Plant growth-promoting rhizobacteria mediate soil hydro-physical properties: An investigation with Bacillus subtilis and its mutants

Plant growth-promoting rhizobacteria and other soil bacteria have the potential to improve soil hydro-physical properties and processes through the production of extracellular polymeric substances (EPS). However, the mechanisms by which EPS mediates changes in soil properties and processes remain incompletely understood, partly due to variations in EPS composition produced under different environmental conditions. In this study, we investigated the influence of different bacterial traits on intrinsic soil properties and processes of evaporation and infiltration using sand treated with the wild-type Bacillus subtilis variant (UD1022) and its two mutant variants, eps-$eps<^>{-}$-tasA-$tasA<^>{-}$ and srfAC-$AC<^>{-}$. The eps-$eps<^>{-}$-tasA-$tasA<^>{-}$ mutant suppresses EPS production through alterations in the eps and tasA genes, while the srfAC-$AC<^>{-}$ mutant lacks the gene for surfactin production. Experimental results confirmed that the solution viscosity of the eps-$eps<^>{-}$-tasA-$tasA<^>{-}$ mutant was the lowest and the solution surface tension of the srfAC-$AC<^>{-}$ mutant was the highest among the three tested bacteria strains. The distinct intrinsic properties of EPS produced by these bacterial strains resulted in varied hydro-physical responses in the treated sand. Key influences included modifications in wettability, hydraulic decoupling (or mixed wettability), and aggregation, which collectively led to reduced evaporation rates and heterogeneous water distribution during infiltration in the bacteria-treated sands. Our findings advance the understanding of the role bacterial EPS play in vadose zone hydrology and offer insights for the development of sustainable strategies for increasing water retention, supporting crop production in arid regions, and facilitating land restoration.