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 approximate to 6hours and 10days, 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 7min 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.