The increase of CO2 uptake partially offset by stimulated N2O and CH4 emission under nitrogen enrichment: A quantitative meta-analysis

Background/Question/Methods: Anthropogenic N enrichment of ecosystems, mainly from fuel combustion and fertilizer application, alters physiological function and biogeochemical cycling of ecosystems in a way that leads to altered flux of biogenic greenhouse gases (GHGs). Our meta-analysis of 314 observations across 109 independent studies evaluated the effect of N addition on the flux of three major GHGs: carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O). The objective was to quantitatively synthesize data from agricultural and non-agricultural terrestrial ecosystems across the globe and examine whether factors, such as ecosystem type, N addition level and chemical form of N addition influence the direction and magnitude of GHG fluxes. 

Results/Conclusions: Results indicate that N addition increased forest ecosystem C content (EC), marginally increase soil organic carbon of agricultural land, but had no significant effect on net ecosystem CO₂ exchange (NEE) for non-forest natural ecosystems. Across all ecosystems, N addition increased CH₄ emission, reduced CH₄ uptake and increased N₂O emission. There was no consistent relationship between the amount of N fertilization and the magnitude of GHG flux, which is likely due to the influence of interacting microclimate factors that vary greatly over small spatial and temporal scales. Overall, our study indicated N addition caused a small increase in forest ecosystem C sequestration, which is partially offset by increases in CH₄ and N₂O emission from multiple types of ecosystems.