Carbon dioxide fluxes over bermudagrass, native prairie, and sorghum

Abstract

The Bowen ratio/energy balance (BREB) method was used to measure 30 min water vapor and carbon dioxide (CO 2) fluxes over three fields dominated by different C 4 grasses (bermudagrass, tallgrass native prairie, and sorghum) at the Blackland Research Center, Temple, TX. Fluxes were related to biotic and abiotic phenomena. Carbon accumulation rates calculated from BREB measurements were compared with those determined from plant biomass measurements. Thirty min CO 2 fluxes were measured continuously from 22 February to 22 November (days 53–326) in 1993 and from 3 March to 22 November (days 62–326) in 1994. Soil CO 2 fluxes were measured periodically in each field and from a bare soil. In all fields, average daily evapotranspiration varied from less than 1 mm day −1 in March and November to about 5 mm day −1 in June and July. The CO 2 fluxes, which were affected by leaf area, radiation, and soil water content, were near zero in each field in the fall and spring and were maximum in the summer (positive toward surface). Total annual CO 2 flux in the bermudagrass, which was sprigged (planted with grass stolons) in April 1993, was −0.4 and 2.8 kg of CO 2 m −2 year −1 (which are equivalent to −0.1 and 0.8 kg of carbon (C) m −2 year −1) in 1993 and 1994, respectively. The larger flux in the second year was due to greater bermudagrass leaf area. Corresponding annual CO 2 fluxes were 0.2 and 0.3 kg m −2 year −1 (0.05 and 0.08 kg C m −2 year −1) for the prairie, and 0.9 and −0.3 kg m −2 year −1 (0.2 and −0.09 kg C m −2 year −1) for the sorghum. Bare soil CO 2 flux was a small fraction of soil CO 2 flux from all fields, suggesting that a considerable amount of soil CO 2 flux was due to root respiration. The dry matter accumulation rate calculated for each year and field from CO 2 flux measurements was within 20% of the rate calculated from gravimetric biomass measurements. The prairie and sorghum fields were in approximate equilibrium for C storage because estimated annual CO 2 fluxes were near zero. By contrast, the bermudagrass field was a large C sink, primarily in the roots, during the second year. This substantiates other evidence that conversion from continuously cultivated cropland to grassland could create a short-term C sink.