LAUSR

Natural abundance of carbon (C) and nitrogen (N) stable isotope ratios (δ13 C and δ15 N) has been used to indicate ecosystem C and N status and cycling; however, use of this approach to infer plant and microbial N preference under projected ecosystem N enrichment is limited. Here, we investigated natural abundance δ13 C and δ15 N of five dominant plant species, and soil δ15 N of microbial biomass and available N forms under N addition in a meadow steppe. Additional N, applied as urea, led to decreases in δ15 N of soil NO3 - (δ15 Nnitrate , from 3.0 to 0.4‰) and increases in δ15 N of soil NH4 + (δ15 Nammonium , from -1.3 to 11‰) and dissolved organic N (δ15 NDON , from 8.5 to 15‰) that reflected increased net nitrification rates, a possible increase in NH3 volatilization, and greater availability of the three N forms. An overall increase in δ15 N of soil total N (δ15 NTN ) from 7.1 to 7.9‰ indicated accelerated and greater openness of soil N cycling that was also partially revealed by enhanced net N mineralization rates. Plant δ15 N, which ranged from -1.8 to 2.1‰, generally decreased with N addition, indicating a greater reliance on soil NO3 - under N-enrichment conditions. Nitrogen addition decreased δ15 N of microbial biomass N (from 14 to 2.8‰), possibly due to a shift in preferential N form (DON to NO3 - ), that indicated a convergence of plant and microbial preferential N forms and an increase in plant-microbial N competition. Microbes were thus more flexible than plants in the use of different forms of N. Addition of N decreased plant litter δ13 C, while plant species δ13 C remained unaffected, likely due to a shift in the abundance of dominant species with a greater proportion of biomass coming from δ13 C-depleted species. Enrichment factor (the difference in plant δ15 N relative to δ15 NTN ) of four non-legume species was negatively related to soil inorganic N availability, net nitrification rate, and net N mineralization rate, and was proven to be a good indicator of ecosystem N status. Our study highlights the importance of natural abundance of 15 N as an indicator of plant-microbial N competition and ecosystem N cycling in meadow steppe grasslands under projected ecosystem N enrichment.