 water extract. Negev (Israel), which in turn may be accompanied by a thicker For the Negev samples, we included an OM characteriza- organic coating of mineral grains. tion via FT-IR. For recording FT-IR spectra, we used 1 mg of ground, desiccated soil (< 0.5 mm) mixed with 80 mg of potas- MATERIAL AND METHODS sium bromide and dried over night over silicagel in an exsicca- tor (Ellerbrock et al., 1999). The mixture was pressed into a The first study site is located at Sekule (southwestern pellet by applying a pressure of 980.7 MPa for 10 min. Infrared Slovakia) with a mean annual precipitation of 550 mm and absorbance spectra of OM were collected in the wave number −1 carbonate-free inland sand dunes as parent material. Because of range of 4,000–400 cm with 16 scans per spectrum. The spec- sand mining for building purposes, an artificial glade arose, tra were smoothed (boxcar moving average algorithm, factor 45) where biocrusts cover the sandy soils. The biocrusts at freshly and corrected for baseline shifts using WIN-IR Pro 3.4 software disturbed areas are thin, algae- and cyanobacteria-dominated (Digilab, Massachusetts, USA). For a detailed description on crusts while thick, moss-dominated crust cover the soil at less FT-IR spectra of the Sekule samples see Drahorad et al. (2020). disturbed areas. For a detailed description of the forest glade Water repellency (WR) measurements included water side and the occurring biocrust species, see Lichner et al. drop penetration time (WDPT) test. It is the fastest in-situ (2013). The second study site is located in the Negev dunes method for assessing the persistence of the actual WR of the (Israel) 25 km north of Nizzana and 12 km south of the town of undisturbed biocrusts. A drop of distilled water (approx. 50 µL) Yevul and is characterized by a mean annual precipitation of is placed on the soil surface and the time that it takes for com- approx. 170 mm and carbonate containing sand as parent mate- plete surface penetration is recorded. Since water only enters rial. Biocrusts stabilize the dunes with thin cyanobacterial the soil if the contact angle between water and soil is less than crusts at the south-exposed slopes and thicker, moss-dominated 90°, the WDPT test is a measure of the time required until the crusts dominate at the wetter, north-exposed dune slopes. For a contact angle reaches values below 90° and thus, of the persis- detailed description of biocrust OM composition and occurring tence of WR rather than its intensity (Iovino et al., 2018; Letey cyanobacterial species see Drahorad et al. (2013a) and et al., 2000). We use the terms ´actual WR` for field-moist Hagemann et al. (2015). samples in natural position and ´potential WR` as the maximum possible WR for samples that were dried by 105 °C, according Sampling of biocrusts and underlying soil to Dekker and Ritsema (1994). A second method that we used for the analysis of WR was At both study sites, we sampled biocrusts in two depths, in- the measurement of the contact angle via the Wilhelmy Plate cluding algae-and cyanobacterial-dominated biocrusts at freshly Method (WPM). The WPM allows the determination of the disturbed areas in Sekule and the southexposed slopes in the advancing and receding contact angle and is theoretically suited 370 Water repellency decreases with increasing carbonate content and pH for different biocrust types on sand dunes – 2– to measure contact angles between 0° and 180° (Bachmann et C (36.00 u), PO (62,97 u) in negative ion mode were used al., 2003). Briefly, we used disturbed samples (< 2 mm) to for internal mass calibration. Surface analysis of the soil parti- create a thin layer of soil particles on a glass slide using double- cles turned out to be challenging due to their heterogeneous sided adhesive tape. When present, aggregates were gently surface properties and the particulate character of the sample crushed in a mortar. Despite the coarse texture of the samples, system. In addition, the topography of the particles has a signif- we decided not to grind them in order to avoid the breaking of icant impact on mass