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a Leibniz-Zentrum für Agrarlandschafts- und Landnutzungsforschung (ZALF) e.V., Institut für Bodenlandschaftsforschung, Eberswalder Strasse 84, 15374 Müncheberg, Germany
b Universität Hannover, Institut für Bodenkunde, Herrenhäuser Strasse 2, 30419 Hannover, Germany
* Corresponding author (hgerke{at}zalf.de).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIAL AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: A, absorption band indicating hydrophobic functional groups • B, absorption band indicating hydrophilic functional groups • CA, contact angle • FT-IR, Fourier-transform infrared • NMR, nuclear magnetic resonance • Nt, total N • PY, sodium pyrophosphate soluble • SOC, soil organic C • SOM, soil organic matter • W, water soluble
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIAL AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Water repellence of soils and clay fractions was found to be positively correlated with SOM content (e.g., Chenu et al., 2000; Jaramillo et al., 2000; Mataix-Solera and Doerr, 2004). However, such relations could not always be confirmed (e.g., Horne and McIntosh, 2000). To explain the conflicting results, Negre et al. (2002) suggested considering the macromolecular behavior of SOM that depends on chemical composition and three-dimensional spatial structure of the organic matter. However, the relation between SOM composition and soil wettability is largely unknown or was hypothesized based on results obtained with indirect methods. Soil organic matter composition was analyzed to improve understanding of effects of soil management and fertilization (e.g., Hayes and Clapp, 2001; Stevenson, 1994) rather than of soil physical properties such as wettability.
Soil organic matter is composed of the hydrophobic carbon backbone and of functional groups (e.g., Jenkinson, 1988), which both affect soil wettability in a characteristic way (e.g., Ma'shum and Farmer, 1985; Hayes and Clapp, 2001). The class of water repellent substances in SOM includes aliphatic constituents (DeBano, 1981; Capriel, 1997) and waxes (Franco et al., 2000). For classes of pure chemical substances, the amount of hydrophilic C=O-groups (i.e., O and N containing hydroxyl and carboxyl groups) relative to that of hydrophobic CH-groups determines the hydrophobic character of SOM, and a larger carbon backbone allows for more complex three dimensional arrangements of organic molecules (Morrison and Boyd, 1983). Under dry conditions, the hydrophilic groups within SOM will closely approach each other (Ma'shum and Farmer, 1985) if the carbon backbone is large enough. This results in a coiling of molecules, which form a hydrophobic outside around a hydrophilic center (Sposito, 1989; Stevenson, 1994). Vice versa under moist conditions, the center of the SOM molecules becomes hydrophobic (e.g., Yates and von Wandruszka, 1999). The conformation of SOM is also affected by pH value, salt content, and ion composition of solution (e.g., Sposito, 1989; Stevenson, 1994; Falbe and Regitz, 1998).
In contrast to pure chemical substance classes, the ‘umbrella’ term SOM describes a complex mixture that is different in composition, structure, wettability, and solubility (Hayes and Clapp, 2001). Soil organic matter may be grouped in (i) water soluble components, which are assumed to be located on particle surfaces after drying processes (e.g., Falbe and Regitz, 1998); (ii) potentially soluble components such as sodium pyrophosphate extracts, which are assumed to be mainly adsorbed onto clay minerals (Sposito, 1989); and (iii) insoluble components, which are assumed to be not accessible inside of soil aggregates or at inactive particle surfaces (Hayes and Clapp, 2001). The spatial arrangement of the hydrophobic components within SOM strongly affects wettability (Roy and McGill, 2000) and is again depending on SOM-mineral interactions.
As for pure chemical substances, it may also be assumed for SOM, that the ratio between hydrophilic and hydrophobic functional groups (e.g., C=O/CH ratio) could be used for indicating wettability. However, in previous attempts to define a hydrophobicity index that includes such relations, defined chemical substance classes were still be assumed (e.g., Akamatsu and Fujita, 1992; Gross and Logan, 1995).
While the wettability of individual particles depends on SOM molecular structure and composition, the wettability of a soil depends on the composition and spatial arrangement of the differently hydrophobic particles (e.g., Bachmann et al., 2001). A direct measurement of the soil CA is generally not possible because of the irregular surface topography. Consequently, indirect methods like the capillary rise method (e.g., Adamson, 1990; Michel et al., 2001) or the Wilhelmy plate method (Bachmann et al., 2003) are increasingly applied.
