Natural abundance of ¹³C and ¹⁵N provides evidence for plant–soil carbon and nitrogen dynamics in a N‐fertilized meadow

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 N_nitrate , from 3.0 to 0.4‰) and increases in δ¹⁵ N of soil <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mtext>NH</mml:mtext><mml:mn>4</mml:mn><mml:mo>+</mml:mo></mml:msubsup></mml:math> (δ¹⁵ N_ammonium , from -1.3 to 11‰) and dissolved organic N (δ¹⁵ N_DON , 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 N_TN ) 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 because of a shift in preferential N form (DON to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mtext>NO</mml:mtext><mml:mn>3</mml:mn><mml:mo>-</mml:mo></mml:msubsup></mml:math> ), 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, whereas plant species δ¹³ C remained unaffected, likely because of 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 N_TN ) of four nonlegume 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.