Decreased precipitation frequency increases nitrogen loss across degraded grasslands in a two-year mesocosm experiment

Abstract

Soil inorganic nitrogen (N) is frequently lost from grasslands through gaseous emissions and leaching, both strongly regulated by precipitation, reducing soil nutrient availability and influencing offsite environments. Amid declining precipitation frequency and widespread grassland degradation, these losses warrant greater attention, yet their magnitude and mechanisms remain poorly understood. Using a two-year mesocosm experiment with intact soil-plant monoliths collected across a Leymus chinensis grassland degradation gradient, we tested how moderate and extreme decreases in precipitation frequency affected N loss and explored the underlying mechanisms. Reduced frequency at both levels increased total N loss by enhancing N mineralization and weakening the coupling between plant N demand and soil microbial N supply potential, thereby promoting NH 3 volatilization and N leaching. In contrast, N 2 O emissions, the weakest N loss pathway, were suppressed via increased N 2 O-reducing gene nosZ abundance. However, effects varied over time and showed some differences across degradation levels and degrees of decreased precipitation frequency. Compared with the first year, the effect of reduced frequency on N loss was markedly attenuated in the second year. Nitrogen loss increased along the degradation gradient, with this increasing trend being further amplified under extreme reductions in precipitation frequency. Greater N loss was associated with lower soil inorganic N pools, and this relationship became more pronounced with grassland degradation across precipitation regimes. These findings suggest altered rainfall regimes may push degraded grasslands toward a more loss-prone and less resilient N cycle, while increasing the risks of water pollution and air quality deterioration.