resolution of the obtained spectra. There- sand grains, which would have resulted in the creation of new fore, five regions of each sample set were analyzed in spec- surfaces that likely cause an underestimation of the CA. We trometry mode for comparison. The corresponding mass images measured the advancing contact angle with five repeated meas- were used to set regions of interest (ROI), which were defined urements for each sample, using a dynamic contact angle tensi- by a threshold of 10%–90% pixel intensity of the total ion ometer (DCAT11, Dataphysics, Filderstadt, Germany). image to select the particle areas. By normalization to the total It should be noted that one possible source of error that can ion signal intensity, we minimized the topographic effect not lead to the overestimation of CA for the Sekule samples may only on the mass resolution but also on the signal intensity and have been the coarse texture and the fact that samples were not enabled a comparison of selected mass signals. The mass imag- ground prior to CA analysis. While the Negev samples con- es shown in Fig. 4 were recorded in imaging mode with delayed tained more fine particles, which are likely to have covered the extraction for good mass and good lateral resolution (Henss et complete adhesive tape, this was not the case for the Sekule al., 2018). For the images, areas of 500 x 500 µm with 1024 x samples. Containing fewer silt and clay-sized mineral particles, 1024 pixels were scanned. More detailed information on SIMS it may have been the case that some spots on the adhesive tape measurements can be found elsewhere (Vickermann and Gil- between larger sand grains were exposed to the water, which more, 2009). Data analysis was performed with the “Sur- may have led to an overestimation of the CA for this sample. faceLab 7.1.1” software (ION-TOF GmbH). For surface characterization and the description of min- Data management and statistics included the calculation of eral grain coatings, we analyzed single sand grains by using significant differences between the measured biocrust parame- Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS). ters on a level of significance of 5%, using a 1-way ANOVA. Therefore, we isolated mineral grains of moss-dominated bi- As the data-set was too small for a MANOVA including bi- ocrusts via density separation in deionized water and trans- ocrust depths, types and sampling areas as single fixed factors, ferred them on an adhesive copper tape. The ToF-SIMS meas- we reclassified these samples in 12 equal factors, allowing the urements were performed with a TOF.SIMS M6 instrument calculation of a 1-way ANOVA. Post Hoc Scheffé test was (ION-TOF GmbH, Muenster, Germany) equipped with a 30 used, as the data set was unbalanced. Pearson´s correlation keV Bi-cluster primary ion gun, as well as with a gas cluster coefficient was used to describe correlations (Statistica 14). ion beam (GCIB) and a dual source column (DSC) for sputter- 3+ ing. All analyses were carried out with Bi primary ions, with a RESULTS AND DISCUSSION cycle time of 75 µs (imaging mode) or 120 µs (spectrometry mode) in positive and negative ion mode. Charge compensation The TC samples at different sampling sites show significant was done with low energetic electrons. Mass spectra were differences in pH, carbonate content, texture and CA (p < 0.001) recorded using the spectrometry mode on an analysis area of (Table 1). Interestingly, TC samples of the same biocrust type 2 12 –2 500 x 500 µm keeping a dose density limit of 10 ions cm . A show comparable contents of TOC and N at the two study sites mass resolution of FWHM m/Δm > 3500 at m/z 29.00 (CHO ) (Table 1 and Figure 1). This is surprising, as the humid ecosys- in positive ion mode and m/Δm > 1800 at m/z 26.00 (CN ) in tem in Sekule should in general favor higher biomass accumu- 2+ 3+ negative ion mode was achieved. The signals H , CH (15.02 u), lation due to higher amounts of annual precipitation. With + + + + Na (22.99 u), K (38.96 u), C H (41.04 u), C H (43.05 u) in increasing annual precipitation and available moisture, biocrust 3 5 3 7 – – – positive ion mode and H (2.01 u), C (12.01 u), C (24.00 u), biomass increases (Kidron et al., 2014; Lichner et al., 2018). 