Towards Quantifying the Key Drivers of Nitrous Oxide Emissions in Agricultural Crop Production.

Nitrous oxide emissions in agricultural fields are primarily associated with the denitrification process in humid soil moisture regimes. Laboratory incubations have demonstrated that oxygen, nitrate and carbon availability are key drivers of nitrous oxide emissions, however the relative importance of these drivers in controlling emissions in the field is unclear. The analysis used data collected from a field experiment comparing seven two-year potato rotations and a series of fertilizer N rate and timing treatments on potato. Cumulative growing season nitrous oxide emissions were compared with indices of carbon availability (cumulative growing season carbon dioxide emissions), of nitrate availability (Nitrate Exposure, calculated as the sum of daily soil nitrate concentration in the surface soil over the monitoring period), and of aeration (average water-filled pore space). For different N fertility treatments in the same crop rotation (i.e. potato grown after barley), over 80% of the variation in growing season nitrous oxide emission can be explained by Nitrate Exposure. In contrast, for potato grown in different crop rotations, Nitrate Exposure alone explained only 24% of the variation in nitrous oxide emissions. This was increased to 54% when both Nitrate Exposure and carbon availability were considered. In the rotation crop phase of the rotations, Nitrate Exposure alone explained only 20% of the variation in nitrous oxide emissions, whereas a multiple regression including both Nitrate Exposure and carbon availability explained 67% of nitrous oxide emissions. While mitigation practices for nitrous oxide emissions commonly focus on N management alone, these findings suggest that carbon availability also plays a key role in determining nitrous oxide emissions from agricultural fields.