Soil nitrous oxide measurement methods: Collar adaptors and comparison of manual versus automatic chamber headspace sample collection.

Manually operated non-flow-through non-steady-state chambers have been a dominant method for soil measurements of nitrous oxide (N2O) emission due to low cost, simplicity, and adaptability to different experimental designs. However, the use of automated dynamic non-steady-state chambers that deliver measurements in real-time has increased in the literature. A systematic comparison of these two methods could assist with integration between the rich soil N2O knowledge established using manual chamber + gas chromatography (MC-GC) versus the results increasingly generated by automatic chambers + trace gas analyzers (AC-TGAs). The geometry of the system, a research-specific requirement, can also affect the performance of chamber systems. Automatic chambers with site-specific geometry could not be available or the cost could be prohibitive due to the advanced technology of such systems. The use of low-cost geometry adaptors is an alternative, but their utilization must be preceded by performance validation. In this technical note, we compared daily N2O fluxes obtained with an AC-TGA system versus MC-GC. The comparison was also performed between two prototypes of rectangular adaptors (ADP-1 and ADP-2) and the standard round collar supplied with AC-TGA. The regression approach returned a slope of 0.98 (confidence interval [CI] = 0.97-1.00) for AC-TGA versus MC-GC, indication of potential for generation of comparable datasets. The rectangular geometry adaptors demonstrated to be an alternative to meet research-specific requirements in N2O studies. The prototype ADP-1 resulted in equivalent fluxes to measurements performed with the standard round collar (slope = 1.00 [CI = 0.99-1.03]), but prototype ADP-2 underestimated fluxes from 1% to 11%.