2 2 Table 1. Basic biocrust and soil characteristics at Sekule (Slovakia) and Negev dunes (Israel) in three sampling depths (TC 0–2 mm; SC 2– 20 mm; TS 20–100 mm; n = 7/6/4 for Sekule and n = 3 for Negev dunes). Particle size distribution by wet sieving (n = 4 Sekule, n = 3 Negev). All values are means, standard deviation in parenthesis. Different upper case letters denote significant differences between sam- pling depths within the same biocrust type at each sampling site, different lower case letters denote significant differences between biocrust types at the same sampling depth and sampling site and different numbers denote significant differences between comparable biocrusts types at different sampling sites. sand particles fine particles Area type depth pH Carbonates N 2000–630 µm 630–200 µm 200–63 µm < 63 µm [weight-%] [%] [%] A,a,1 A,a,1 A,a,1 A,a,1 A,a,1 A,a,1 A,a,1 Sekule algae TC 4.8 (±0.1) 0.0 0.04 (±0.01) 0.01 (±0.02) 32.43 (±10.84) 57.42 (±9.93) 3.01 (±0.32) A,b,1 A,a,1 A,b,1 A,a,1 A,a,1 A,a,1 A,a,1 SC 4.9 (±0.1) 0.0 0.02 (±0.01) 0.01 (±0.01) 32.35 (±8.17) 61.89 (±7.67) 3.10 (±0.85) A,b,1 A,a,1 A,b,1 A,a,1 A,a,1 A,a,1 A,a,1 TS 4.9 (±0.1) 0.0 0.01 (±0.00) 0.02 (±0.02) 29.07 (±2.30) 65.15 (±3.81) 3.31 (±0.77) A,a,1 A,a,1 B,a,1 A,a,1 A,a,1 A,a,1 A,a,1 moss TC 4.4 (±0.1) 0.0 0.09 (±0.02) 0.00 (±0.00) 28.39 (±9.31) 44.69 (±8.19) 4.69 (±1.29) A,b,1 A,a,1 A,b,2 A,a,1 A,a,1 A,a,1 A,a,1 SC 4.6 (±0.1) 0.0 0.03 (±0.01) 0.03 (±0.01) 34.75 (±15.22) 50.28 (±16.36) 3.93 (±0.35) B,b,1 A,a,1 A,b,2 A,a,1 A,a,1 A,a,1 A,a,1 TS 4.8 (±0.1) 0.0 0.01 (±0.00) 0.03 (±0.03) 37.16 (±13.46) 51.38 (±15.98) 3.68 (±1.23) A,b,2 A,a,2 A,a,1 A,a,2 A,a,2 A,b,2 A,a,1 Negev cyano TC 7.4 (±0.1) 2.6 (±0.2) 0.03 (±0.01) 4.61 (±1.15) 58.56 (±2.40) 33.83 (±2.68) 10.13 (±1.42) A,b,2 A,a,2 A,b,1 A,a,2 A,a,2 A,b,2 A,a,1 SC 8.0 (±0.4) 1.7 (±0.2) 0.01 (±0.00) 3.33 (±0.67) 56.57 (±3.93) 37.00 (±3.85) 5.75 (±0.74) B,b,2 A,a,2 A,b,1 A,a,2 A,a,2 A,a,2 A,a,1 TS 8.4 (±0.3) 1.0 (±0.2) 0.01 (±0.00) 2.34 (±0.60) 58.08 (±1.80) 36.27 (±1.66) 5.76 (±2.36) A,b,2 A,b,2 B,a,1 A,a,2 A,a,2 A,a,2 B,b,2 moss TC 7.1 (±0.2) 7.3 (±1.1) 0.10 (±0.02) 3.30 (±0.82) 57.97 (±3.68) 34.05 (±3.37) 26.92 (±2.09) A,b,2 A,b,2 A,b,2 A,a,2 A,a,1 A,a,2 A,b,2 SC 7.6 (±0.2) 5.4 (±2.2) 0.04 (±0.01) 4.08 (±1.42) 54.28 (±1.65) 37.70 (±2.91) 14.94 (±2.38) B,b,2 B,b,2 A,b,2 A,a,2 A,a,1 A,a,2 A,a,2 TS 8.0 (±0.2) 4.2 (±0.5) 0.02 (±0.01) 2.96 (±0.79) 57.65 (±2.34) 35.71 (±1.78) 11.44 (±4.36) 371 Sylvie Laureen Drahorad, Vincent J. M. N. L. Felde, Ruth H. Ellerbrock, Anja Henss Fig. 1. Variability plot showing the total organic carbon (TOC) of two biocrust types (algae/cyano; moss) in three sampling depths (TC 0–2 mm; SC 2–20 mm; TS 20–100 mm) in Sekule and the Negev. Letters indicate a significant difference (p < 0.05) between sampling depths within the same biocrust type (a), a significant difference between the biocrust types at the same sampling depth and sampling site (b) or a significant difference between comparable biocrusts types at different sampling sites (c). We therefore assume that the available moisture is lower in of Sekule (Drahorad et al., 2020). The FT-IR spectra are Sekule, as water infiltrates fast and the biocrusts dry rapidly characterized by the same bands as the previously recorded for due to the high content of sand sized particles (> 95%). In con- data on biocrusts in Sekule (Figure 2). This included the bands –1 trast, higher amounts of fine sized particles (Table 1) increase relevant for the WR of soils at the wavenumbers at 2925 cm + –1 –1 the water holding capacity and wetness duration, likely favor- 2858 cm (aliphatic C-H) and 1635 cm (C=O, aromatics) ing a higher biomass build-up in cyanobacterial Negev (Ellerbrock et al., 2005). This similarity in the spectra is in biocrusts (Kidron et al., 2009). accordance with FT-IR data of biocrusts sampled in humid climate (Fischer et al., 2013). Nierop et al. (2001) did show a Total organic carbon and pH-values of biocrusts comparable