Monday, August 3, 2009 - 3:20 PM

COS 3-6: Denitrification to N2 and N2O in a pasture peatland: A test of the 15N2O pool dilution approach

Wendy H. Yang1, Yit Arn Teh2, and Whendee L. Silver1. (1) University of California, Berkeley, (2) University of St. Andrews

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.