Methods to analyze SOM composition include FT-IR or nuclear magnetic resonance (NMR) spectroscopy (e.g., Hesse et al., 1984; MacCarthy and Rice, 1985). In FT-IR spectra, absorption bands at distinct wave numbers indicate the presence of functional groups with known chemical compositions and properties. The intensity of the aliphatic (CH) absorption band in DRIFT FT-IR spectra was used to estimate the hydrophobic character of soil samples (Capriel et al., 1995; Capriel, 1997; McKissock et al., 2003). The analysis of SOM conformation requires the combined use of several spectroscopic (e.g., x-ray, NMR, FT-IR) and chromatographic methods (e.g., Hesse et al., 1984).
In this paper, the chemical composition of SOM functional groups of differently soluble SOM fractions is compared with wettability data of these soils. The objective is to analyze effects of SOC content, SOM composition and solubility, SOC/clay ratio, and SOM-mineral interaction on CA values to improve the understanding of SOM for transport and retention processes. We studied forest soils since extraction of soluble SOM fractions is easier (Ellerbrock et al., 1999b) and more linked to vegetation (Quideau et al., 2001) as compared with arable soils where SOM reflects effects of management practices (Gerzabek et al., 1997, 2001).
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MATERIAL AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIAL AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The Chorin site is located about 12 km northeast of the city of Eberswalde in eastern Germany within a deciduous forest area dominated by beech trees (Fagus sylvatica). The coarse-textured soil is classified as Cambisol according to FAO with hydromorphic features (endostagnic) in the subsoil. Soil samples were collected in April 2002 from 0- to 5-, 5- to 10-, 10- to 13-, 13- to 18-, 40- to 60-, and 60- to 70-cm depth. While the Ah1 and Ah2 are relatively similar in texture and pH, they differ strongly in the content of organic C (i.e., 37.9 and 13.4 g kg–1 for Ah1 and Ah2; see Table 1). Still, a considerable content of SOC (0.92 g kg–1) was observed in 60- to 70-cm depth. Highest C/N ratios of 18 and 19 in the topsoil are reflecting effects of beech litter. For the subsoil horizons, where Nt contents are close to the analytical detection limit, the C/N ratios are not considered. The pH values (3.8–4.2) are relatively uniformly distributed with depth. Clay content is relatively low (55 g kg–1) in all topsoil horizons and increases considerably in the subsoil.
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The Waldstein site, located northeast of the city of Bayreuth, is a coniferous forest that is dominated (about 90%) by Norway Spruce (Picea abies). The soil is classified as Podzol (FAO). Soil samples were from 0- to 10-, 10- to 12-, 14- to 28-, 32- to 53-, and 57- to 68-cm depth. Clay contents are relatively uniform (about 100 g kg–1) except for the Bh horizon (164 g kg–1). The SOC content decreases in the sequence Bh > Bhs > AhE > BwC > C (Table 1). The pH values (2.9–4.3) are increasing with depth.
Chemical Characterization and Organic Matter Extraction
Soil samples were air-dried and passed through a 2-mm sieve. The pH was determined from a 5-g soil sample in 0.025 dm3 of 0.0l M calcium chloride solution with a SCHOTT pH electrode (Schott Instrument, Woburn, MA). Organic C content (SOC) was calculated as the difference of total C and carbonate carbon. Carbonate carbon was determined after application of phosphoric acid by gas-chromatographic analysis of carbon dioxide evolution. The content of total C in the soil samples was determined by elemental analysis (CNS 2000, LECO Ltd., Mönchengladbach, Germany) as carbon dioxide via infrared detection after dry combustion at 1250°C (DIN ISO 10694, 1994) in duplicate. The detection limits are 0.1 g kg–1 for SOC and 0.09 g kg–1 for Nt. Soil texture was determined by wet-sieving and sedimentation with Köhn-Pipette method after organic C destruction with H2O2 and chemical dispersion using Na4P2O7 (Hartge and Horn, 1992).
