Nutrient Management Research Database

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) tominimumtillage 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 ha1 day1 and 0 to 23.7 g N ha1 day1, 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.

This research was conducted on a 30 ha irrigated and laser-leveled field irrigated with furrow irrigation.  The site was first managed under standard tillage (ST) and then converted to no-till and the field was seeded with maize and then wheat, followed by sunflower and then chickpea.  In 2003 the field was split into two with one half managed under standard tillage  and the other under no-till. Ammonium nitrate was band applied at planting  with an additional broadcast of 8-24-6.  A second application of N was side dressed in late May.   

Sampling chambers were established in the seed bed, the crop row, the furrow, and over the side dress fertilizer band to measure CO₂, N₂0 and NO fluxes at the beginning of the experiment.  Fluxes of CO₂ and N₂0 were measured monthly during fallow seasons and biweekly during growing seasons.  NO was also measured monthly for a shorter duration.

Highlights of results include:  

• Tillage did not effect CO₂ or N₂O emissions at the field scale.
• There was an increase in N₂O following fertilizer N application and irrigation events.
• There were large spatial variations in N2O emissions, which presents a challenge in calculating emissions at the field scale.