Global divergent responses of soil heterotrophic respiration to nitrogen deposition

Numerous studies have demonstrated that soil heterotrophic respiration (RH) is significantly influenced by the atmospheric nitrogen (N) deposition. However, the response ratios (ΔRH) exhibit substantial spatial heterogeneity worldwide. Here, based on a hierarchical meta-analysis of 912 paired observations from global N addition experiments, we show that this heterogeneity in ΔRH can be explained by the divergences in the ecological characteristics of the sample sites. The mean annual precipitation (MAP) and mean annual temperature (MAT) at the sample sites exhibited linear negative effects on ΔRH, especially in regions with high MAP (> 700 mm) or MAT (> 17 °C). ΔRH significantly increased in alkaline soils (pH > 7.5) but decreased in acidic soils (pH RH significantly increased in soils with high total organic carbon (TOC > 15 g/kg) or low available N contents (NH4+ 11 g/kg). Additionally, N addition altered soil properties, such as soil microbial biomass, pH, TOC, TN, C:N ratio, NH4+, and NO3−. The alternations in soil microbial biomass (ΔBiomass) and pH linearly affected ΔRH at the global scale, whereas others linearly impacted ΔRH only under certain threshold levels. Structural equation modeling demonstrated that the ΔBiomass, which directly and ultimately drives the variation in ΔRH, was simultaneously modulated by climatic conditions, initial soil properties, and their alternations after N addition. Moreover, these factors significantly interacted with each other, and their relative importance to ΔRH varied across sample sites. Using machine learning, we predicted the global ΔRH following a 10% increase in atmospheric N deposition and observed significant heterogeneity (–32.3% to +142%). Our findings suggest that when accurately evaluating soil RH to N deposition, the levels of sample site characteristics and their interactions should be comprehensively considered.