pattern of polysaccharides in marine algae, soil algal mats and moss-covered dune sand. In general, bacterial vs. The TCs show lower pH values as compared to SC and TS fungal materials show the same pattern in C NMR-spectra samples (with the exception of the TC of the algae biocrust at with high proportions of alkyl-C structures and polysaccharides Sekule). This is in accordance with findings for biocrust- (Kögel-Knabner, 2002). NMR-spectra of biocrusts from the covered soils located in humid regions of eastern Germany (pH Negev and eastern Germany showed similar OM composition decrease from 4.8 to 4.2) or in semiarid regions within the and also similarities to cell spectra of algae (Fischer et al., Negev (pH decrease from 8.6 to 7.6) (Fischer et al., 2010; Keck 2013). More studies on the OM composition are needed to et al., 2016). The biocrust samples from both sites, Negev and reveal general (i.e. global) patterns. Nevertheless, for the Sekule, show a decrease in pH from the cyanobacterial/algae comparison between Negev and Sekule biocrusts we assume biocrust to the moss-dominated biocrust. This trend was also that the biocrust OM composition is not the relevant driver for found for a development from algae to moss-dominated bi- differences in WR. ocrusts in the Netherlands (pH decrease from 4.8 to 4.2) Differences in the FT-IR spectra were pronounced for the –1 (Nierop et al., 2001). bands 1085 and 1033 cm (polysaccharides, silicates and clay The accumulation of TOC is significantly higher in the TC minerals) showing a double peak for Negev biocrusts compared of moss-dominated biocrusts at both sites as compared to the to Sekule biocrusts. This reflects the higher amount of fine SC and TS underneath and as compared to the algae- or cyano- particles and therefore likely higher clay mineral content in bacterial-dominated biocrust (Figure 1). For Sekule, the two these samples. Moreover, the second difference between the –1 examined biocrust types represent an early and a late succes- spectra are bands near 1430 and 875 cm , which are character- sional development stage, respectively. As late successional istic for carbonates (Smidt et al., 2002). For the Negev bi- –1 biocrusts show higher gross photosynthesis than early succes- ocrusts, the reduction in absorbance of the spectrum at 875 cm sional biocrusts, that can induce higher overall TOC accumula- in the order cyanobacterial TC>cyanobacterial SC>moss tion in these biocrust (Miralles et al., 2018). TC >moss SC is in line with the carbonate concentrations de- termined (Table 1). The results demonstrate that FT-IR can also Organic matter characterization of Negev dune biocrust be used for carbonate concentration measurements in biocrusts as already shown for other carbonate-containing soils (Tatzber FT-IR was used to describe the OM of the biocrusts at the et al., 2007). Negev site as already done in an earlier work for the biocrusts 372 Water repellency decreases with increasing carbonate content and pH for different biocrust types on sand dunes Fig. 2. FT-IR spectra of cyanobacterial and moss-dominated biocrust of the Negev (green = cyanobacterial topcrust; red = cyano- bacterial subcrust; violet = moss-dominated topcrust; blue = moss-dominated subcrust). Insert for comparison: mean FT-IR spectra of algae and moss top- and subcrusts at Sekule, original data see: Drahorad et al., 2020). Arrows indicate differences in FT-IR spec- tra with bands indicating higher clay content (black arrow) and bands indicating the occurrence of calcium carbonate (grey arrows). Table 2. Intensity of water repellency (contact angle) in the examined biocrusts at Sekule (Slovakia) and Negev dunes (Israel) in three sampling depths (TC 0–2 mm; SC 2–20 mm; TS 20–100 mm; n = 7/6/4 for Sekule and n = 3 for Negev) and persistence of water repellency (actual repellency) of the undisturbed biocrusts in situ (n = 10). Different upper case letters denote significant differences between sampling depths within the same biocrust type at each sampling site, different lower case letters denote significant differences between different biocrust