Impacts of human alteration of the nitrogen cycle in the US on radiative forcing

Nitrogen cycling processes affect radiative forcing directly through emissions of nitrous oxide (N2O) and indirectly because emissions of nitrogen oxide and ammonia (NH3) affect atmospheric concentrations of methane (CH4), carbon dioxide (CO2), water vapor (H2O), ozone (O-3) and aerosols. The emissions of N2O are mostly from agriculture and they contribute to warming on both short and long time scales. The effects of and NH3 on CH4, O-3, and aerosols are complex, and quantification of these effects is difficult. However, the net result on time scales of decades is likely one of cooling, which becomes less significant on longer time scales. Deposition of N onto ecosystems also affects sources and sinks of N2O, CH4, and CO2, but the dominant effect is changes in carbon (C) stocks. Primary productivity in most temperate ecosystems is limited by N, so inputs from atmospheric deposition tend to stimulate plant growth and plant litter production, leading in some cases to significant C sequestration in biomass and soils. The literature reviewed here indicates a range of estimates spanning 20-70 kg C sequestered per kg N deposited in forests, which are the dominant C sinks. Most of the sequestration occurs in aboveground forest biomass, with less consistency and lower rates reported for C sequestration in soils. The permanency of the forest biomass sink is uncertain, but data for the fate of forest products in the US indicate that only a small fraction of enhanced forest biomass C is sequestered in long-term harvest products or in unmanaged forests. The net effect of all of these N cycle processes on radiative forcing in the US is probably a modest cooling effect for a 20-year time frame, although the uncertainty of this estimate includes zero net effect, and a modest warming for a 100-year time frame. We know that N-cycling processes are important and that biotic feedbacks to climate change are unlikely to be properly modeled or assessed without including C-N interactions. However, due to the complexity of biological processes involving C-N-climate interactions, biogeochemical models are still poorly constrained with respect to ecosystem responses to impacts of N deposition and climate change. Only recently have N-cycling processes been incorporated into Earth system models for C-N interactions. The robustness of these models remains to be demonstrated. Much work remains for improving their representation in models used to simulate climate forcing scenarios.