Nitrogen enrichment enhances the dominance of grasses over forbs in a temperate steppe ecosystem

Chinese grasslands are extensive natural ecosystems that comprise 40 % of the total land area of the country and are sensitive to N deposition. A field experiment with six N rates (0, 30, 60, 120, 240, and 480 kg N ha⁻¹ yr⁻¹) was conducted at Duolun, Inner Mongolia, during 2005 and 2010 to identify some effects of N addition on a temperature steppe ecosystem. The dominant plant species in the plots were divided into two categories, grasses and forbs, on the basis of species life forms. Enhanced N deposition, even as little as 30 kg N ha⁻¹ yr⁻¹ above ambient N deposition (16 kg N ha⁻¹ yr⁻¹), led to a decline in species richness. The cover of grasses increased with N addition rate but their species richness showed a weak change across N treatments. Both species richness and cover of forbs declined strongly with increasing N deposition as shown by linear regression analysis (p<0.05). Increasing N deposition elevated aboveground production of grasses but lowered aboveground biomass of forbs. Plant N concentration, plant δ¹⁵N and soil mineral N increased with N addition, showing positive relationships between plant δ¹⁵N and N concentration, soil mineral N and/or applied N rate. The cessation of N application in the 480 kg N ha⁻¹ yr⁻¹ treatment in 2009 and 2010 led to a slight recovery of the forb species richness relative to total cover and aboveground biomass, coinciding with reduced plant N concentration and soil mineral N. The results show that N deposition induced changes in soil N transformations and plant N assimilation that are key to changes in species composition and biomass accumulation in this temperate steppe ecosystem