Biofilms reshape soil-water dynamics with enhanced vapor adsorption and hysteresis reversal under dry conditions

Bacterial biofilms are crucial for microbial survival during drought by modifying soil-water dynamics. Their influence on water vapor transport in dry soils remains unexplored. This study systematically investigates biofilm-mediated changes in soil water vapor sorption (SWVS) dynamics across a wide matric potential range (−400 MPa to −10 MPa). Using soils of varying textures inoculated with wild-type and mutant strains of Pseudomonas putida and Bacillus subtilis , we characterized SWVS isotherms and concomitant changes in soil and biofilm properties. Key findings reveal that active cells and biofilms persist in dry soils (water activity a w < 0.93), altering SWVS behavior through two distinct mechanisms: (1) Enhanced SWVS capacity (10–49% increase across soils) resulting from biofilm hydration through swelling and extensive water retention within extracellular polymeric substance (EPS) matrices; (2) Hysteresis reversal at critical water activity values ( a w < 0.6), where biofilm-dominated adsorption exceeds particle surface desorption. Mechanistic analysis demonstrates that EPS components influence SWVS through functional groups (-COOH and -OH) with strong water-binding affinity, EPS swelling, and suppression of cation/surface hydration, which physically obscures localized mineral surfaces. A conceptual framework for vapor dynamics in biofilm-colonized soils is proposed, and such results address a gap in understanding biological controls on soil hydrology under drought.