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 NO x 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 ([almost equal to]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 52.4kgCha⁻¹ day⁻¹ and 0 to 23.7gNha⁻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.