The air-dried and <2-mm sieved forest soil samples from Chorin, Steigerwald, and Waldstein were sequentially extracted. In a first step, SOM(W) and in a second step, the SOM(PY) were obtained. For the water extraction, 5 g of soil was mixed with 0.05 dm3 deionized water (Nierop and Buurman, 1998). The mixture was shaken for 24 h using a roller mixer (SRT2, Steward Scientific, UK) at room temperature. The solid residue was separated by centrifuging (1400 x g for 35 min). The solution was filtered through a 0.45-µm membrane filter (Schleicher and Schuell, Dassel, Germany). For the sodium pyrophosphate extraction, the remaining solid residue from water extraction was mixed with 0.05 dm3 of 0.1 M Na4P2O7 solution in deionized water (Ellerbrock et al., 1999a) and shaken for 6 h at room temperature (Hayes, 1985). The solid residue was separated by centrifuging (1400 x g for 35 min) and filtered at 0.45 µm. The remaining solution was adjusted with 1 M HCl to pH = 2, to precipitate SOM(PY). Precipitation was completed after 12 h and the mixture was centrifuged (1400 x g for 30 min). Both water and Na4P2O7 extracts were washed free of salts by using a dialysis membrane with a pore size of 2.5 to 3 nm (NADIR, Roth, Karlsruhe, Germany), and freeze-dried.
Fourier-Transform Infrared Analysis
The SOM fractions were analyzed with a BioRad FTS 135 (BioRad Corp., Hercules, CA). The KBr technique (Celi et al., 1997) was applied to obtain absorption spectra of organic matter in a range of wave numbers between 3900 and 400 cm–1. Here, 0.5 mg of either air-dried finely ground soil (i.e., for analyzing total SOM composition) or freeze-dried water- and Na4P2O7–soluble extracts [i.e., for analyzing composition of SOM(W) and SOM(PY)] was mixed with 80 mg of KBr and finely ground in an agate mortar. The resulting mixture was dried for 12 h over silica gel in a desiccator to standardize the water content. For all spectra, 16 scans were performed at a resolution of 1 cm–1 (Ellerbrock et al., 1999a). The replicate error is generally within the line thickness used in the figures (Ellerbrock and Gerke, 2004).
The FT-IR spectra were analyzed only at two absorption bands that indicate the hydrophobic (CH-groups) and hydrophilic (CO-groups) functional groups. For hydrophobic methyl and methylene groups, the CH-bands occur at 2920 cm–1 (asymmetric stretch) and at 2860 cm–1 (symmetric stretch) (Capriel et al., 1995); both bands were here combined to a single one (3020– 2800 cm–1) and denoted as absorption Band A. Hydrophilic C=O-groups occur at 1640 to 1615 and 1740 to 1720 cm–1 (Celi et al., 1997; Günzler and Böck, 1990). Here, we used slightly different bands 1640 to 1620 and 1740 to 1710 cm–1 to exclude a possible overlap with C=C and amid bands, and denoted both as absorption Band B. The OH-bands were not considered because they could possibly reflect differences in water contents. The heights of absorption Bands A relative to those of Bands B (A/B ratio) in the FT-IR spectra were computed using BioRad WINIREZ (BioRad Corp., Krefeld, Germany) software. Ash spectra subtraction was not required (Senesi et al., 2001) since the regions of Bands A and B are not affected by Si–O–Si bands (Günzler and Böck, 1990).
Capillary Rise Method
Wettability of soil samples was determined with the capillary rise method (Adamson, 1990). The CA 
(°) was calculated with the Washburn equation, which is derived from Poiseuille's law of liquid flow through a cylindrical capillary (Washburn, 1921):
 | [1] |
L is the surface tension of the liquid (mJ m–2), 
is the viscosity of the liquid (Pa s), and t is time (s). From Eq. [1] an expression for the mass increase, w (kg), of the soil column during the capillary rise process can be derived (Siebold et al., 1997) as:
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is the liquid density (Mg m–3) and C is a geometry factor (m5) that reflects porosity and tortuosity of the capillaries and depends on particle size and bulk density, and can be formulated (e.g., Michel et al., 2001) as:
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is a tortuosity approximation of the capillaries and n is the number of capillaries with a mean radius 
. The empirical factor C in Eq. [3] was evaluated independently in measurements using the same soil with identical bulk density and using n-hexane (L = 18.4 mJ m–2 at 20°C) as a reference liquid, which completely ( = 0°) wets the soil particles. The main source of error associated with the capillary rise method is based on the requirement to analyze two samples of identical bulk densities. This problem was minimized by using the following procedure: a glass tube was filled with 2 g of the air dry <2-mm sieved relatively homogenized soil with a sintered glass plate at the base. The sintered glass plate was covered by filtration paper. The soil was compacted in a reproducible way by dropping the glass tube sample 20 times onto the table from a height of approximately 1.5 cm by using an eccentric disc device.
