Impacts of nitrogen deposition on soil nitrous oxide emissions in global forests

Nitrous oxide (N2O) in the atmosphere contributes substantially to climate warming and causes stratospheric ozone depletion. As an external nitrogen (N) input, N deposition has a potential to increase N2O emissions from natural soils and thus partially offset the cooling effect due to N-induced carbon sequestration. By synthesizing the results of N addition experiments in literature, we assessed the effects of N deposition on soil N2O emissions across boreal, temperate, subtropical and tropical forest biomes. Nitrogen additions increased soil N2O emissions but the effect is much weaker in boreal forests as compared to the other three forest biomes. The N2O emission factors (i.e., the increased N2O-N emission as a fraction of added N) were estimated to be 0.2‰, 5.1‰, 6.7‰, and 3.9‰ in boreal, temperate, subtropical and tropical forests, respectively. The derived emission factors were about a tenth of the ratios of ambient N2O-N emission versus N deposition. These ratios have often been denoted as N2O emission factors in literature, but this is inappropriate as natural N2O emissions are affected by many other factors, such as background soil N fertility and biological N fixation. Nitrogen deposition resulted in total biome-scale emissions of 0.001, 0.039, 0.049, 0.079TgN2O year−1 in boreal, temperate, subtropical and tropical forests, respectively. On a global scale, N deposition caused an overall emission of 0.17TgN2O year−1 from forest soils, being less than a tenth of the total N2O emissions from forest soils and offsetting 5% of the cooling effect by CO2 sequestration due to N deposition. Uncertainties in current understanding of soil N2O emissions in forests are discussed and several key research needs are highlighted.