Effects of precipitation regime and N-availability on the dynamics of plant-microbial and soil biogeochemical cycling responses to rewetting

prod 2019-99a SPE EA BIOmE INRAInternational audienceIntroduction. Shifts in the frequency and magnitude of rain events (precipitation regime) associated with climate change may affect plant morphological and physiological strategies as well as soil microbial activity. Objectives. The objective of this study was to determine i) how precipitation history shapes the response dynamics of soil bacterial and fungal communities to rewetting, as well as plant-microbial competition for N, and ii) how the N status of the system may modulate the effect of precipitation regime. Materials & methods. The legacy effects of 12 weeks of contrasting precipitation (frequent or infrequent watering, equal total water input) and N inputs was assessed in soil mesocosms on wheat plants, soil microbial communities and N cycling. The legacy effects on the response to rewetting of potentially active (rRNA-based) bacterial and fungal communities was then documented over 29h, by sequencing phylogenetic marker genes, and following the dynamics of plant-microbial coupling and competition for N using 13C-CO2 and 15N-NH4 labeling. Results. Precipitation and N input history had contrasting effects on plant physiology, fungal:bacterial ratio, microbial community composition and plant-microbial coupling (reduced C transfer to microbes), but showed no effect on microbial response to rewetting. After an initial short-term response, potentially active soil microbial communities changed little, regardless of precipitation or N input history. Upon rewetting, microbes outcompeted plants for N but plant competitiveness increased over time, in particular after a history of more favorable conditions. Soil CO2 efflux upon rewetting was higher in systems with a history of frequent precipitation inputs and was not modulated by N availability. Conclusion. We suggest that by altering C and N dynamics between plants and soil microbes and reducing soil C sequestration potential, more extreme fluctuations in soil moisture may affect future ecosystem functioning