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SOIL PHYSICS

Significance of Wettability-Induced Changes in Microscopic Water Distribution for Soil Organic Matter Decomposition

Leibniz University of Hannover, Herrenhaeuser Str. 2, 30419 Hannover, Germany

^* Corresponding author (goebel{at}ifbk.uni-hannover.de).

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The significance of soil water-repellent properties has been discussed with respect to water dynamics and distribution; however, there are indications of their importance also for C stabilization processes. Water-repellent aggregates, for example, have been shown to protect soil organic matter (SOM) due to their stability against water slaking. Soil wettability can act as a key factor for SOM decomposition as it controls the microbial availability of water and nutrients. The main objective of this study was therefore to investigate the impact of wettability on C release by linking the wetting properties in terms of the contact angle to soil respiration parameters. For this, the wetting properties of two topsoil samples (an Orthic Luvisol and a Dystric Cambisol) were altered by the addition of particles that were hydrophobized by treatment with dichlorodimethylsilane (DCDMS). Additionally, aggregates were created to assess whether artificial aggregation also can contribute to SOM protection. Environmental scanning electron microscopy revealed a locally confined distribution of water for the DCDMS-treated material, compared with untreated soil where water was uniformly distributed. Measurements indicated an increasing contact angle with increasing amount of DCDMS-treated particles in the mixtures. With increasing contact angle, C release decreased, suggesting that wettability-induced changes in water distribution can significantly affect the decomposition of SOM. Respiration from artificial aggregates, however, was not reduced compared with the corresponding homogeneous material. We conclude that wettability is an important factor for SOM decomposition as it governs the spatial distribution and availability of water necessary for microbial activity.

Abbreviations: DCDMS, dichlorodimethylsilane • ESEM, environmental scanning electron microscopy • SOC, soil organic carbon • SOM, soil organic matter

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The impact of water repellency on infiltration, surface runoff, erosion, preferential flow, and the three-dimensional distribution and dynamics of soil moisture on the profile scale is well documented (Doerr et al., 2000). To date, however, only a few studies have focused on the impact that soil wettability may have for the stability of SOM against microbial decomposition (Goebel et al., 2005). Water repellency is caused by low solid-surface free energy of the soil particles, resulting in a weak attraction between the solid and liquid phases (Roy and McGill, 2002). Soil minerals, in general, have high-energy surfaces (Rodriguez et al., 1997), but under natural conditions they are often covered by films of adsorbed organic molecules with low surface free energy (Doerr et al., 2000). With respect to the progressive warming of the earth's atmosphere, caused by an increasing release of greenhouse gases, soils as a sink for CO₂ have moved increasingly into the center of interest (Lal et al., 1999). In addition to the amount of C that can be stored in soils, the stability of SOM is also of central importance for the potential that soils can have for C sequestration.

In this context, Goebel et al. (2005) have pointed out that soil wettability may be related to the stability of SOM against microbial decomposition. Soil water repellency was shown to reduce water infiltration into aggregates (Zhang and Hartge, 1992; Goebel et al., 2005) and may reduce the effects of air slaking due to entrapped air compression and, consequently, can enhance aggregate stability (Zhang and Hartge, 1992; Hallett and Young, 1999). Furthermore, there are indications that SOM that is encapsulated in aggregates is more protected against microbial decomposition (Hassink and Whitmore, 1997; Piccolo and Mbagwu, 1999). From these findings, it can be concluded that water-repellent aggregates effectively protect SOM. Besides this indirect effect of water repellency, however, which is mediated by aggregate stability, hydrophobic soil properties may also directly affect the decomposition processes of SOM.

An important controlling factor for SOM decomposition is the distribution and availability of water, nutrients, and O₂. Water acts as a key factor, as it is crucial for all microbial uptake mechanisms (Marschner and Kalbitz, 2003). It governs convective nutrient transport, enzyme diffusion, and microbial motility, and is important for the aeration status of the soil (Or and Friedman, 2002). Another requirement for SOM decomposition is the microbial accessibility of SOM (bioavailability) (Gaillard et al., 1999). These considerations emphasize the importance of the three-dimensional distribution and arrangement of microorganisms, SOM, water, and nutrients within the soil matrix, which may either enable an optimal supply of water, nutrients, and O₂ and an intimate contact between SOM and microorganisms, or lead to a physical separation between SOM, water, and nutrients and the microorganisms. For example, Ladd et al. (1993) showed how soil structure can confer the stability of SOM by limiting reactions between the substrate and enzymes. In this context, the wetting properties of a soil can play an important role, as many of the processes mentioned above depend on the solid–liquid interaction, which in turn is determined by the surface free energies of the involved phases.

