Additive Effects of Multiple Global Change Drivers on Terrestrial Nitrogen Cycling Worldwide

Global change has dramatically altered the Earth's biogeochemical cycles. However, the interactive effects of multiple global change factors (GCFs) on terrestrial nitrogen (N) cycling worldwide remain unclear, limiting the ability to predict how future global change will affect the global N cycle. We conducted a meta-analysis of 108 published articles to evaluate the main and interactive effects of elevated CO₂, N addition, warming, and altered precipitation on soil N pools (NH₄ ⁺, NO₃ ^-, and organic N) and transformation rates (N mineralization, nitrification, and denitrification) across terrestrial ecosystems. Results showed that single GCFs impacted the soil N cycle in different directions and magnitudes, with N addition and increased precipitation having the strongest positive effects on N pools and transformation rates, respectively. Moreover, the positive effects of N addition on the soil N cycle were generally enhanced when combined with other GCFs. Although the interactions of multiple GCFs were commonly additive (66.2%-83.3%), both synergistic (10.5%-15.1%) and antagonistic (2.8%-18.9%) effects were also observed. The types of treatment and ecosystem, geographic location, and climate all regulated the responses of soil N pools to GCFs to some degree, while only the types of treatment and ecosystem significantly affected the response of soil transformation rates to GCFs. These findings emphasize the importance of considering interactive effects among GCFs on terrestrial N cycling and highlight the necessity of incorporating these interactions into Earth system models for accurate predictions of N cycling responses to global changes.