Effects of simulated nitrogen deposition on N fate in a temperate steppe: Evidence from multi-year ¹⁵N isotope study

Enhanced atmospheric deposition of reactive nitrogen (N) could alter the N cycle in terrestrial ecosystems. However, the fate of deposited N under different levels of N deposition is still not fully understood, and the long-term dynamics and redistribution of deposited N within steppe ecosystems remain uncertain. Using ¹⁵N tracers, we investigated N fate across varying N addition rates (0, 2, 5, 10, and 25 g N m^-2 yr^-1) over 1 month, 1 year and 8 years in a temperate steppe in Inner Mongolia, China. Under ambient N deposition, over 94 % and 87 % of added ¹⁵N was retained after 1 month and 1 year, respectively, with soil acting as the primary sink (>50 %). The N saturation effect was primarily observed in plants, with N retention stabilizing or declining above 5 g m^-2 yr^-1. The N addition also altered the distribution of soil N, with most added nitrate accumulating in deeper soil layers (20-30 cm), and enhancing microbial N retention in the short term. Over eight years, the overall ¹⁵N recovery dropped to less than 21 % across different N addition levels. Additionally, N addition significantly modified the inter-annual variation in N retention. Our study shows that N deposition significantly affects N retention and distribution in a steppe ecosystem. Added N was mainly retained in the soil, with plants showing signs of N saturation. Nitrate largely accumulated in deeper soil layers. The plant-litter-soil continuum faces risks of N loss under sustained high deposition, threatening ecosystem N sustainability.