We hypothesized that water-repellent properties alter the three-dimensional water distribution in the soil matrix and can cause the formation of dry domains. The impact on water distribution can be effective on different spatial scales, from the profile (decimeters to meters) down to the primary particle scale (nanometers to micrometers). Directly coupled to the water distribution are nutrient transport and the diffusion of enzymes and O₂. Completely dry domains, for example, will be excluded also from the nutrient supply. In water-repellent soils, water tends to form droplets rather than continuous films on the particle surfaces. Even if water films are present, however, their thickness may be significantly reduced with increasing water repellency (Churaev, 2000). This leads to a reduction of water flow and can reduce enzyme diffusion, which may be completely interrupted when film thickness becomes smaller than 1 to 2 µm (Derjaguin and Churaev, 1986).

To date, no study has been done to investigate the relevance of wettability-induced effects on microscopic water distribution to the decomposition of SOM. The objectives of this study were, therefore, to assess the impact of wettability on soil water distribution and arrangement and to evaluate its significance for SOM decomposition by relating the wetting properties of soil material (expressed as soil-water contact angle) to respiration parameters. To do this, the wettability of natural soil material was systematically altered by the addition of C-free material (<63 µm) consisting of different proportions of hydrophobic and hydrophilic particles. Additionally, aggregates were created from some of these mixtures to assess the significance of aggregation for SOM protection. The resulting mixtures (homogeneous material and aggregates) were incubated and SOM decomposition was quantified by CO₂ respiration measured by gas chromatography.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
This study used soil material sampled from A horizons of a silty agricultural soil developed from loess (an Orthic Luvisol from Banteln, Lower Saxony, Germany) and a sandy forest soil developed from Triassic sediments (a Dystric Cambisol from Steinkreuz, Bavaria, Germany). Both soils were sieved to <630 µm and aggregates were gently crushed. Additionally, we used C-free material sieved to <63 µm, termed silt in the following. This material originated from the A horizon of a Gleyic Luvisol treated with H₂O₂ to remove organic matter. Some basic physical and chemical properties are shown in Table 1.

View this table: [in this window] [in a new window]   Table 1. Selected physical and chemical properties and measured contact angles () of the soils.

where n is the number of repeating monomer units. The O atoms of PDMS can interact with polar groups of soil particles by electrostatic forces. Simultaneously, the methyl (–CH₃) groups are oriented outward from the particles, resulting in highly hydrophobic surfaces (Pfeiffer and Weis, 2002). After 24 h, the material was intermixed with deionized water to ensure the complete reaction of the DCDMS. This was necessary because previous investigations showed that intermixing DCDMS-treated and untreated silt may change the wettability of the untreated material if the reaction was not complete, which would lead to poorly defined mixtures. After ageing the material for at least 3 mo, DCDMS-treated and untreated silt were mixed in different proportions to get mixtures of different wettabilities. These mixtures were then mixed in a 1:1 proportion with the natural soils from Banteln and Steinkreuz. The mixing procedure consisted of (i) adding the materials (approximately 100 g) to a polyethylene beaker, (ii) stirring the material for 1 min with a glass rod, and (iii) shaking the material for 3 min by hand. The resulting portions of DCDMS-treated particles in the mixtures are given in Table 2. To prepare aggregates of different wettability, the mixtures of the Banteln series were completely suspended in deionized water and air dried. After a drying period of 1 wk the resulting aggregates (4–6.3 mm) were fractionated by dry sieving and one part was again homogenized. This ensured that aggregates and homogeneous samples consisted of the same material.

