Vegetation Restoration Constrained by Nitrogen Availability in Temperate Grasslands in Northern China

Grazing exclusion using fencing has been considered an effective means of vegetation restoration in degraded grasslands. Increased plant growth during recovery requires more nitrogen (N), which is a major limiting factor in northern China. It remains unclear whether soil N supply in this region can support long-term vegetation restoration. In this study, a field inventory was conducted in seven temperate grasslands in northern China. At each site, grassland outside of the fencing experienced continuous grazing, whereas that within the fencing was protected. Results showed that grazing exclusion significantly increased aboveground biomass, species richness, and the Shannon–Wiener diversity index by 126.2%, 42.6%, and 18.8%, respectively. Grazing exclusion reduced the concentrations of nitrate and total inorganic N by 51.9% and 21.0%, respectively, suggesting that there may be a mismatch between N supply and plant demand during the growing season. The aboveground biomass, species richness, and Shannon–Wiener diversity index in the restored grasslands were positively correlated with legume dominance within the community. These results indicate that the vegetation restoration in temperate grasslands could be constrained by soil N availability, which may be supplemented through biological N fixation.