LAUSR

Shifts in the frequency and magnitude of rain events (precipitation patterns) associated with climate change may impact ecosystem nitrogen and carbon cycling through effects on plant physiology and soil microbial activity. Here, we determined how the combination of temporal irrigation distribution and N supply affects plant-microbial C and N dynamics in microcosms with winter wheat. First, we investigated legacy effect of 12 weeks of contrasting irrigation distribution (frequent and small versus infrequent and large water inputs) and N inputs (high versus low) on plant biomass, organic and inorganic N pools, and potential nitrification and denitrification rates. Second, we investigated legacy effects of these treatments on C and N fluxes in plants and microbes over 29 h after a rewetting event, using 13C-CO2 and 15N-NH4 labeling. We found that irrigation distribution and N input led to significant differences in plant responses and soil C input, setting the scene for the rewetting response. Immediately after rewetting, microorganisms outcompeted plants for soil mineral N. However, over time, the net outcome of competition improved for plants regardless of water or N input history. Further, we found that a history of frequent irrigation led to more productive plants (biomass and rate of photosynthesis) and increased their net competitiveness for N over microorganisms. This suggests that the shift toward more extreme fluctuations in soil moisture predicted by climate forecasts for most temperate zones may have negative implications for plant productivity due to altered N dynamics between plants and soil microorganisms.