Belowground-mediated and phase-dependent processes drive nitrogen-evoked community changes in grasslands

Enhanced deposition of atmospheric nitrogen (N) leads to loss of plant diversity in grassland ecosystems. Numerous theories have provided potential explanations for the negative effects of N enrichment on plant diversity. However, the relative importance of each mechanism and the time scales of responses for the different functional groups remain unclear. We investigated the temporal responses of plant community in a temperate steppe to N enrichment by linking aboveground to belowground processes using a series of field N-addition and greenhouse experiments. The N enrichment-induced declines in plant diversity of grasslands were phase-based, functional group-dependent, and driven by three belowground processes. The rapid accumulation of NH4+-N by N addition inhibited photosynthetic rates of broad-leaf-non-rhizomatous forbs, contributing to loss of these N-sensitive species  during early phase of N enrichment (≤3 years). The N-induced changes in this phase were independent of soil...