Effects of simulated nitrogen deposition on microbial dynamics: Altered nitrogen fixation and ammonia oxidation in...

Nitrogen-limited drylands are highly sensitive to environmental changes, with biological soil crusts playing a key role in biological nitrogen fixation. Ammonia oxidation, the rate-limiting step in nitrification, is essential for nitrogen retention in soils. Despite sustained high global nitrogen deposition, the impacts of varying nitrogen addition levels on nitrogen fixation, ammonia oxidation by crusts, and nitrogen cycling remain poorly understood. Crusts were sampled in April and October from plots in the Gurbantunggut Desert, where nitrogen had been applied for 13 years, to assess nitrogen fixation, ammonia oxidation rates, and the functional microbial community structure. Results indicated nitrogen addition reduced nitrogen fixation, with the highest nitrogen addition (3.0 g N m² yr^-1) causing a ∼60 % decline, suggesting a microbial shift towards reliance on added nitrogen. In contrast, low nitrogen addition (0.5 g N m^-2 yr^-1) enhanced ammonia oxidation by 293 %, likely due to the alleviation of substrate limitations. However, higher nitrogen addition (N1.5, N3.0) led to a reduction in ammonia oxidation, with microorganisms such as Scytonema, Nitrososphaera, and Nitrosopumilus playing key roles in both processes. Notably, nitrogen fixation rates were generally lower during the dry season, while ammonia oxidation increased under N0 treatment. Nitrogen addition influenced the nitrogen fixation and ammonia oxidation capacities of the crusts, heightening the risk of nitrogen loss but diminishing the influence of prolonged drought. These findings underscore the need to consider nitrogen levels and seasonal dynamics when managing soil nitrogen processes to maintain the stability of dryland ecosystems and the nitrogen cycle, it is essential to reduce anthropogenic nitrogen deposition and mitigate the growing impacts of drought.