Nature and Extent of Directly Measured Denitrification Losses from Some Irrigated Vegetable Crop Production Units

Abstract Directly measured denitrification N flux from field study sites in irrigated vegetable production units, ranged from 0.024 to 1.92 kg N ha −1 day −1 subsequent to irrigation, and values up to 3.60 kg N ha −1 day −1 were observed after rain. The N denitrification flux was measured as nitrous oxide (N 2 O) evolution from locations within each study site treated with actylene (C 2 H 2 ) to establish C 2 H 2 concentrations close to 1% (vol/vol) in the soil pore space. Denitrification N flux showed a pronounced dependence on soil‐water suction. Peak fluxes were observed for suctions between 50 and 100 mbars and were associated with field‐measured Eh (redox potential) values between 400 and 650 mV. Flux decreased to < 0.096 kg N ha −1 day −1 when suction increased above 250 mbars and Eh returned to values > 650 mV. Nitrate in extracts of soil cores sampled concurrently with measurements of gas evolution appeared to have little effect on the magnitude of the N flux. Fluxes of at least 1.20 kg N ha −1 day −1 were observed for nitrate levels as different as 2 and 33 µg N/g. The major impact of increased soil nitrate concentrations was in shifting the distribution of denitrification products in favor of N 2 O. Annual dentrification N loss from each study site, estimated from direct measurements, compared favorably with the indirect estimate based on the imbalance in the N budget. Direct estimates ranged from 95 to 233 kg N ha −1 year −1 (14 to 52% of N input), and coupled with other directly measured inputs and outputs of N provided an 80 to 95% accountability of the N applied. Nitrous oxide emission comprised 13 to 20% of the total denitrification loss at sites with pH above 7, but values of approximately 30% were observed at more acid sites.