Denitrification to N2 and N2O in a pasture peatland: A test of the 15N2O pool dilution approach

Background/Question/Methods

We used a new method to measure denitrification to nitrous oxide (N₂O) and dinitrogen (N₂) in a pasture peatland in the Sacramento-San Joaquin River Delta, a major N₂O source. Soil N₂ emissions are difficult to measure against high atmospheric N₂ concentrations, so ecosystem nitrogen losses as N₂ are uncertain. The isotope pool dilution method, involving ¹⁵N₂O additions to a surface flux chamber, provides field measurements of N₂O and N₂ emissions with minimal soil disturbance. Following a major rain event, we tested the method on two landforms (crown and slope) along a micro-topographical gradient. We enriched the chamber headspace N₂O by 1-15 atom % ¹⁵N-N₂O and increased concentrations by up to 100 ppb. We used SF₆ as a conservative tracer to account for physical losses of ¹⁵N₂O. Dinitrogen production via N₂O consumption was determined empirically from biological ¹⁵N₂O loss. Gross N₂O production was estimated using a least-squares modeling approach. Observed net N₂O flux was calculated from the change in N₂O concentration over time; predicted net N₂O flux was estimated as the difference between gross N₂O production and consumption. Gravimetric water content and nitrate (NO₃^-) concentrations were measured from soil cores (0-10 cm depth) taken from each quadrant of the chamber footprint.

Results/Conclusions

Net N₂O emissions ranged from 0.25 – 346 mg N m^-2 d^-1. At low to intermediate N₂O/N₂ ratios (< 50), predicted net N₂O fluxes matched observed values (R²= 0.99). A linear regression of predicted and observed ¹⁴N₂O concentrations produced a slope of 0.99 with R²= 0.99. Gross N₂O production rates were 11.30 ± 4.98 and 0.82 ± 0.08 mgN m^-2 d^-1 at the crown and slope sites, respectively; gross N₂O consumption rates were 0.28 ± 0.13 and 0.18 ± 0.09 mgN m^-2 d^-1 at the crown and slope, respectively. Large NO₃^- pools supported high N₂O production and favored N₂O production over N₂. Our results show that the ¹⁵N₂O pool dilution technique can accurately estimate N₂O production and consumption at low to intermediate N₂O/N₂ ratios, and suggest that N₂O emissions are the dominant denitrification N loss pathway in this ecosystem.