Heterogeneous responses of soil nitrous oxide emissions to nitrogen addition: A global meta-analysis

Abstract

Many projects have demonstrated that nitrogen (N) addition accelerates the emission of nitrous oxide (N₂O) from soils. However, the response ratios of N₂O emission to N (ΔE_N2O) exhibit significant spatial heterogeneity across global sites. Here, a meta-analysis was performed to explain the heterogeneity of ΔE_N2O through the characteristics of sample sites based on the data from 108 field experiments. Results showed that the Mean Annual Precipitation (MAP) and Mean Annual Temperature (MAT) of sample sites exerted significantly positive effects on ΔE_N2O on a global scale, particularly in regions with high MAP (> 1,100 mm) or MAT (> 15 °C). However, ΔE_N2O was affected by soil pH only in the sites with acidic soils (pH < 5.5). In addition, ΔE_N2O was significantly affected by soil properties in soils with high total organic carbon (> 50 g/kg), high carbon-to-nitrogen (C:N) ratio (> 15), or low total N (< 2.0 g/kg). Among all these factors, C:N ratio and pH, and ambient N deposition rate were the most important. Further analysis revealed that increased MAP led to a decrease in C:N ratio, while elevated ambient N deposition rate resulted in a serious decline in pH and C:N ratio, indicating that environmental conditions may indirectly affect ΔE_N2O through directly influencing soil properties. In sum, when comprehensively evaluating soil N₂O emissions induced by atmospheric N deposition, the characteristics of sample sites (environmental conditions and soil properties), as well as their direct and/or indirect effects should be systematically considered.