Nitrogen addition (NH4NO3) mitigates the positive effect of warming on methane fluxes in a coastal bog

Abstract

Methane (CH4) fluxes in boreal peatlands could be impacted by global warming and N deposition. However, the interaction of these factors remains poorly understood and has not been considered in coupled climate-carbon models. In this study, manipulated warming and N deposition (ammonium nitrate) were conducted for five years in a coastal bog in western Newfoundland, Canada. Consistent with previous studies, warming significantly increased CH4 emission by 52%, which might be due to the increase of microbial activity and vegetation growth. Although N addition alone did not alter CH4 emissions, it significantly reduced the positive effect of warming on CH4 fluxes. We attributed this finding to three possible reasons: (1) N addition caused a shift in root allocation from deep to shallow roots, which decreased labile substrates for CH4 production under warming; (2) warming promoted vegetation uptake of ammonium (NH4+), which alleviated the inhibition of NH4+ to methanotrophs and subsequently increased CH4 oxidation under N addition; and (3) the combination of warming and N addition increased maintenance respiration of vascular plants compared with warming alone, thus reducing net primary production and root exudates for CH4 production. In addition, we found that soil temperature, soil moisture, DOC and TN were the main controls on CH4 fluxes. Our results indicate that the combined effect of N deposition and warming should not be ignored. Otherwise, the emission of CH4 in bogs will be overestimated under future scenarios of climate warming and N deposition increase. Our study can be further applied as empirical evidence for the parameterization of temperature-dependence and nutrient-dependence of CH4 fluxes in biogeochemical models of boreal peatlands.