Trade-offs on carbon and nitrogen availability lead to only a minor effect of elevated CO2 on potential denitrification in soil

The global surplus of reactive nitrogen (Nr) and elevated atmospheric CO2 are both major threats to current and future ecosystem integrity. Denitrification is the primary sink for Nr by the conversion to inert N2 and is also the predominant source of N2O emission. Yet, it is unclear how and to what extent soil denitrification would respond to elevated CO2 (eCO2), which is a major part of the uncertainties on the estimation of terrestrial nitrogen (N) cycle budget under the influence of climate changes. Herein, we provided quantitative results of a global metaanalysis with 127 observations that included data on soil potential denitrification rates (PDR) and denitrifying gene abundance, and 305 observations that included data on N2O emission and soil available N content under eCO2. Averaged across all studies, we found that globally eCO2 had an overall minor effect on soil PDR, but showed a large variation with different edaphic and environmental factors. The response was primarily explained by soil organic carbon (SOC) and mean annual precipitation (MAP), i.e., the higher SOC content or MAP, the lower response of soil PDR to eCO2. Further, eCO2 significantly decreased soil NO3 - content (-15.3%) and denitrifying bacteria (-31.3%), but increased nirS (14.7%), napA (24.4%) genes and N2O emission (34.1%). We speculate that eCO2 has both positive and negative effects on soil potential denitrification, i.e., a higher soil C input induced by eCO2 supplies more energy for denitrifiers and thus increases denitrification in N-rich ecosystems, while unbalanced C:N input decreases soil NO3 - content, causing substrate deficiency in N-limiting ecosystems under eCO2. Whilst the response of soil PDR to eCO2 is governed by trade-offs on soil C and N availability, this may serve as a buffer on soil available N content in terrestrial ecosystems under eCO2. Collectively, based on our findings, we built a comprehensive conceptual model defining the key regulators, which has significant implications for expanding the understanding and predicting the responses of the terrestrial N cycle to eCO2 in future scenario evaluation.