Determination of Contact Angles
Contact angles were measured with the Wilhelmy plate method (Bachmann et al., 2003). All samples were air dried for 3 wk in open glass containers at a relative humidity between 60 and 70%. After drying, a single-grain layer was prepared by sprinkling the sample material on a double-adhesive tape covering a glass slide and gently pressing the material to establish a firm connection with the tape. The slide was attached to an electronic balance (DCAT 11, DataPhysics, Filderstadt, Germany) and lowered into water. The wetting force F_w (N) was measured by recording the sample weight during the immersion process. After correcting for the buoyancy force, the advancing dynamic contact angle (°) was evaluated according to (Bachmann et al., 2003)

[1]

where _lv (N m^–1) is the surface tension of the liquid and l_w (m) is the wetted length (perimeter) of the immersed sample. Mean contact angles for each sample were calculated from three replicate measurements. Bachmann et al. (2003) gave further details.

It is noted that the measured contact angle may be affected by surface roughness, which leads to deviations from the equilibrium contact angle measured on a smooth flat surface of the same material (Wenzel, 1936). Experimental observations have shown that, with increasing surface roughness, the advancing contact angle generally increases (Grundke, 2001). The preparation procedure ensured, however, that all samples contained the same particle size distribution. Consequently, possible effects of roughness were effective for all samples to the same extent and did not bias the comparison between the mixtures.

Incubation Experiments
Measurements used 10 g of soil material (air dry). Water was added by a pipette to adjust initial gravimetric water contents to 20%. Each sample was put in a glass flask (300 mL) equipped with a septum at the top. To minimize evaporation losses from the soil, each flask was supplied with a polyethylene reservoir filled with 10 mL of deionized water. The reservoir was perforated at the top to allow vapor exchange with the sample. All samples were incubated in duplicate at 20°C under exclusion of light for 40 d (Banteln series) and 16 d (Steinkreuz series). Gas aliquots of 5 mL were taken from the headspace of each incubation flask with a medical syringe. Measurements were performed at intervals of 3 and 4 d, and each incubation flask was sampled twice. Before sampling, the temperature and pressure within the glass flask were measured. To prevent inhibition effects due to CO₂ accumulation, each incubation flask was aired after the measurements. Water loss at the end of the incubation experiments was approximately 1% (Steinkreuz series) and 3% (Banteln series) from the initial water content.

Carbon dioxide release was quantified by gas chromatography (PerkinElmer, Auto System XL, TCD, Ueberlingen, Germany). The molar amount of CO₂ was calculated using the ideal gas equation. From the initial soil organic carbon (SOC) content and the loss of C after each time step, the percentage of remaining carbon, C_rem (%), was calculated. The percentage of released carbon, C_rel (%), at the end of the incubation experiment was calculated from the initial SOC content and the cumulative loss of C. The SOC contents were measured by dry combustion and infrared detection of CO₂ (CNS analyzer, LECO, CNS-2000, Moenchengladbach, Germany).

A two-component first-order decay model with two different mineralization rates was fitted to the measured data (after Qualls and Haines, 1992, modified):

[2]

where t is the time (d), (100 – b) and b are the initial percentages of the rapidly and slowly decaying SOM pools (%), respectively, and k₁ and k₂ are the mineralization rate constants of the two pools (d^–1).

Diffuse Reflectance Infrared Fourier- Transform Spectroscopy
To prove the effectiveness of the DCDMS treatment, diffuse reflectance infrared Fourier-transform (DRIFT) spectroscopy (IFS 88, Bruker, Karlsruhe, Germany) was applied. Measurements were performed with DCDMS-treated (20 mL DCDMS kg^–1) and untreated material. A clear increase of surface CH₃ groups after the treatment was indicated by a prominent peak at 2965 cm^–1 in the DRIFT spectrum.

Environmental Scanning Electron Microscopy
The impact of the DCDMS treatment on the water distribution was visualized by dynamic wetting experiments performed with environmental scanning electron microscopy (ESEM; Quanta 200, FEI Co., Eindhoven, the Netherlands). The measurements were performed in a gaseous environment by using water vapor at chamber pressures in the range of 0.13 to 2.66 kPa. For the measurement, a small amount of soil material was placed on a Peltier cooling stage, which allows adjustment of the sample temperature. At constant temperature (3°C), the relative humidity was controlled by the chamber pressure. For the wetting experiments, the pressure was increased until condensation of water on the particle surfaces took place.

