Mowing and phosphorus affect plant diversity and soil carbon and nitrogen storage under nitrogen enrichment in the semi-arid alpine steppe

Plant diversity strongly affected the grassland productivity and soil C and N storage. Nitrogen enrichment weakens the stability of the ecosystem through decreased biodiversity. Mowing is a viable management tool for the maintenance of diversity in grasslands. However, it is unclear how plant diversity affects soil C and N storage in mowing (or phosphorus) under N enrichment in the semi-arid alpine steppe. In the present study, a 3-year trial has investigated the effects of nitrogen, mowing (or phosphorus), and their interactions on plant diversity and soil C and N storage in a semi-arid alpine steppe in Qilian mountains, northern China. We found that aboveground biomass (AGB) was significantly increased in N (nitrogen added alone) and NP (both nitrogen and phosphorus added), significantly decreased in M (mowing) across the three years (2019–2021) (P < 0.05), and significantly increased by 21.4% in the third year in MN (nitrogen added at the time of mowing). Plant diversity was significantly decreased in N and NP, but had no significant effect under M, MN, and P across the three years. We found that SOC stock was significantly decreased by 13% in the 0–10 cm soil layer in N, while increased by 6% at the 10–20 cm soil layer in MN. In contrast, NP had no significant effect on SOC stock and TN stock, while both NO3–N and AP significantly increased. M increased the relative biomass of sedge and forb, while N increased the relative biomass of grass and decreased the relative biomass of forb. Forb is positively correlated with C storage and negatively correlated with N storage, while the grass is the opposite. Therefore, the results indicated that mowing modulated the effects of N addition on diversity loss and carbon stock loss. In contrast, phosphorus is ineffective for the negative effect of nitrogen on grassland species diversity.