Carbon dioxide fluxes in a semiarid environment with high carbonate soils

Carbon dioxide is increasing in the atmosphere due to human activities. Over long time periods semiarid soils have sequestered inorganic carbon to accumulate the third largest global carbon pool. The hypothesis for this study was 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 over 4 years in southeastern Arizona. Aboveground biomass and soil samples taken in spring and fall were analyzed to determine seasonal changes in carbon content. Contrary to the hypothesis, both sites were found to be losing carbon annually. Absence and presence of rainfall were important carbon flux driving forces. 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 a loss of 128 g C m-2 from organic and inorganic sources. Average annual daytime CO2 flux from the brush site was a loss of 26 g C m-2, while the grass site had a gain of 86 g C m-2. Based on measured average annual aboveground biomass data and estimates of belowground biomass, the brush site sequestered 80 g C m-2 and the grass site 135 g C m-2 into biomass during the growing season. Analysis of combined inorganic soil carbon data from both sites showed a significant seasonal difference with more in the fall season than in spring. The average annual fall season soil inorganic carbon was 22.5 g kg-1 and the spring season was 19.4 g kg-1 to a depth of 30 cm. This significant seasonal difference indicated that 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.