Effects of increasing organic nitrogen inputs on CO2, CH4, and N2O fluxes in a temperate grassland.

Understanding future climate change requires accurate estimates of the impacts of atmospheric nitrogen (N) deposition, composed of both inorganic and organic compounds, on greenhouse gas (GHG) fluxes in grassland ecosystems. However, previous studies have focused on inorganic compounds and have not considered the potential effects of organic N sources. Here, we conducted a grassland experiment that included organic, inorganic N, and a mix of them at a ratio of 4:6, with two input rates, to study N inputs induced CO 2 , CH 4, and N 2 O fluxes, as well as the potential abiotic and biotic mechanisms driving the fluxes. We found that N compositions significantly affected fluxes each of the three GHGs. Greater organic N decreased the impacts of N addition on CO 2 and N 2 O emissions, caused primarily by low rates of increase in substrates (soil available N) for production of CO 2 and N 2 O resulting from high ammonia volatilization rather than changes in microbial activity. Also, greater organic N slightly stimulated CH 4 uptake. Nitrogen composition effects on CO 2 emissions and CH 4 uptake were independent of N input rates and measurement dates, but N 2 O emissions showed stronger responses to inorganic N under high N addition and in June. These results suggest that future studies should consider the source of N to improve our prediction of future climate impact of N deposition, and that management of N fertilization can help mitigate GHG emissions. Image 1 • CO 2 emissions increased with higher levels of inorganic N input. • Increase of organic N attenuated the impacts of N input on N 2 O emissions. • Organic N addition slightly stimulated CH 4 uptake compared to mixed N addition. • Effects of N forms on CO 2 and CH 4 were independent of N addition levels. [ABSTRACT FROM AUTHOR]