Trajectories and thresholds of multitrophic diversities and multiple functions in response to nitrogen enrichment.

Global nitrogen deposition has profound impacts on ecosystem biodiversity and functions. However, the response trajectories and thresholds of the diversities of multiple trophic groups and multiple functions to nitrogen enrichment remain unclear. Here, combining a field experiment with a global-scale meta-analysis of 92 nitrogen experiments in grasslands, we show that plant diversity follows a decrease-then-plateau pattern with increasing nitrogen amount, while the diversities of soil bacteria, fungi, protists, and invertebrates all exhibit a stable-then-decrease pattern. For ecosystem functions, plant productivity and ecosystem carbon uptake both follow an increase-then-plateau pattern, while soil organic matter decomposition is best fitted with a stable-then-increase curve and plant-microbe mutualism with a decrease-then-plateau pattern. Accordingly, the nitrogen thresholds differ substantially between ecosystem attributes. Furthermore, the mechanisms driving the diversity and function responses are highly attribute-dependent. Our findings suggested that the attribute-specific trajectories and thresholds should be incorporated into models forecasting ecosystem feedbacks to nitrogen enrichment.