After compaction, the tube was attached to an electronic balance and brought into contact with the respective test liquid. The experiments were performed with a precision tensiometer (DCAT 11, Data Physics, Filderstadt, Germany; resolution 0.00001 x g). The weight gain of the sample in contact with the liquids was recorded at a rate of 30 measurements per second. The C factor was determined from the slope of the hexane absorption rate in the linear range of the w2(t) function. Preliminary experiments showed that differences in the slope of the hexane absorption curves for different samples of the same soil were relatively small. For every soil, one sample was measured against hexane and two against water. The calculation of the CA was performed according to Siebold et al. (1997) from the ratio of the slopes of the water (i.e., arithmetic means of the two curves) and the hexane absorption rates in the linear range of the graph w2 = f(t). The room temperature was recorded (±0.1°C precision) during each measurement to adjust the values of 
, , and of the test liquid (Eq. [2]). Standard deviations of CA measurements for the different homogenized samples of the same soil were in the range of 5 to 6° (see Goebel et al. [2004] for further details). Here, three wet-dry sequences were measured.
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RESULTS |
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For the upper four horizons of the Chorin soil, the A/B ratios of SOM(W) are at a level of about 0.2 (Table 2). The A/B ratios of SOM(PY) from Chorin are higher than those of SOM(W). The smallest A/B ratios were found for bulk soil. For extracts as well as for bulk soil, largest A/B ratios are in the Bw horizon. The (Ah)Bw horizon shows lowest A/B ratio of SOM (PY) and also the smallest value of the CA. In the FT-IR spectra of SOM(W) from Steigerwald samples, the A/B ratios increase with depth except for the subsoil horizon (Table 2). For SOM(PY), the largest values of the A/B ratio were found for the Bgw1 and the smallest for the Ah horizon. The differences in A/B ratio between the horizons are larger for SOM(PY) than for SOM(W), however, these differences are generally smaller than those observed for Chorin samples. In the FT-IR spectra of SOM(W) and SOM(PY) from Waldstein samples, the largest A/B ratios occur in the C horizon while for bulk soil, it is in the AhE horizon (Table 2). The A/B ratios of SOM(W) are comparable with those of SOM(PY). As for the extracts from the other soils, the smallest values of the A/B ratio were found for the humic topsoil.
Contact Angles
The contact angles of the Chorin samples (Table 2) vary between 52.1 and 78.1°. Such subcritical water repellence values (Hallett et al., 2001) indicate that the samples are not fully wettable but still not hydrophobic. The largest value of the CA (78.1°) was measured for the Ah1 horizon with the highest SOC content (Table 1). However, for the Cg horizon, where SOC content (0.9 g kg–1) is smallest, CA was also relatively high (72.6°). Almost the smallest CA (53.8°) was measured for the Ah2 horizon with the second largest SOC content.
For Steigerwald soil, the CA values vary between 59.2 and 88.3° (Table 2). These values indicate a larger difference in wettability between soil horizons as compared with Chorin soil. The highest CA (88.3°) was measured again for the Ah sample with the highest SOC content. Corresponding with Chorin soil, a relatively high CA (83.8°) was found for the subsoil (2C horizon) with the smallest SOC content. The smallest CA (59.2°), however, was measured for the Bw horizon, where the SOC content is ten times larger than in the 2C horizon (Table 1).