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Impact of Dichlorodimethylsilane Treatment on Wettability and Water Distribution
Figure 1 shows ESEM images of the DCDMS-treated silt with a contact angle of 136° (left) and untreated silt with a contact angle of 0° (right). The images were taken just at the moment when condensation of water took place (at vapor pressures of 0.79 and 0.76 kPa, respectively). In contrast to the untreated material, which was instantly completely wetted, the water on the hydrophobic material only condensed as drops (indicated by white arrows) on certain spots where wettability was relatively greater. It also turns out that the water drops are not interconnected with each other but locally confined. This illustrates the impact of hydrophobic surfaces on water distribution on the microscale. It becomes obvious that under similar conditions (e.g., same water contents), water may be excluded from considerably large domains.

View larger version (80K): [in this window] [in a new window]   Fig. 1. Environmental scanning electron microscopy images showing the distribution of condensed water on hydrophobic silt (treated with 42 mL dichlorodimethylsilane kg–1) with a contact angle () of 136° (left), and corresponding untreated silt with a of 0° (right). The white arrows indicate spots where water condensed on the hydrophobic silt and droplets were formed.

View larger version (17K): [in this window] [in a new window]   Fig. 2. Contact angle () as a function of the portion of hydrophobic silt particles (treated with 20 mL dichlorodimethylsilane [DCDMS] kg–1) in mixtures of hydrophilic and hydrophobic silt.

View this table: [in this window] [in a new window]   Table 3. Contact angle (), mineralization rate constants (k), pool sizes, and percentage of released carbon (Crel).

View larger version (27K): [in this window] [in a new window]   Fig. 3. Cumulative CO2–C releases as functions of time for the Banteln (A, aggregates; H, homogeneous material; containing 0, 25, or 50% hydrophobic particles) and the Steinkreuz mixture series (containing 0, 10, 20, 30, or 40% hydrophobic particles). Given also is the contact angle () and the cumulative CO2–C release of the natural soil material (NM).

The mineralization rate constants k₁ and k₂ (Eq. [2]) and the pool sizes of the rapidly and slowly decaying components are given in Table 3. With one exception (SK_20), the fit of the regression model was good, with R² > 0.920 (P < 0.001). For the homogeneous variants of Ba_25 and Ba_50, however, the fitting parameters were not significant. For the aggregates of the Banteln series, both mineralization rate constants k₁ and k₂ indicated a clear tendency of smaller values corresponding with larger amounts of hydrophobic particles. In contrast, the Steinkreuz series did not show a clear tendency. The largest k values were found for SK_10 and SK_20, and both smaller and larger amounts of hydrophobic particles resulted in smaller k values. Figure 3 clearly indicates, however, that also for the Steinkreuz soil the amount of released C continuously decreased with increasing portion of hydrophobic particles.

It seems that the mineralization rate constants may lead to wrong conclusions for the Steinkreuz series and should therefore be interpreted with caution. For this reason, we used the percentage of released C (C_rel) as an additional parameter (Table 3). The relation between contact angle and percentage of released C is depicted in Fig. 4 . For both soils, the amount of released C markedly decreased with increasing contact angle, clearly indicating the effect of wettability on SOM decomposition.

View larger version (16K): [in this window] [in a new window]   Fig. 4. Percentages of released C at the end of the incubation experiments as a function of contact angle.

View larger version (18K): [in this window] [in a new window]   Fig. 5. (a) Log-transformed percentages of remaining carbon (Crem) in mixtures containing 0, 10, 20, 30, or 40% hydrophobic particles as a function of time, and (b) percentage of rapidly decomposable SOM as a function of contact angle () for the Steinkreuz mixture series.

View larger version (87K): [in this window] [in a new window]   Fig. 6. Conceptual model of the impact of wettability on the decomposition of soil organic matter, illustrating how the presence of hydrophobic particles in the soil matrix may affect the water distribution and film thickness and the diffusion of enzymes and nutrients, which in turn influence the microbial decomposition of the soil organic matter.

Impact of Aggregation on Carbon Release
In contrast to the study of Goebel et al. (2005), showing that aggregation may contribute to the stabilization of SOM, the results of this study showed no clear trend (Fig. 3). Only for Ba_0 was C release from aggregates slightly smaller than from the homogeneous material, but this difference was statistically not significant. For the Ba_50 mixture, C release was even higher for the aggregates. This can be explained by the fact that this study used artificial aggregates for which typical formation processes were absent. Consequently, processes that may lead to a separation or inclusion of SOM within particular domains of the aggregates (Piccolo et al., 1999) could not be effective for artificial aggregates. It is rather more likely that SOM is more or less uniformly distributed within the aggregates. The protective effect of aggregation apparently depends more on separation processes of SOM within the aggregate than on a simple inclusion of SOM.

