Impact of bacterial biomass on soil particle wettability under various moisture conditions

Abstract

Prolonged droughts render the development of soil water repellency (SWR), which in turn impacts the infiltration and distribution of water through the soil profile, exposing soil microorganisms to water stress. Soil microorganisms (particularly necromass) significantly contribute to soil organic matter (SOM), which is believed to be the source of water repellency in soil. SWR-induced water stress is hypothesized to increase bacterial cell surface hydrophobicity (CSH). Interaction of bacterial cells with soil particles will change the wetting properties of the particles, with stronger impact of stressed cells. Furthermore, variations in soil moisture can lead to increased SWR associated with shifts in microbial abundance and community structure. Growth of all the strains investigated was inhibited by both matric and osmotic stress, and in both growth conditions. However, changes in cell surface hydrophobicity (CSH) varied between different stress types and growth conditions. The CSH of B. subtilis and P. fluorescens increased with increasing stress level, R. erythropolis and M. pallens exhibited generally high but constant contact angle (CA) at all stress levels, while A. chlorophenolicus A6 and N. aromaticivorans exhibited rather inconsistent response to growth conditions and type of stress. In none of the experiments, the CA exceeded 110°. This contact angle thus seems to represent an upper limit for CSH. In an association with quartz minerals, B. subtilis and P. fluorescens rendered the surface of the minerals hydrophobic. The degree of initial hydrophobicity of the CMAs was significantly higher with osmotically stressed cells. However, the high degree of water repellency did not persist, probably due to biomass loss. The response of the two soils to fluctuations in water content (WC) varied, depending on water content and the initial level of SWR, with no changes observed in CA and community composition of the initially more hydrophobic soil. Changes were observed in moderately hydrophobic soil, particularly under lower WC. The results reported in this dissertation show the significant role of bacterial surface hydrophobicity in the development of SWR. While stress induced CSH has only a short-term impact on water repellency in soil, Prolonged dry conditions lead to a shift towards more adapted microbial community with higher CSH.