Environmental memory of microbes regulates the response of soil enzyme kinetics to extreme water events: Drought-rewetting-flooding

How environmental history mediates the response of microbial activities to dry-wet cycle (D/W) in monsoon regions is poorly investigated. In this study, soil from Red River Delta of Vietnam was incubated for 52 days, which included 14 days at 20% water holding capacity (WHC), followed by 14 days rewetted to 60% WHC (rewetting) or 100% WHC (flooding).The kinetics of & beta;-glucosidase, chitinase and acid phosphomonoesterase enzymes, microbial respiration, microbial biomass C and P were measured. The activities of all soil enzymes (Vmax) decreased by 20-42% due to drought but partially or totally recovered after two weeks of rewetting or flooding. The substrate affinity (Km) of all measured enzymes were fluctuating at initial drought and then dropped down by 33-55% after 14 days prolonged drought. Chitinase was the only enzyme showing the decoupling of Vmax and Km at the onset of drought, indicating different factors regulating enzyme synthesis and substrate affinity for the N demands of microorganisms. Pulse rewetting significantly reduced catalytic efficiency (Ka) but boosted turnover time (Tt) of chitinase and & beta;-glucosidase. Rewetting prolonged Tt as compared to flooding by the end of the experiment. Moreover, both soil basal respiration and substrate induced respiration were significantly inhibited by drought and recovered to the control level after rewetting. In conclusion, soil with history of D/W triggers the recovery of microbial activities after D/W cycles and rewetting has a great impact on enzyme systems, while flooding induces more effects on enzyme synthesis.