Below-ground-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 above-ground to below-ground 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 below-ground 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 of 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 (similar to 4-9 years). When N addition proceeded longer than similar to 10 years, N enrichment stimulated below-ground meristem differentiation and rhizome growth of the rhizomatous species, leading to the dominance by rhizomatous sedges/grasses in the community at the 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