Seasonal and interannual variability in 13C composition of ecosystem carbon fluxes in the U.S. Southern Great Plains

The delta 13C value of terrestrial CO2 fluxes (delta(bio)) provides important information for inverse models of CO2 sources and sinks as well as for studies of vegetation physiology, C-3 and C-4 vegetation fluxes, and ecosystem carbon residence times. From 2002-2009, we measured atmospheric CO2 concentration and delta 13C-CO2 at four heights (2 to 60 m) in the U.S. Southern Great Plains (SGP) and computed delta(bio) weekly. This region has a fine-scale mix of crops (primarily C-3 winter wheat) and C-4 pasture grasses. delta(bio) had a large and consistent seasonal cycle of 6-8 parts per thousand. Ensemble monthly mean delta(bio) ranged from -25.8 +/- 0.4 parts per thousand (+/- SE) in March to -20.1 +/- 0.4 parts per thousand in July. Thus, C-3 vegetation contributed about 80% of ecosystem fluxes in winter-spring and 50% in summer-fall. In contrast, prairie-soil delta 13C values were about -15 parts per thousand, indicating that historically the region was dominated by C-4 vegetation and had more positive delta(bio) values. Based on a land-surface model, isofluxes (delta(bio) x NEE) in this region have large seasonal amplitude because delta(bio) and net ecosystem exchange (NEE) covary. Interannual variability in isoflux was driven by variability in NEE. The large seasonal amplitude in delta(bio) and isoflux imply that carbon inverse analyses require accurate estimates of land cover and temporally resolved 13CO(2) and CO2 fluxes.