Soil microbial activity and community composition: Impact of changes in matric and osmotic potential

► For the first time, we show the combined effects of salinity and soil water content on soil microbial activity and community composition. ► Water potential (WP) was a better measure of microbial stress than salinity (EC) or soil water content. ► The reduction of cumulative respiration at a given WP was similar in the two soils with a 50% reduction compared to the control (optimal water content, no salt added) at WP −3 MPa. ► In both soils, the microbial biomass decreased by 35-50% as WP decreased to about −2 MPa but then remained stable with further decreases, suggesting that a proportion of the biomass is tolerant to low WP.