Nitrous Oxide Evolution from Irrigated Land

Nitrous oxide (N2O) fluxes from irrigated land cropped to vegetables in a coastal valley in California ranged from 0.0038 to 1.06 kg N/ha per day. For all sites, maximum fluxes (0.24 to 1.06 kg N/ha per day) were observed during the first post‐fertilization irrigation and were followed by a trend to lower values (0.048 to 0.144 kg N/ha per day) towards the end of cropping despite total fertilizer N inputs of 176 to 528 kg N/ha. The N2O flux showed a pronounced dependence on irrigation events with maximum fluxes associated with soil‐moisture contents between 18 and 22% and soil‐water suctions between 75 and 150 mbars. The magnitude of N2O flux during periods of high moisture content appeared to depend primarily upon soil nitrate concentration which declined during the cropping cycle. Data for N2O flux, soil moisture content, air‐filled porosity, and soil‐water suction suggested that denitrification rather than nitrification was the major process involved in N2O production at each field site. Mean N2O fluxes for each site coupled with soil‐water suction data for a 12‐month period suggested annual emissions of N2O ranging from 19.6 to 41.8 kg N/ha.