Global effects on soil respiration and its temperature sensitivity depend on nitrogen addition rate

Soil respiration (Rs) plays a crucial role in regulating carbon (C) cycling in terrestrial ecosystems and the atmospheric carbon dioxide (CO₂) concentration. However, the impact of nitrogen (N) addition on Rs and its temperature sensitivity (Q₁₀) is poorly understood. Here, we used 1413 paired observations from 261 studies to explore how N addition affects Rs and its Q₁₀ across terrestrial ecosystems on a global scale. Nitrogen addition increased autotrophic respiration in croplands, deserts and shrublands, but decreased it in grasslands, forests (including coniferous and deciduous forests) and wetlands. In contrast, N addition increased heterotrophic respiration in grasslands, wetlands and deserts, but decreased its Q₁₀ across different ecosystem types other than deserts. Nitrogen addition decreased (p < 0.05) the overall Rs and its Q₁₀ by 19.5 and 32.1%, respectively. The decrease in Rs in response to N addition might be attributed to a reduction in heterotrophic respiration. Moreover, the Rs and its Q₁₀ decreased while soil organic C content increased with the increasing rate of N addition up to 100–150 kg ha⁻¹ yr⁻¹. The results suggest that 100–150 kg N ha⁻¹ yr⁻¹ was the optimum N addition rate to maintain the soil as a C sink. In addition, soil pH, C:N ratio and respiratory quotient were significant predictors of soil respiration and its Q₁₀. Our study highlights that the N addition rate-dependent effects on Rs and its Q₁₀, and soil organic C should be incorporated into global C models to improve the assessment of N addition effects on the global C cycle.