Seasonal and interannual variability in 13C composition of ecosystem carbon fluxes in the U.S. Southern Great Plains
Authors
Margaret S. Torn — Lawrence Berkeley National Laboratory
Joseph A. Berry — Carnegie Institution for Science
Sebastien Christophe Biraud — Lawrence Berkeley National Laboratory
William Riley — Lawrence Berkeley National Laboratory
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Category
Atmospheric State & Surface
Description
The δ13C signature of terrestrial carbon fluxes (δbio) provides an important constraint for inverse models of CO2 sources and sinks, insight into vegetation physiology, C3 and C4 vegetation productivity, and ecosystem carbon residence times. From 2002–2009, we measured atmospheric CO2 concentration and δ13C–CO2 at four heights (2 to 60 m) in the U.S. Southern Great Plains (SGP) and computed δbio weekly. This region has a fine-scale mix of crops (primarily C3 winter wheat) and C4 pasture grasses. δbio had a large and consistent seasonal cycle of 6–8 permil. Ensemble monthly mean δbio ranged from −25.8 ± 0.4 permil (±SE) in March to −20.1 ± 0.4 permil in July. Thus, C3 vegetation contributed about 80% of ecosystem fluxes in winter-spring and 50% in summer-fall. In contrast, prairie-soil δ13C values were about –15 permil, indicating that historically the region was dominated by C4 vegetation and had more positive δbio values. Based on a land-surface model, isofluxes (δbio × NEE) in this region have large seasonal amplitude because δbio and net ecosystem exchange (NEE) co-vary. Interannual variability in isoflux was driven by variability in NEE. The large seasonal amplitude in δbio and isoflux implies that carbon inverse analyses require accurate estimates of land cover and temporally resolved 13CO2 and CO2 fluxes.