types at the same sampling depth and sampling site and numbers denote significant differences between comparable biocrusts types at different sampling sites. Area crust type depth contact angle [°] actual repellency / WDPT [s] A,a,1 Sekule algae TC 111.75 (±11.92) very hydrophilic 0 (±0) A,a,1 SC 109.90 (±10.29) – B,a,1 TS 88.39 (±7.26) – A,b,1 moss TC 135.01 (±8.99) moderately hydrophobic 294 (±14) B,a,1 SC 113.63 (±4.37) – C,a,1 TS 93.54 (±4.24) – A,a,2 Negev cyano TC 29.07 (±2.87) very hydrophilic 0(±0) A,a,2 SC 27.43 (±13.65) – A,a,2 TS 29.40 (±6.50) – A,b,2 moss TC 86.70 (±11.75) very hydrophilic 0(±0) A,b,2 SC 62.50 (±11.08) – B,a,2 TS 38.77 (±9.55) – Actual repellency and intensity of WR of biocrusts tial WR resistance for algae biocrusts at Sekule study site. Two treatment effects explain this difference. First, the concept of The WDPT of the undisturbed biocrust in-situ shows that potential WR includes drying of samples and the potential WR only one biocrust is moderately hydrophobic (Table 2). Early of soil samples increases with increasing drying temperature biocrusts on Sekule sand and biocrust growing on carbonate (Dekker et al., 2001; Diehl et al., 2009). Moreover, the disturb- containing sands of the Negev do not show an actual WR per- ance itself during sampling has a profound influence. Graber et sistence. These values are below WR values of sandy soil sur- al. (2006) identified strong differences of WR between dis- faces under various European pine forests (< 433 s) (Iovino et turbed and undisturbed samples in sandy soils. They hypothe- al., 2018) and below the actual repellency found on non- sized that the reason for these changes relate to differences in calcareous sand dunes in the Netherlands (600–3600 s) (Dekker surface roughness, pore size distribution, pore connectivity, et al., 2001). Compared to these rather low values of the actual bulk density and changes in the distribution and orientation of repellency, the CA data shows a moderate to very strong poten- the substances that are responsible for repellency. Moreover, 373 Sylvie Laureen Drahorad, Vincent J. M. N. L. Felde, Ruth H. Ellerbrock, Anja Henss even very thin layers of fine particles covering biocrusts chang- of OM increase as well, and so does their WR (Drahorad et al, es the WR on the surface (Cania et al., 2020; Fischer et al., 2020; Gypser et al., 2016; Lichner et al., 2018). For both study 2010). For example, sand burial of moss-dominated biocrusts in sites, TOC and CA are highly correlated (r = 0.81 Sekule and r the Tengger desert was reported to decrease WR (Jia et al., = 0.83 Negev). This confirms part one of our hypothesis on OM 2020). These results show the importance of the biocrust sur- dynamics, namely that an increase in biocrust OM induces an face structure and the in-situ integration in the ecosystem for increase in WR. Plotting the complete data set confirms visual- real field site WR. Therefore, measurements of intact biocrusts ly that the Sekule biocrusts show higher WR for samples with a are relevant for the evaluation of water flow pathways in bi- similar TOC content compared to Negev biocrusts (Figure 3). ocrust covered ecosystems. Therefore, we assume that within each study site, the differ- In contrast to the actual repellency determined by WDPT, ences in WR result from a higher overall amount of OM in the the intensity of WR shows CA of above 100° for both biocrust moss-dominated biocrusts as compared to the algal- types at Sekule and CA up to almost 90° for the moss- /cyanobacterial crusts. This effect may be stronger at the Sekule dominated biocrust of the Negev. Here, differences between site, as Wang et al. (2010) found that soil organic carbon af- crust types are most obvious. While the values for all depths of fected WR stronger in soils that were classified as repellent, the cyanobacterial biocrust of the Negev do not show any dif- while texture and pH had a higher impact on WR in wetta- ferences, for the moss-crust a clear increase from 38.77° to ble/non-repellent soils. 