Carbon Dioxide Fluxes on Walnut Gulch Experimental Watershed

Carbon dioxide is increasing in the atmosphere, presumably from human activities. Many soils on Walnut Gulch Experimental Watershed (WGEW) in southeastern Arizona contain carbonates that have accumulated over long periods of time. The hypothesis is that these soils are maintaining this carbon pool under present climatic conditions and are a sink for some of the increasing atmospheric carbon. Bowen ratio systems were used to measure CO2 fluxes from a brush and a grass community with different soil types on WGEW. Contradictory to the hypothesis, the two sites were found to be losing carbon annually. The brush site with higher inorganic carbon in the soil, had an average annual loss of 144 g C m-2 and the grass site 127 g C m-2. Based on measured aboveground biomass data and estimates of belowground biomass, the brush site took up 80 g C m-2 and the grass site 135 g C m-2 of organic carbon during the growing season. Inorganic soil carbon analysis showed a significant seasonal difference with more in the fall season. The average fall season soil inorganic carbon was 2.24 % and the spring season was 1.96 % to a depth of 30 cm. This significant seasonal difference indicated some of the measured CO2 fluxes were into and out of the inorganic carbon pool. The source of carbon for the measured annual losses from these sites was concluded to be from the large inorganic carbon pool with carbon cycling through both the organic and inorganic pools at the sites