The CA values of the Waldstein soil samples vary between 58 and 84° (Table 2). Here, the largest value of the CA (84°) was measured for the C horizon with the lowest SOC content (1.7 g kg–1). The smallest CA (58°) was found for the BwC horizon. The sample with the highest SOC content (Bh horizon) has also a high CA (80°). Smaller CA values were measured on samples from Bhs and AhE horizons.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIAL AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Let us assume that SOM is adsorbed onto mineral surfaces via polyvalent cations such as Ca2+ or Fen+ (Fig. 5) . In case of a ‘low’ SOM/mineral ratio (i.e., the number of SOM molecules relative to the sorption sites of surface-active mineral particles) (Fig. 5 left), the hydrophilic groups of SOM molecules are directed toward the mineral surface. Under such conditions, the hydrophobic groups can be more effective with respect to soil wettability than indicated by their relative amount in SOM composition (A/B ratio) determined by FT-IR. With increasing SOM content, the sparse flat structure of the SOM molecules (Clapp et al., 2001) changes to a dense upright orientation (i.e., ‘medium’, Fig. 5 center). ‘Upright’ indicates that not all hydrophilic groups are able to contact the mineral surface such that both hydrophilic and hydrophobic functional groups appear at the outer surface. Thus for a ‘medium’ SOM/mineral ratio, soil wettability may correspond with A/B ratios determined by FT-IR. In case of larger SOM/mineral ratios (‘high’, Fig. 5 right), the surface of the organic matter molecular layer can interact with ‘surplus’ SOM-molecules if polyvalent cations are present. The outer surface again becomes more hydrophobic since the hydrophilic functional groups of secondary molecular layers initially tend toward the inside and the carbon backbones toward the outside. With further increasing SOM content, molecular structures may become more complex (e.g., Sposito, 1989). This simplified scheme suggests that also for a large SOM/mineral ratio, soil wettability may differ from the A/B ratios determined by FT-IR. Similar interactions, which are known from surface chemistry (Falbe and Regitz, 1998), could probably be transferred to composite soil systems (Stevenson, 1994).
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For A/B* of SOM(PY), no correlation with the corresponding CA values was found (Fig. 6a) . Values of the G-factor were then optimized such that for each soil type, a separate and nearly linear relation between A/B*G and CA was obtained when keeping G = 1 for samples with SOC around 10 g kg–1 (Fig. 6b). The selected G values for SOM(PY) resulted in a similar dependence of functional group effectiveness on SOC/clay ratio (Fig. 7a) as conceptualized in Fig. 5 that seems to be independent of soil type. For all three soils, SOC/clay ratios of about 0.2 corresponded with SOC contents of about 10 g kg–1 for a G factor of around 1. For SOC/clay ratios > 0.2 and SOC contents mostly >10 g kg–1, a factor of G < 1 (i.e., toward about 0.8), and for ratios <0.2, values of G > 1 (i.e., to about 40), were required to obtain the assumed linear relation between A/B*G and CA.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIAL AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The analyses in this paper suggest that soil wettability can be better interpreted if SOM composition and its ‘effectiveness’ are considered. The ratio between hydrophilic and hydrophobic SOM functional groups (A/B ratio) is increasing corresponding with CA values for bulk soil. For soluble SOM fractions, the A/B ratios (i.e., reflecting the ‘quality’ of SOM) are not sufficient to explain the CA data. Improvements can be obtained by weighting the A/B ratios with SOC content (i.e., reflecting the ‘quantity’ of SOM functional groups), relating the A/B ratios to the clay mineral content (i.e., reflecting the ‘sorption status’ of SOM), and considering the ‘effectiveness’ of SOM functional groups depending on the spatial orientation of SOM molecules at mineral surface (i.e., reflecting the ‘spatial structure’ of SOM). Values of the effectiveness factor, G, larger than unity account for those hydrophilic groups, which are not surface-active with respect to wettability. The G factor can be described by an exponential function of the SOC/clay ratio with values of about 1 for medium SOC contents of about 10 g kg–1 for all three forest soils. The concept allows separating between a more soil type dependent relation of weighted SOM composition and a more general function reflecting the effects of SOM and mineral contents.
The soluble SOM fractions are not as much related to soil wettability as SOM of bulk soil. Although the mobile fraction SOM(W) may preferably accumulate at surfaces of SOM-mineral complexes, for instance, during wet-dry cycles, the extraction methods applied here are not able to separate the SOM fractions proportional to their local spatial positions. It is evident that A/B ratios determined from FT-IR spectra of SOM fractions give a relatively simplified description of the relation between SOM composition and soil wettability. More detailed analyses, for instance, of spatial distributions and orientations of functional groups at SOM–mineral complex surfaces are required to further improve the understanding of SOM functions in soil.
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ACKNOWLEDGMENTS |
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Received for publication February 24, 2004.
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TOPABSTRACTINTRODUCTIONMATERIAL AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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CA = 31.1 + 334.1 x (A/B) – 476.3 x (A/B)2; r2 = 0.62; samples C/Bw, Bsh, and E/Ah are not included in the regression.