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
In this study, we related the wetting properties of artificially altered soil material in terms of the advancing contact angle with respiration parameters to assess the direct effect of wettability on the decomposition of SOM. The use of DCDMS as a hydrophobizing agent was shown to be very effective in rendering the particle surfaces hydrophobic. Environmental scanning electron microscopy images clearly showed the impact of wettability on water distribution and availability. With increasing portion of hydrophobic particles, the material becomes increasingly more water repellent. We prepared two mixture series of hydrophobic particles and natural soil material (A horizon) from an agricultural (Banteln) and a forest site (Steinkreuz). For both mixture series, an increasing portion of hydrophobic particles resulted in smaller C release rates, indicating the direct impact of wettability on the decomposition of SOM. For one series (Steinkreuz), however, the impact of wettability seemed to be effective particularly for the initial incubation stage. Comparison with results from natural soils suggests that the intermixing of natural soil material and DCDMS-treated particles seems to have no detrimental effect on soil microbial activity. In contrast to natural aggregates, the artificial aggregates used in this study do not provide any physical protection for SOM against microbial decomposition. We conclude that wettability-induced changes in water distribution (and water potential) and the resulting effects on nutrient availability and enzyme diffusion have the potential to effectively alter the conditions for microbial decomposition of SOM.

	ACKNOWLEDGMENTS

 
Financial support provided by the "German Research Foundation DFG" (Priority program "Soils as source and sink for CO₂–Mechanisms and regulation of organic matter stabilization in soils," SPP 1090, BA 1359/5-1) for this study is greatly appreciated.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.

Received for publication May 17, 2006.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

