Abstract
Abstract. Carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) are the most important anthropogenic greenhouse gases (GHGs). Variation in soil moisture can be very dynamic, and it is one of the dominant factors controlling the net exchange of these three GHGs. 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. A previously developed automated chamber system for measuring CO2 flux from soils was configured to run in line with a new quantum cascade laser (QCLAS) instrument that measures N2O and CH4. Here we present data from a forested wetland in Maine and an agricultural field in North Dakota, which provided examples of both net uptake and production for N2O and CH4. The objective was to provide a range of conditions in which to run the new system and to compare results to a traditional manual static-chamber method. The high-precision and more-than-10-times-lower minimum detectable flux of the QCLAS system, compared to the manual system, provided confidence in measurements of small N2O uptake in the forested wetland. At the agricultural field, the greatest difference between the automated and manual sampling systems came from the effect of the relatively infrequent manual sampling of the high spatial variation, or "hot spots", in GHG fluxes. Hot spots greatly influenced the seasonal estimates, particularly for N2O, over one 74-day alfalfa crop cycle. The high temporal frequency of the automated system clearly characterized the transient response of all three GHGs to precipitation and demonstrated a clear diel pattern related to temperature for GHGs. A combination of high-frequency automated and spatially distributed chambers would be ideal for characterizing hot spots and "hot moments" of GHG fluxes.
Highlights
The production and transport of CO2, CH4, and N2O in soils is strongly affected by changes in soil temperature and moisture through diel cycles, wet-up and dry-down events, management practices, seasonal patterns, and interannual variation in climate (Davidson and Schimel, 1995; Borken et al, 2006; Davidson et al, 2006)
The microbial processes of nitrification and denitrification are the dominant sources of N2O (Firestone and Davidson, 1989), and these soil microbial processes are subject to rapid responses to wetting and thawing events (Davidson, 1992)
Calculations of minimum detectable fluxes (MDFs) of N2O and CH4 for manual chamber-based measurements from North Dakota were made following the methodologies developed by Parkin et al (2012) as this methodology fit with the manual sampling technique (Sect. 2.3)
Summary
The production and transport of CO2, CH4, and N2O in soils is strongly affected by changes in soil temperature and moisture through diel cycles, wet-up and dry-down events, management practices, seasonal patterns, and interannual variation in climate (Davidson and Schimel, 1995; Borken et al, 2006; Davidson et al, 2006). The microbial processes of nitrification and denitrification are the dominant sources of N2O (Firestone and Davidson, 1989), and these soil microbial processes are subject to rapid responses to wetting and thawing events (Davidson, 1992). The balance between release and uptake of CH4 from soils is dependent largely on soil moisture status, which can change rapidly with precipitation events. Fluxes associated with precipitation events are difficult to study if humans must be present to make measurements immediately before, during, and after storms
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