Automated online measurement of N2, N2O, NO, CO2, and CH4 emissions based on a gas-flow-soil-core technique

• A technical system is improved to measure soil N2, N2O, NO, CO2 and CH4 emissions.
• Two methods are applied for measuring NO concentrations of different levels.
• Synchronization and automation enable more accurate quantification of the gases.

The gas-flow-soil-core (GFSC) technique allows to directly measure emission rates of denitrification gases of incubated soil cores. However, the technique was still suffering some drawbacks such as inadequate accuracy due to asynchronous detection of dinitrogen (N2) and other gases and low measurement frequency. Furthermore, its application was limited due to intensive manual operation. To overcome these drawbacks, we updated the GFSC system as described by Wang et al. (2011) by (a) using both a chemiluminescent detector and a gas chromatograph detector to measure nitric oxide (NO), (b) synchronizing the measurements of N2, NO, nitrous oxide (N2O), carbon dioxide (CO2) and methane (CH4), and (c) fully automating the sampling/analysis of all the gases. These technical modifications significantly reduced labor demands by at least a factor of two, increased the measurement frequency from 3 to 6 times per day and resulted in remarkable improvements in measurement accuracy (with detection limits of 0.5, 0.01, 0.05, 2.3 and 0.2 μg N or C h−1 kg−1 ds, or 17, 0.3, 1.8, 82, and 6 μg N or C m−2 h−1, for N2, N2O, NO, CO2, and CH4, respectively). In some circumstances, the modified system measured significantly more N2 and CO2 and less N2O and NO because of the enhanced measurement frequency. The modified system distinguished the differences in emissions of the denitrification gases and CO2 due to a 20% change in initial carbon supplies. It also remarkably recovered approximately 90% of consumed nitrate during incubation. These performances validate the technical improvement, and indicate that the improved GFSC system may provide a powerful research tool for obtaining deeper insights into the processes of soil carbon and nitrogen transformation during denitrification.