Natural abundance of <sup>13</sup>C and <sup>15</sup>N provides evidence for plant–soil carbon and nitrogen dynamics in a N‐fertilized meadow
Ruzhen Wang, Josep Peñuelas, Li Tian, Heyong Liu, Hui Wu, Yuge Zhang, Jordi Sardans, Yong Jiang
Abstract
Abstract Natural abundance of carbon (C) and nitrogen (N) stable isotope ratios (δ 13 C and δ 15 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 δ 13 C and δ 15 N of five dominant plant species, and soil δ 15 N of microbial biomass and available N forms under N addition in a meadow steppe. Additional N, applied as urea, led to decreases in δ 15 N of soil NO 3 − (δ 15 N nitrate , from 3.0 to 0.4‰) and increases in δ 15 N of soil (δ 15 N ammonium , from −1.3 to 11‰) and dissolved organic N (δ 15 N DON , from 8.5 to 15‰) that reflected increased net nitrification rates, a possible increase in NH 3 volatilization, and greater availability of the three N forms. An overall increase in δ 15 N of soil total N (δ 15 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 δ 15 N, which ranged from −1.8 to 2.1‰, generally decreased with N addition, indicating a greater reliance on soil NO 3 − under N‐enrichment conditions. Nitrogen addition decreased δ 15 N of microbial biomass N (from 14 to 2.8‰), possibly because of a shift in preferential N form (DON to ), 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 δ 13 C, whereas plant species δ 13 C remained unaffected, likely because of a shift in the abundance of dominant species with a greater proportion of biomass coming from δ 13 C‐depleted species. Enrichment factor (the difference in plant δ 15 N relative to δ 15 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.