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 CO₂, CH_4, and N₂O 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 CO₂ and N₂O emissions, caused primarily by low rates of increase in substrates (soil available N) for production of CO₂ and N₂O resulting from high ammonia volatilization rather than changes in microbial activity. Also, greater organic N slightly stimulated CH₄ uptake. Nitrogen composition effects on CO₂ emissions and CH₄ uptake were independent of N input rates and measurement dates, but N₂O 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.