Acclimation of the Nitrogen Cycle to Changes in Precipitation

Abstract

Climate change drivers elicit ecosystem responses that vary through time. We propose that the mismatch between equilibrium and transient ecosystem response defines ecological acclimation. As precipitation is expected to shift under climate change, we asked how both increases and decreases in water availability elicit ecological acclimation of the N cycle and how long does this acclimation take. Using foliar δ15N as a proxy for N cycling processes, we found that the slope of foliar δ15N decreases with mean annual precipitation across continents. However, within a desert grassland, slopes of interannual foliar and soil δ15N increased with precipitation amount. Using precipitation manipulation field experiments, we then assessed trends in foliar and soil δ15N as duration of the precipitation manipulation increased, from 5 to 14 years. When parsed temporally, the δ15N-precipitation slopes showed initially increasing trends that decreased after 14 years of precipitation manipulation. The difference in directionality of spatial δ15N-precipitation slope compared to within-site, experimental δ15N-precipitation slopes revealed potential acclimation of the N cycle. Furthermore, we estimated rates of ecological acclimation—defined as convergence time for within-site δ15N-precipitation slopes to match the global model—to range from 11 to 18 years. We conclude N cycling is changing with precipitation amount and duration of the altered precipitation regime. We hypothesize that fast and slow ecological mechanisms—such as microbial processes and shifts in plant species dominance, respectively—explain ecological acclimation of N cycling responses to climate change. As a result, spatial models must be interpreted with caution when forecasting future responses to climate change.