Use of stable isotopes in the study of CO₂ fluxes in shortgrass steppe

Abstract

The shortgrass steppe in northeastern Colorado has brief periods of water availability, and as a consequence, distinct carbon isotopes of C3 and C4 plants. We investigated 1) what environmental controls are responsible for the dynamics of δ13C of ecosystem respiration (δ13CR) and how plant functional groups contribute to δ13CR, and 2) what environmental controls are responsible for δ18O of ecosystem water pools and respiratory fluxes and how plant functional groups interact with different environmental conditions (moist versus dry). Last, we determined whether or not isotopic fluxes of 13CO2 and C18O16O could be used for partitioning NEE into gross photosynthesis and respiration. We hypothesized that time lags between pulse precipitation events, net CO2 exchange (NEE) and δ13C of ecosystem respiration (δ13CR) are related to antecedent moisture conditions. Time lags of two weeks occurred between a rainfall event and a NEE response after extended dry periods. During extreme dry periods, soil respiration measurements indicated older plant carbon compounds were emitted. Plant responses related to precipitation events seem to correlate well with variations of δ13CR. Dominant influence of very enriched δ18O of soil surface water caused decoupling between δ18O of atmospheric CO2 and inverse CO2 concentrations during the dry seasons. The changes in δ18O of leaf water pools and respiratory CO2 fluxes responded rapidly to pulse precipitation. Isotopic δ18O partitioning suggested that soil respiration accounted for about 89 % of the total ecosystem respiration, and the strong contribution was more pronounced in dry conditions. Photosynthesis did not alter the C18O16O signal during prolonged dry conditions. It appeared that water vapor and carbon were weakly coupled during rainy sampling periods. Restricted usage of partitioning with carbon isotopic analysis was also revealed in dry condition during July 2001. Despite the limitations, photosynthetic CO2 fluxes partitioned by both stable isotopes agreed well, and the relationship was stronger when isotopic disequilibrium between photosynthetic assimilation and δ13CR was strong.