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Keywords:

  • denitrification;
  • N2O yield;
  • nitrous oxide;
  • pool dilution;
  • Sacramento–San Joaquin Delta

Abstract

Soils are both a major source and sink of nitrous oxide (N2O), but the proportion of soil N2O production released to the atmosphere (termed the N2O yield) is poorly constrained due to the difficulty in measuring gross N2O production. The quantification of gross N2O fluxes would greatly improve our ability to predict N2O dynamics across the soil-atmosphere interface. We report a new approach, the 15N2O pool dilution technique, to measure rates of gross N2O production and consumption under laboratory and field conditions. In the laboratory, gross N2O production and consumption compared well between the 15N2O pool dilution and acetylene inhibition methods whereas the 15NO3 tracer method measured significantly higher rates. In the field, N2O emissions were not significantly affected by increasing chamber headspace concentrations up to 100 ppb 15N2O. The pool dilution model estimates of 14N2O and 15N2O concentrations as well as net N2O fluxes fit observed data very well, suggesting that the technique yielded robust estimates of gross N2O production. Estimated gross N2O consumption rates were underestimated relative to rates calculated as the difference between gross and net N2O production rates, possibly due to heterogeneous and/or inadequate tracer diffusion to deeper layers in the soil profile. Gross N2O production rates were high, averaging 8.4 ± 3.2 mg N m−2 day−1, and were most strongly correlated to mineral nitrogen concentrations and denitrifying enzyme activity (R2 = 0.73). Gross N2O production rates varied spatially, with the highest rates in soils with the best drainage and the highest mineral N availability. Estimated and calculated N2O consumption rates constrained the average N2O yield from 0.70 to 0.84. Our results demonstrate that the 15N2O pool dilution technique can provide well-constrained estimates of N2O yields and field rates of gross N2O production correlated to soil characteristics, improving our understanding of terrestrial N2O dynamics.