Estimating Greenhouse Gas Fluxes by Gas Chromatography and the Photoacoustic Infrared Detection System.

Practical methodologies for estimating greenhouse gas (GHG) fluxes are needed in environmental studies to assess the impacts of agricultural practices on GHG emissions and C sequestration. The conventional static-chamber methodologies for measuring GHG, involve manual gas sampling, and analysis by gas chromatography (GC) while the photoacoustic infrared system (PAS), gives direct readings of GHG concentrations, eliminating the need for gas storage and analyses.

The CO₂, N₂O and CH₄ fluxes from wheat fields in India were measured from static chambers in 32 plots(P) with 4 nitrogen(N) and 2 residue(S) treatments, on 23 sampling dates(D), by 2 methods(M): 1) manually taking 4 periodic gas samples at 10 min intervals and analyzing the gas concentrations by GC and 2) automatically recording 10 GHG concentrations at 1.2 min intervals using the PAS.

The 95% confidence limits of the slopes(emission rates) analyzed by regression analyses showed that GC and PAS estimates of CO₂ and N₂O fluxes within each plot, varied significantly (P<0.05) from each other in only 6-7%, of the 736 PxD measurements.

The SAS mixed procedure with repeated/group effect showed that for both N₂O and CO₂ fluxes, MxSxN interactions were not significant, but MxN and MxS were, for N₂O and CO₂ respectively. These results indicate that the 2 methods exhibited similar trends for SxN but not for S and N means. GC values were generally higher for N₂O but lower for CO₂ than those of the PAS. However, GC data also showed greater variability than those of the PAS such that no significant differences were found between GC and PAS estimates for N₂O and CO₂ in all except 1 of the 8 SxN treatments averaged over 4 reps and 23D.

Both methods showed no significant CH₄ emission on all sampling dates after the PAS-CH₄ concentrations were corrected for changes in water vapor over time.