Regional asymmetries in dissolved organic carbon response to nitrogen addition rather than soil organic carbon and microbial biomass carbon

As the most active carbon pool, soil dissolved organic carbon (DOC) plays a critical role in soil organic carbon (SOC) cycling. Due to the coupling of nitrogen (N) and carbon cycles, N addition can impact carbon cycling in terrestrial ecosystems, particularly the soil DOC. However, the impacts of N addition on soil DOC remain lack between humid and non-humid regions. This study undertook to collect 1132 paired observations from 103 independent studies to quantify the response pattern of DOC to N addition in humid (554 observations) and non-humid (574 observations) regions and identify the driving factors regulating N-induced effects. Our results revealed asymmetrical effect of nitrogen addition on soil DOC across humid and non-humid regions rather than microbial biomass carbon (MBC) and SOC. N addition significantly decreased soil DOC (-2.49%) in humid regions, while increasing it (7.30%) in non-humid regions. The effect size of soil DOC decreased linearly with that of MBC to SOC ratio in humid regions, while it increased linearly in non-humid regions. The effect size of soil DOC was positively correlated with the initial MBC and negatively correlated with the initial soil pH in humid regions, while the opposite trend was the case in non-humid regions. Seasonal variability of precipitation remained a significant driving factor for the effect size of soil DOC, when the influences of temperature, soil properties, and N addition rates were removed. Furthermore, the initial SOC content was a primary driving factor for the effect size of soil DOC in humid regions, while the N addition rates was a primary driving factor for the effect size of soil DOC in non-humid regions. Overall, these findings have important practical and policy implications for optimizing the management of soil carbon pools, modeling global carbon balance, and addressing climate change, particularly under different climatic conditions.