Warming and altered precipitation rarely alter N addition effects on soil greenhouse gas fluxes: a meta-analysis

Background Changes in soil greenhouse gas (GHG) fluxes caused by nitrogen (N) addition are considered as the key factors contributing to global climate change (global warming and altered precipitation regimes), which in turn alters the feedback between N addition and soil GHG fluxes. However, the effects of N addition on soil GHG emissions under climate change are highly variable and context-dependent, so that further syntheses are required. Here, a meta-analysis of the interactive effects of N addition and climate change (warming and altered precipitation) on the fluxes of three main soil GHGs [carbon dioxide (CO 2 ), methane (CH 4 ), and nitrous oxide (N 2 O)] was conducted by synthesizing 2103 observations retrieved from 57 peer-reviewed articles on multiple terrestrial ecosystems globally. Results The interactive effects of N addition and climate change on GHG fluxes were generally additive. The combination of N addition and warming or altered precipitation increased N 2 O emissions significantly while it had minimal effects on CO 2 emissions and CH 4 uptake, and the effects on CH 4 emissions could not be evaluated. Moreover, the magnitude of the combined effects did not differ significantly from the effects of N addition alone. Apparently, the combined effects on CO 2 and CH 4 varied among ecosystem types due to differences in soil moisture, which was in contrast to the soil N 2 O emission responses. The soil GHG flux responses to combined N addition and climate change also varied among different climatic conditions and experimental methods. Conclusion Overall, our findings indicate that the effects of N addition and climate change on soil GHG fluxes were relatively independent, i.e. combined effects of N addition and climate change were equal to or not significantly different from the sum of their respective individual effects. The effects of N addition on soil GHG fluxes influence the feedbacks between climate change and soil GHG fluxes.