High-Frequency, Automated Measurements of CO2, N2O and CH4 from Forest and Agricultural Soils

Carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) are the most important anthropogenic greenhouse gases. Soils are the dominant natural source of N2O, and fertilized agricultural soils are a major source of increasing anthropogenic N2O. Anthropogenic sources of CH4 include rice cultivation, while wetlands are a significant natural source, and upland soils are a natural CH4 sink. While most anthropogenic CO2 is derived from fossil fuel combustion, a significant fraction is from land use change, including a portion from loss of soil carbon. Soils play a central role as sources and sinks of the three most important anthropogenic greenhouse gases of the 21st century, CO2, CH4, and N2O. Variation in soil moisture can be very dynamic, and it is one of the dominant factors controlling soil aeration, and hence the balance between aerobic (CO2 producing) and anaerobic (CH4 producing) respiration. The production and consumption of N2O is also highly dependent on spatial and temporal variation in soil moisture. Although technologies for high frequency, precise measurements of CO2 have been available for years, methods for measuring soil fluxes of CH4 and N2O at high temporal frequency have been hampered by lack of appropriate technology for in situ real-time measurements. We utilized a previously developed automated chamber system for measuring CO2 efflux (Licor 6252 IRGA) from soils, and configured it to run in-line with a new model quantum cascade laser (QCL) system which measures N2O and CH4 (Aerodyne model QC-TILDAS-CS). Chambers, with corresponding soil water content and soil temperature sensors, were deployed at Howland Forest ME, in an upland soil and nearby swamp in 2011 at in an agricultural field in Mandan ND in 2012. Each chamber flux, soil moisture and temperature, are measured at an hourly frequency. This high frequency measurement rate allows for the characterization of seasonal and diel trends in CO2, CH4 and N2O as well as any rapid, transient response to wet up events. Opportunities for mitigation efforts to reduce emissions of CO2, CH4 and N2O could be missed due to lack of understanding of transient spikes in emissions of these gases in response to rapidly changing environmental conditions across agricultural and forest landscapes. This new QCL system provides an opportunity to characterize these types of responses which will further help to constrain estimates of annul fluxes and future green house gas modeling efforts.