The Effects of N Enrichment on Microbial Cycling of Non-CO2 Greenhouse Gases in Soils-a Review and a Meta-analysis.

Abstract

Terrestrial ecosystems are typically nitrogen (N) limited, but recent years have witnessed N enrichment in various soil ecosystems caused by human activities such as fossil fuel combustion and fertilizer application. This enrichment may alter microbial processes in soils in a way that would increase the emissions of methane (CH4) and nitrous oxide (N2O), thereby aggravating global climate change. This review focuses on the effects of N enrichment on methanogens and methanotrophs, which play a central role in the dynamics of CH4 at the global scale. We also address the effects of N enrichment on N2O, which is produced in soils mainly by nitrification and denitrification. Overall, N enrichment inhibits methanogenesis in pure culture experiments, while its effects on CH4 oxidation are more complicated. The majority of previous studies reported that N enrichment, especially NH4+ enrichment, inhibits CH4 oxidation, resulting in higher CH4 emissions from soils. However, both activation and neutral responses have also been reported, particularly in rice paddies and landfill sites, which is well reflected in our meta-analysis. In contrast, N enrichment substantially increases N2O emission by both nitrification and denitrification, which increases proportionally to the amount of N amended. Future studies should address the effects of N enrichment on the active microbes of those functional groups at multiple scales along with parameterization of microbial communities for the application to climate models at the global scale.