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This article in SSSAJ

  1. Vol. 44 No. 3, p. 505-511
    Received: Apr 4, 1979
    Accepted: Jan 8, 1980

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Nature and Extent of Directly Measured Denitrification Losses from Some Irrigated Vegetable Crop Production Units1

  1. J. C. Ryden and
  2. L. J. Lund2



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 (N2O) evolution from locations within each study site treated with actylene (C2H2) to establish C2H2 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 N2O. 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.

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