Understanding the sources and mitigation potential of nitrous oxide in agriculture

Abstract:

More than half of the global warming potential of GHG emissions from agriculture is attributed to nitrous oxide (N₂O).. Many factors control the production and release of N₂O from soils. In addition to fertilizer N, soil N, moisture and carbon availability control N₂O emissions. In addition, a previously overlooked factor, iron, was recently found to be the most significant factor influencing N₂O production. Controlled by soil and management factors, N₂O production is attributed to multiple pathways, including ammonia oxidation (AO), denitrification, and abiotic chemical reactions. Ammonia oxidation or nitrifier activity N₂O production, is a well known pathway, but it significance to total N₂O production is also highly debated and soil conditions influencing its production are poorly understood. Studies in a variety of crops in California strongly suggest that this pathway contributes substantially to N₂O emissions. It is well established that denitrification primarily occurs under O₂- limiting conditions, while N₂O produced from AO is also influenced by soil O₂ content, with maximum production occurring at low O₂ levels (~0.5%). Since emission of N₂O can arise from both AO and denitrification activities at low O₂ concentrations, it is difficult to discern the importance of each pathway under various soil conditions and management. Furthermore, both the N form and concentration are determinants of nitrifier N₂O production. The nitrifier denitrification pathway has been shown to dominate over nitrifier nitrification and nitrification coupled denitrification pathways. Irrigation, rainfall, and fertilization events stimulate microbial activity, including AO and denitrification that produces N₂O and although limited, these events contribute to the majority of annual emissions. This uncertainty and complexity surrounding N₂O production pathways has hampered the development of practices to reduce N₂O emissions. As agricultural production intensifies in developing countries, mitigation strategies will increasingly rely on an understanding of how N₂O is produced in agricultural soils.