Experimental evidence shows minor contribution of nitrogen deposition to global forest carbon sequestration

Human activities have drastically increased nitrogen (N) deposition onto forests globally. This may have alleviated N limitation and thus stimulated productivity and carbon (C) sequestration in aboveground woody biomass (AGWB), a stable C pool with long turnover times. This ‘carbon bonus’ of human N use partly offsets the climate impact of human‐induced N(2)O emissions, but its magnitude and spatial variation are uncertain. Here we used a meta‐regression approach to identify sources of heterogeneity in tree biomass C‐N response (additional C stored per unit of N) based on data from fertilization experiments in global forests. We identified important drivers of spatial variation in forest biomass C‐N response related to climate (potential evapotranspiration), soil fertility (N content) and tree characteristics (stand age), and used these relationships to quantify global spatial variation in N‐induced forest biomass C sequestration. Results show that N deposition enhances biomass C sequestration in only one‐third of global forests, mainly in the boreal region, while N reduces C sequestration in 5% of forests, mainly in the tropics. In the remaining 59% of global forests, N addition has no impact on biomass C sequestration. Average C‐N responses were 11 (4–21) kg C per kg N for boreal forests, 4 (0–8) kg C per kg N for temperate forests and 0 (−4 to 5) kg C per kg N for tropical forests. Our global estimate of the N‐induced forest biomass C sink of 41 (−53 to 159) Tg C yr(−1) is substantially lower than previous estimates, mainly due to the absence of any response in most tropical forests (accounting for 58% of the global forest area). Overall, the N‐induced C sink in AGWB only offsets ~5% of the climate impact of N(2)O emissions (in terms of 100‐year global warming potential), and contributes ~1% to the gross forest C sink.