Tillage and seasonal emissions of CO2, N2O and NO across a seed bed and at the field scale in a Mediterranean climate

Whereas the contribution of agriculture to the emissions of greenhouse gases (GHGs) is well known, especially of NOx gases following the application of N-fertilizer additions, quantitative estimates across fields remain uncertain. Here, we quantified CO2, N2O, and NO emissions from an irrigated field under standard tillage and in a field recently converted (≈5 years) to minimum tillage in Yolo County, California, under a Mediterranean climate. We focused on the spatiotemporal variation of GHG emissions among positions across a seed bed and at the field scale. Seasonal CO2 and N2O fluxes ranged from 4.6 to 52.4 kg C ha−1 day−1 and 0 to 23.7 g N ha−1 day−1, respectively. There was a significant seasonal pattern of CO2 emissions as a function of crop growth, while the level of CO2 flux rates varied annually by crop type and the previous year's soil C inputs. The seasonal N2O emissions coincided with N fertilization placement and irrigation events. With the exception of immediately after N fertilizing, NO emissions were on average 2–33 times lower than N2O emissions. Whereas gross effects of tillage and position in the seed bed on CO2 and N2O emissions were not significant, the emissions were significantly different in a specific seed bed position because of an interaction between tillage and position in the seed bed. For example, N2O fluxes in the side dress position were significantly greater than fluxes from other seed bed positions, and were further accentuated by a significant tillage effect. At the field scale, soil-water content and temperature were generally related to both optimum CO2 and N2O emissions, but the relationships were highly variable. The results suggest that position-specific variations and interaction with tillage should be accounted for to improve the estimates of GHG emissions from irrigated soils.