62.50° and finally 86.70° can be observed from TS to SC and Based on the conclusion that an increasing OM content is finally to TC. Different CA between the different sampling inducing a higher WR for biocrusts at the same study site but depths, which show the effect of OM accumulation by the crust that OM amount or composition do not explain the differences organisms, are obvious for all but the cyanobacterial crust from between the study sites, two effects may explain the differences the Negev. The fact that differences between crust types in in the WR. First, the biocrusts at the Negev reveal a higher pH Sekule are very low (and in fact are only significant in the case value than the biocrusts in Sekule and pH and CA show a of the TC vs. TS in the moss-dominated biocrust) may be indic- strong negative correlation (r = –0.76 Sekule and r = –0.73 ative for the effect of texture and pH at this study site. Soils Negev). Studies on WR and pH on semiarid alkaline soils below the Sekule biocrusts have a coarser texture and a more showed lower persistence of WR compared to acidic soils acidic pH compared to the Negev soils. (Mataix-Solera et al., 2007). In their meta-analysis, Zheng et al. (2016) also reported a negative correlation between pH and WR, Relation between WR, TOC and pH value of biocrusts while soil organic carbon generally correlates positively with WR. In both ecosystems, algae- or cyanobacterial-dominated bi- Deprotonation of surface sites and the changes in OM ocrusts show lower TOC content and CA than moss-dominated confirmation are the mechanisms that explain the effect of pH biocrusts. This trend is in line with earlier studies on WR of changes on WR (Diehl et al., 2010; Doerr et al., 2000). Doerr et biocrusts. As biocrusts develop, their thickness and the amount al. (2000) state that this effect is strong enough to explain all Fig. 3. Correlation between contact angle and amount of total organic carbon (TOC) for all samples at the study site Sekule (Slo- vakia; n = 34) and the Negev (Israel; n = 18). Dashed lines showing the regression bands (level of confidence 0.95). 374 Water repellency decreases with increasing carbonate content and pH for different biocrust types on sand dunes changes in WR. We doubt this for the examined biocrusts as the intensity refers to an increased amount of Ca and most likely of changes between the study sites are very strong. As the correla- CaCO for the Negev sample. Unfortunately, carbonate frag- tion between H -concentration and TOC is very high (r = 0.75 / ments cannot be assigned specifically as there is an overlap of – – – – r = 0.91, Sekule/ Negev), it is not possible to separate the effect CO /Si and CO /SiO . Due to the sample roughness, the mass 3 2 pH has on the detected WR. Nevertheless, at the Negev site, the resolution is not sufficient to differentiate between these over- amount of carbonates has a higher correlation with CA (r = lapping peaks. Nonetheless, the SiO signal is also significant- 0.81) than the pH (r = –0.62). This indicates that the amount of ly increased for the Negev sample, which is certainly due to the – – carbonates has a higher effect on WR than the pH value. In our signal overlap of CO and SiO . This is supported by the 3 2 opinion, this highlights the more relevant second variable influ- higher C content of the Negev biocrust samples (Table 1) and encing the differences in WR between the study sites, namely accounts for a higher fraction of carbonate-bound C in the texture. The Negev biocrusts are composed of a high amount of sample from the Negev site. Figure 4 shows exemplary mass finer particles compared to Sekule biocrusts. In general, WR images of particles from both sites in positive and negative ion decreases with particle size (González-Peñaloza et al., 2013). In mode. Beside the particular structure, the overlay of different this study, the dunes at Sekule show significantly higher mass signals shows clearly the heterogeneous composition of amounts of coarse and middle-sized sand grains, while the the surface layer. It can be seen that the organic fragments – + Negev biocrusts show significantly higher amounts of fine represented by CN