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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).

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