Sampling guidelines and analytical optimization for direct greenhouse gas emissions from tropical rice and upland cropping systems

We describe a modified manual closed-chamber approach with detachable lid and vertically stackable chambers for sampling followed by simultaneous analysis of nitrous oxide (N₂O) and methane (CH₄) for measuring greenhouse gas flux from rice and upland cropping systems in peninsular India. A meta-analysis of leading internationally/regionally recommended approaches to monitor agricultural GHG emissions is presented to put our sampling choices (e.g., chamber design, sampling intensity, sample storage and analytical corrections) into perspective. Given our set-up, the sample retention capacity of polypropylene syringes and crimped glass vials with grey butyl-rubber septa was ∼6 hours and 10 days, respectively; and temperature correction of N₂O and CH₄ concentrations was essential but plant volume correction did not affect the flux rates substantially. Optimization of gas flow rates, pre-column sample retention period, oxygen venting and temperature/current were found to reduce run time from >14 to 7 min per sample and enhance sensitivity by 30–40% while improving analytical precision from 15–30% to < 2% relative standard deviation (RSD). We suggest an alternative to the linear interpolation approach of integrating the area under the N₂O peak because linear interpolation can overestimate the cumulative seasonal N₂O emissions by 50–100%, especially after fertilization and/or rain events.