Biotic regulation of nitrogen gas emissions in temperate agriculture

It is generally assumed that fertilizer addition is the prime driver of nitrogen (N) gas loss from modern cropping systems. This assumption has its basis in observations of nitrous oxide (N₂O, an important greenhouse gas) emissions, and is contrary to theory from unmanaged ecosystems, where N losses are controlled by plant physiological influence on the soil environment. However, dinitrogen (N₂) emissions are likely a major N loss pathway in both managed and unmanaged ecosystems, but these emissions are very difficult to measure. We directly measured N₂ and N₂O emissions from two temperate agricultural systems over the course of the growing season to test when total N gas losses are highest. We hypothesized that N₂ emissions mirror those of N₂O, with the largest flux immediately after fertilization, early in the growing season. Instead, we found that N₂ emissions were highest at the end of the growing season, and were most strongly correlated with soil moisture, which increased after plant senescence. Dinitrogen emissions were an order of magnitude larger than N₂O. Thus, while N₂O emissions were highest following fertilization, overall N gas loss was greatest at the end of the growing season. These data suggest that total N gas losses are high and have different temporal patterns from N₂O fluxes. Understanding the magnitude and controls over these losses are important for understanding and managing the N cycle of temperate agricultural systems.