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.