Autumn nitrogen enrichment destabilizes ecosystem biomass production in a semiarid grassland.

Nitrogen (N) deposition decreases the temporal stability of ecosystem aboveground biomass production (ecosystem stability). However, little is known about how the responses of ecosystem stability differ based on seasonal N enrichment. By adding N in autumn, winter, or growing season, from October 2014 to May 2020, in a temperate grassland in northern China, we found that only N addition in autumn resulted in a significantly positive correlation between ecosystem mean aboveground net primary productivity (ANPP) and its standard deviation and significantly reduced ecosystem stability. Autumn N-induced reduction in ecosystem stability was associated with the vanished negative effect of community-wide species asynchrony (asynchronous dynamics among populations to environmental perturbations) on the standard deviation of ecosystem ANPP in combination with the emerged positive effect of dominance (Simpson's dominance index that indicates the relative weight of dominant species in a community). Our findings indicate that autumn N addition might overestimate the negative effect of annual atmospheric N deposition on ecosystem stability, suggesting that to better evaluate the influence of N deposition in temperate grasslands, both field experiments and global modeling should consider not only the annual N load but also its seasonal dynamics. Moreover, further studies should pay more attention to the alteration in the ecosystem temporal deviations, which might be more sensitive to human-induced environmental changes.