• Bachmann, J., S.K. Woche, M.-O. Goebel, M.B. Kirkham, and R. Horton. 2003. Extended methodology for determining wetting properties of porous media. Water Resour. Res. 39(12):1353, doi:10.1029/2003WR002143.[CrossRef]
• Bauters, T.W.J., D.A. DiCarlo, T.S. Steenhuis, and J.-Y. Parlange. 1998. Preferential flow in water-repellent sands. Soil Sci. Soc. Am. J. 62:1185–1190.[Abstract/Free Full Text]
• Bear, J. 1972. Dynamics of fluids in porous media. Elsevier, New York.
• Bisdom, E.B.A., L.W. Dekker, and J.F.T. Schoute. 1993. Water repellency of sieve fractions from sandy soils and relationships with organic material and soil structure. Geoderma 56:105–118.[CrossRef][Web of Science]
• Churaev, N.V. 2000. Liquid and vapor flows in porous bodies. Topics in Chem. Eng. 13. Gordon and Breach Sci. Publ., Amsterdam.
• Derjaguin, B.V., and N.V. Churaev. 1986. Properties of water layers adjacent to interfaces. p. 663–738. In C.A. Croxton (ed.) Fluid interfacial phenomena. John Wiley & Sons, New York.
• Doerr, S.H., R.A. Shakesby, and R.P.D. Walsh. 2000. Soil water repellency: Its causes, characteristics and hydro-geomorphological significance. Earth Sci. Rev. 51:33–65.[CrossRef]
• Feeney, D.S., J.W. Crawford, T. Daniell, P.D. Hallett, N. Nunan, K. Ritz, M. Rivers, and I.M. Young. 2006. Three-dimensional microorganization of the soil–root–microbe system. Microb. Ecol. 52:151–158.[CrossRef][Web of Science][Medline]
• Gaillard, V., C. Chenu, S. Recous, and G. Richard. 1999. Carbon, nitrogen and microbial gradients induced by plant residues decomposing in soil. Eur. J. Soil Sci. 50:567–578.[CrossRef]
• Goebel, M.-O., J. Bachmann, S.K. Woche, and W.R. Fischer. 2005. Soil wettability, aggregate stability, and the decomposition of soil organic matter. Geoderma 128:80–93.[CrossRef][Web of Science]
• Griffin, D.M. 1981. Water potential as a selective factor in the microbial ecology of soils. p. 141–151. In L.F. Elliott et al. (ed.) Water potential relations in soil microbiology. SSSA Spec. Publ. 9. SSSA, Madison, WI.
• Grundke, K. 2001. Wetting, spreading, and penetration. p. 119–142. In K. Holmberg et al. (ed.) Handbook of applied surface and colloid chemistry. Vol. 2. John Wiley & Sons, New York.
• Hadas, A., T.B. Parkin, and P.D. Stahl. 1998. Reduced CO₂ release from decomposing wheat straw under N-limiting conditions: Simulation of carbon turnover. Eur. J. Soil Sci. 49:487–494.[CrossRef]
• Hallett, P.D., and I.M. Young. 1999. Changes to water repellence of soil aggregates caused by substrate-induced microbial activity. Eur. J. Soil Sci. 50:35–40.[CrossRef]
• Hassink, J., and A.P. Whitmore. 1997. A model of the physical protection of organic matter in soils. Soil Sci. Soc. Am. J. 61:131–139.[Abstract/Free Full Text]
• Kieft, T.L., P.S. Amy, F.J. Brockman, J.K. Fredrickson, B.N. Bjornstad, and L.L. Rosacker. 1993. Microbial abundances and activities in relation to water potential in the vadose zone of arid and semiarid sites. Microb. Ecol. 26:59–78.[CrossRef][Web of Science]
• Ladd, J.N., R.C. Forster, and J.O. Skjemstad. 1993. Soil structure: Carbon and nitrogen metabolism. Geoderma 56:401–434.[CrossRef][Web of Science]
• Lal, R., J.M. Kimble, R.F. Follet, and C.V. Cole. 1999. The potential of U.S. cropland to sequester carbon and mitigate the greenhouse effect. Lewis Publ., Boca Raton, FL.
• Marschner, B., and K. Kalbitz. 2003. Controls of bioavailability and biodegradability of dissolved organic matter in soils. Geoderma 113:211–235.[CrossRef][Web of Science]
• Or, D., and S. Friedman. 2002. Physical factors affecting microbial habitats and activity in unsaturated porous media. Paper no. 809. In World Congr. of Soil Sci., 17th, Bangkok, Thailand. 14–21 Aug. 2002.
• Paul, E.A., and F.E. Clark. 1996. Soil microbiology and biochemistry. 2nd ed. Academic Press, San Diego.
• Pfeiffer, J., and J. Weis. 2002. Silicone—Multitalente aus Sand. Eigenschaften und Anwendungen. (In German.) CLB Chem. Biotech. 53:128–135.
• Piccolo, A., and J.S.C. Mbagwu. 1999. Role of hydrophobic components of soil organic matter in soil aggregate stability. Soil Sci. Soc. Am. J. 63:1801–1810.[Abstract/Free Full Text]
• Piccolo, A., R. Spaccini, G. Haberbauer, and M.H. Gerzabek. 1999. Increased sequestration of organic carbon by hydrophobic protection. Naturwissenschaften 86:496–499.[CrossRef][Web of Science][Medline]
• Qualls, R.G., and B.L. Haines. 1992. Biodegradability of dissolved organic matter in forest throughfall, soil solution, and stream water. Soil Sci. Soc. Am. J. 56:578–586.[Abstract/Free Full Text]
• Rodriguez, M.A., J. Rubio, F. Rubio, M.J. Liso, and J.L. Oteo. 1997. Application of inverse gas chromatography to the study of the surface properties of slates. Clays Clay Miner. 45:670–680.[Abstract]
• Roy, J.L., and W.B. McGill. 2002. Assessing soil water repellency using the molarity of ethanol droplet (MED) test. Soil Sci. 167:83–97.[CrossRef]
• Sollins, P., P. Homann, and B.A. Caldwell. 1996. Stabilization and destabilization of soil organic matter: Mechanisms and controls. Geoderma 74:65–105.[CrossRef][Web of Science]
• Wenzel, R.N. 1936. Resistance of solid surfaces to wetting by water. Ind. Eng. Chem. 28:988–994.[CrossRef]
• Zhang, H., and K.H. Hartge. 1992. Effect of differently humified organic matter on aggregate stability by reducing aggregate wettability. (In German, with English abstract.) J. Plant Nutr. Soil Sci. 155:143–149.[CrossRef]

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