Greenhouse Gas Fluxes from Forested Wetland and Upland Soils

Abstract:

Carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) are the most important greenhouse gases. Soils are the dominant natural source of N₂O, and have been shown to be a small sink under N-limited conditions. Wetlands are a significant natural source of CH₄, and dry upland soils a natural CH₄ sink. Soils release CO₂ produced by both autotrophic (root) and heterotrophic (microbial) respiration processes. 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 (predominantly CO₂ producing) and anaerobic (both CO₂ and CH₄ producing) respiration. The production and consumption of N₂O is also highly dependent on spatial and temporal variation in soil moisture. Howland forest, ME is a mosaic of well drained upland, wetland and small transitional upland/wetland soils which makes for a unique and challenging environment to measure the effects of soil moisture on the net exchange of these important greenhouse gases.

To quantify the flux of CO₂, CH₄ and N₂O from the Howland forest soils, 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). This system allowed for simultaneous, high frequency, continuous measurement of all three greenhouse gases. Fourteen sampling chambers were deployed in an upland soil (8), nearby wetland (3) and a transitional upland/wetland (3). Each chamber was measured every 90 minutes. Upland soils were consistent sources of CO₂ and sinks for CH₄, however the N₂O fluxes were transient between sources and sinks. The wetland soils were consistent sources of high CH₄ emissions, low CO₂ emissions and a consistently small N₂O sink. The transitional upland/wetland soil was a consistent source of CO₂ but was much more transient between CH₄ and N₂O sources and sinks. High frequency CO₂, CH₄ and N₂O flux measurements were used to estimate seasonal flux sums, and the flux estimates from each soil drainage class were then applied to a GIS-based soil map to create a spatially representative estimate of net flux at the landscape scale.