A test of a field-based ¹⁵N-nitrous oxide pool dilution technique to measure gross N ₂O production in soil

Soils are both a major source and sink of nitrous oxide (N ₂O), but the proportion of soil N ₂O production released to the atmosphere (termed the N ₂O yield) is poorly constrained due to the difficulty in measuring gross N ₂O production. The quantification of gross N ₂O fluxes would greatly improve our ability to predict N ₂O dynamics across the soil-atmosphere interface. We report a new approach, the ¹⁵N ₂O pool dilution technique, to measure rates of gross N ₂O production and consumption under laboratory and field conditions. In the laboratory, gross N ₂O production and consumption compared well between the ¹⁵N ₂O pool dilution and acetylene inhibition methods whereas the ¹⁵NO ₃ ^- tracer method measured significantly higher rates. In the field, N ₂O emissions were not significantly affected by increasing chamber headspace concentrations up to 100 ppb ¹⁵N ₂O. The pool dilution model estimates of ¹⁴N ₂O and ¹⁵N ₂O concentrations as well as net N ₂O fluxes fit observed data very well, suggesting that the technique yielded robust estimates of gross N ₂O production. Estimated gross N ₂O consumption rates were underestimated relative to rates calculated as the difference between gross and net N ₂O production rates, possibly due to heterogeneous and/or inadequate tracer diffusion to deeper layers in the soil profile. Gross N ₂O 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 (R ² = 0.73). Gross N ₂O production rates varied spatially, with the highest rates in soils with the best drainage and the highest mineral N availability. Estimated and calculated N ₂O consumption rates constrained the average N ₂O yield from 0.70 to 0.84. Our results demonstrate that the ¹⁵N ₂O pool dilution technique can provide well-constrained estimates of N ₂O yields and field rates of gross N ₂O production correlated to soil characteristics, improving our understanding of terrestrial N ₂O dynamics.