The contribution of microbial activity to soil–water interactions and soil microstructural stability of a silty loam soil under moisture dynamics

Abstract

Soil structure and its stability against external stresses is fundamental to various soil functions and ecosystem services. Particularly soil moisture dynamics and microbial activity can affect soil structure by promoting particle reorientation, cementation and spatio-temporal pore space remodeling. In this context, microbial exudates can serve as interparticulate binding agents and thus contribute to the formation of stable soil structures and basic physico-chemical soil properties. However, detailed interactions and relationships between microbial activity, soil pore water and the porous soil matrix itself are still unclear, especially in the context of dynamic moisture conditions. In this study, we aimed to increase the fundamental understanding of microbially-induced and moisture dynamic-affected soil microstructural stability and soil–water interactions. For this, a sterilized, untreated and substrate-enhanced silty loam soil was subjected either to constant moisture conditions or four consecutive drying-wetting cycles. At defined timepoints, soil samples were investigated for carbon dioxide release (respirometry), phospholipid fatty acid analysis, wetting behaviour (contact angle), rheological characteristics (rheometry) and water entrapment (1H nuclear magnetic resonance relaxometry) to respectively determine microbial activity, community structure and biomass, soil wettability, soil microstructural stability and the frequency distribution of water-filled pore sizes in soil (FDWPS). Results showed that substrate enhancement promoted soil microstructural stability compared to sterilized and untreated soil, whereby all soils reacted dynamically and irreversibly to the adjusted moisture conditions. Significant positive correlations were found between microbial activity, wettability, FDWPS and soil microstructural stability, which indicate that the presence of interparticulate microbial structures augmented the spatio-temporal reorganization effect induced by soil moisture dynamics. The irreversible dehydration of those interparticulate microbial structures resulted in an additional pore space cementation, water redistribution to smaller soil pores and thus a considerably increased microstructural stability due to stronger cohesive and capillary forces. The results help to understand the extent to which soil moisture dynamics can modulate microbial-induced soil processes and basic soil properties.