Net greenhouse gas balance in response to nitrogen enrichment: perspectives from a coupled biogeochemical model

Increasing reactive nitrogen (N) input has been recognized as one of the important factors influencing climate system through affecting the uptake and emission of greenhouse gases ( GHG ). However, the magnitude and spatiotemporal variations of N‐induced GHG fluxes at regional and global scales remain far from certain. Here we selected C hina as an example, and used a coupled biogeochemical model in conjunction with spatially explicit data sets (including climate, atmospheric CO 2 , O 3 , N deposition, land use, and land cover changes, and N fertilizer application) to simulate the concurrent impacts of increasing atmospheric and fertilized N inputs on balance of three major GHG s ( CO 2 , CH 4 , and N 2 O ). Our simulations showed that these two N enrichment sources in C hina decreased global warming potential ( GWP ) through stimulating CO 2 sink and suppressing CH 4 emission. However, direct N 2 O emission was estimated to offset 39% of N‐induced carbon (C) benefit, with a net GWP of three GHG s averaging −376.3 ± 146.4 Tg CO 2  eq yr −1 (the standard deviation is interannual variability of GWP ) during 2000–2008. The chemical N fertilizer uses were estimated to increase GWP by 45.6 ± 34.3 Tg CO 2  eq yr −1 in the same period, and C sink was offset by 136%. The largest C sink offset ratio due to increasing N input was found in S outheast and C entral mainland of C hina, where rapid industrial development and intensively managed crop system are located. Although exposed to the rapidly increasing N deposition, most of the natural vegetation covers were still showing decreasing GWP . However, due to extensive overuse of N fertilizer, C hina's cropland was found to show the least negative GWP , or even positive GWP in recent decade. From both scientific and policy perspectives, it is essential to incorporate multiple GHG s into a coupled biogeochemical framework for fully assessing N impacts on climate changes.