Human nitrogen fixation and greenhouse gas emissions: a global assessment

The net impact of human nitrogen (N) fixation on climate (ignoring short-lived components) mainly depends on the magnitude of the warming effect of (direct and indirect) nitrous oxide (N2O) emissions and the cooling effect of N-induced carbon dioxide (CO2) uptake. N-induced CO2 uptake is caused by anthropogenic N deposition which increases net primary production (NPP) in N-limited ecosystems and thus CO2 sequestration. Nitrogen oxide (NOx) emissions, however, also induce tropospheric ozone (O3) formation, and elevated O3 concentrations reduce NPP and thus plant C sequestration. We estimated global-scale impacts of anthropogenic N fixation on net greenhouse gas emissions using recent data and modelling approaches with respect to N inputs to various ecosystems, N2O emissions in response to N inputs, and C exchange in responses to N inputs (C–N response) and O3 exposure (C–O3 response). The estimated impact of human N fixation is dominated by an increase in N2O emissions equal to 1.02 (0.89–1.15) Pg CO2-C equivalent (eq) yr-1. CO2 uptake due to N inputs to terrestrial and aquatic ecosystems corresponds to net emissions of -0.75 (-0.97 to -0.56) Pg CO2-Ceq yr-1, while the reduction in CO2 uptake by N-induced O3 exposure corresponds to net emissions of 0.14 (0.07–0.21) Pg CO2-Ceq yr-1. Overall, human N fixation causes an increase in net greenhouse gas emissions of 0.41 (-0.01–0.80) Pg CO2-Ceq yr-1. Even considering all uncertainties, it is likely that N inputs lead to a net increase in greenhouse gas emissions.