A test of a field-based ¹⁵N-nitrous oxide pool dilution technique to measure gross N₂O production in soil

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 ¹⁵NO_3⁻ 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.