and C H O are only found in certain areas 3 3 sized sand grains, fine particles (< 63µm) and carbonates (Ta- and hence, the sand grains are not completely covered by OM. ble 1). First results on the particle size distribution of the fine fraction < 63µm show around 5% clay and up to 20% silt with- CONCLUSIONS in this fraction (unpublished data). This material mix reduces WR effectively as shown by McKissock et al. (2000). Moreo- We compared the WR of two biocrusts types on carbonate- ver, Harper et al. (2000) did show that more TOC was needed free and carbonate-containing sand dunes and examined the to induce WR in soils that have clay contents above 5%. In effect of OM, pH and carbonate content on WR. We conclude addition, the Negev biocrusts contain up to 7.3 (±1.1) weight-% that the driving factor for the increase in WR within the carbonates (Table 1). This may have a direct effect on WR as individual sampling sites is the OM content and not the OM well, as addition of powdered lime effectively reduced WR in composition. However, this is only true for comparisons within soils during remediation trials (Roper, 2005). one site, but not among sites. The high differences in potential WR between the study sites is related neither to OM amount, Surface characterization of mineral particles separated nor to changes in OM composition or organic coating from moss-dominated biocrust characteristics. The most relevant factors explaining the lower WR in biocrusts of the Negev are the higher amounts of No significant difference in the composition of organic carbonates and the related higher pH values. Moreover, the fragments can be found in the mass spectra from Sekule and Negev biocrusts show higher amounts of fine particles that are Negev (Figure 4). But for the inorganic compounds an in- likely to reduce WR as well. As carbonates are destructed + + + + creased intensity of Ca , Si , Mg and Fe was found for the during the texture analysis, further studies are needed to sample from the Negev site. The higher detected amounts of identify the textural effect that carbonates may have on + + Mg and Fe may have an influence on WR. Harper et al. (2000) biocrusts’ WR by increasing the amount of fine particles. This found that these minerals reduce WR in soil samples. Moreover, could be done by comparing the effect of siliceous vs. in the Negev samples a higher content of CaPO was detected carbonate mineral particles of the silt fraction on the WR of + – in the negative ion mode. The higher Ca and CaPO signal different soils. 500 x 500 μm² 200.00 μm 500 x 500 μm² 200.00 μm Si+ Si+ C3H3O+ C3H3O+ Fe+ Fe+ 200.00 μm 500 x 500 μm² 500 x 500 μm² 200.00 μm SiO - SiO - 2 2 CN- CN- CaPO - CaPO - 3 3 Fig. 4. Images of TC mineral particles from moss-dominated biocrusts (Negev site = left; Sekule site = right) show an overlay of the + + + – – – Si , C H O and Fe signal in positive ion mode and SiO , CN and CaPO signals in the negative ion mode. The visualizations 3 3 2 3 + – show the heterogeneous surface composition of the particles and proof that the organic crust (represented by C H O and CN in 3 3 green) is not covering the complete particle. 375 Sylvie Laureen Drahorad, Vincent J. M. N. L. Felde, Ruth H. Ellerbrock, Anja Henss Acknowledgements. For partial funding, we thank the DFG (FE weber, P., 2013a. Spatial carbon and nitrogen distribution 218/14-1). Likewise, we are grateful for logistical support by S. and organic matter characteristics of biological soil crusts in Berkowicz (Hebrew University of Jerusalem) and L. Lichner the Negev desert (Israel) along a rainfall gradient. J. Arid (Slovak Academy of Sciences). In addition, we thank D. Environ., 94, 18–26. Steckenmesser, F. 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The Received 30 March 2021 influence of clay type on reduction of water repellency by Accepted 13 July 2021
Journal of Hydrology and Hydromechanics – de Gruyter
Published: Dec 1, 2021
Keywords: Organic matter composition; Surface characteristics; TOF-SIMS; Biocrust; Carbonate content; Water repellency
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