Measurement of nitrous oxide soil fluxes using sorbent-stabilized sampling of flux chambers

A new chamber-based method to measure nitrous oxide soil gas fluxes using an N₂O sorbent is presented. The Greenhouse Gas Reduction through Agricultural Carbon Enhancement network (GRACEnet) protocols require grab samples (typically 25 mL ea.) obtained at multiple intervals throughout chamber deployment (i.e., 0, 15, and 30 min) and linear and nonlinear options to process the raw concentrations, based on goodness-of-fit tests. The new method uses a single, sorbent-stabilized large volume (400 mL) gas sample collected at the end of each chamber deployment (30 min) and assumes a linear concentration increase. Both methods estimate the initial (time 0) gas flux using chamber concentration changes and deployment time. This report presents a side-by-side field test in experimental plots. Samples were independently analyzed by gas chromatography and thermal desorption/gas chromatography for the standard and new method, respectively. Gas concentrations measured by both methods at the end of the chamber deployment and calculated soil gas fluxes were in close agreement (R² = 0.92 and R² = 0.91, respectively). Additionally, four 100 mL samples taken from multiple chambers at the end of the deployment were pooled into a single cartridge to explore the sorbent's potential to further reduce the number of samples. Pooled sample results from four locations correlated well with average chamber deployments (R² = 0.92 and R² = 0.95 for concentrations and soil gas fluxes, respectively). These results suggest sorbent-based sampling can yield soil gas flux data of similar magnitude to interval grab sampling methods. Further testing is required to study the advantages and limitations of the new method.