A comparison of measured and modeled nitrous oxide emissions from flood irrigated alfalfa in California using the DNDC model

Field validation of biogeochemical models under various cropping systems is necessary to advance our understanding of nutrient cycling and to improve global estimates of greenhouse gas emissions from agriculture. There are over 1 million acres of irrigated alfalfa grown in California, the largest producer in the United States, yet no published field studies have measured N2O emissions from irrigated alfalfa. Here we measured N2O flux, soil inorganic N (NO3, NH4), soil temperature, water-filled pore space and harvested yield in California during 2011 in adjacent fields containing alfalfa at two different stand ages (2nd and 5th yr stands). Both alfalfa fields were managed under identical flood irrigation and crop management regimes during the study period. The field measurements were then compared with modeled estimates of each parameter in order to validate the performance of the Denitrification-Decomposition (DNDC) model under these agronomic conditions. Large fluxes of N2O were measured immediately following each irrigation event but decreased rapidly as soils dried. Cumulative annual N2O emissions from the 5th yr alfalfa stand were more than twice as large (5.20 kg N2O-N ha-1) as those in the adjacent 2nd yr stand (2.30 kg N2O-N ha-1), and were due mainly to larger peaks after irrigation events. These annual emissions were notably larger than measurements reported for rain-fed alfalfa in other regions. The DNDC model slightly over-predicted cumulative annual N2O emissions relative to measured values for both the 5th yr (6.5 kg N2O-N ha-1) and 2nd yr (2.7 N2O-N ha-1) alfalfa stands. The model estimates for yield, soil NO3, soil NH4, and water-filled pore space were relatively consistent with field measurements. More precise field data on crop water requirements, soil temperature, N fixation capacity, and maximum yield have the potential to further improve the fit between DNDC model estimates and measured N2O emissions. Annual emission factors (EFs) were calculated according to Intergovernmental Panel on Climate Change (IPCC) methods as the percentage of N lost as N2O-N relative to the total non-harvested biomass N divided by the age of the stand at termination. The resulting EFs calculated from our N2O measurements were 12.6% and 4.2% for the 5th and 2nd year alfalfa stands respectively, which indicates that there is little basis for estimating N2O emissions in flood irrigated alfalfa based on the current IPCC default parameters related to crop biomass. The results further suggest that the accumulation and mineralization of organic matter throughout the duration of the alfalfa cropping cycle gradually increases the annual rate of N2O flux over this period.