1. 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.
2. 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.
3. 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 pH as evidenced by results from application lime to mitigate N-evoked soil acidification. With progression of N addition, manganese (Mn) toxicity to narrow-leaf-non-rhizomatous forbs due to soil acidification-induced Mn²⁺ mobilization in soil accounted for their loss in the second phase of N enrichment (~ 4-9 years). When N addition proceeded longer than ~10 years, N enrichment stimulated belowground meristem differentiation and rhizome growth of the rhizomatous species, leading to the dominance by rhizomatous sedges/grasses in the community at later phase of N enrichment.

Synthesis: The hierarchical mechanisms not only provide a comprehensive explanation for the N enrichment-induced diversity decline in grasslands, but can also facilitate us to understand the differential sensitivities of ecosystems to chronic N enrichment, and predict future ecosystem dynamics.

The dataset was collected from both field and greenhouse experiments.