Wetting and drying cycles drive variations in the stable carbon isotope ratio of respired carbon dioxide in semi-arid grassland

In semi-arid regions, where plants using both C3 and C 4 photosynthetic pathways are common, the stable C isotope ratio (δ13C) of ecosystem respiration (δ13C R) is strongly variable seasonally and inter-annually. Improved understanding of physiological and environmental controls over these variations will improve C cycle models that rely on the isotopic composition of atmospheric CO2. We hypothesized that timing of precipitation events and antecedent moisture interact with activity of C3 and C4 grasses to determine net ecosystem CO2 exchange (NEE) and δ13CR. Field measurements included CO2 and δ13C fluxes from the whole ecosystem and from patches of different plant communities, biomass and δ13C of plants and soils over the 2000 and 2001 growing seasons. NEE shifted from C source to sink in response to rainfall events, but this shift occurred after a time lag of up to 2 weeks if a dry period preceded the rainfall. The seasonal average of δ13CR was higher in 2000 (-16‰) than 2001 (20‰), probably due to drier conditions during the 2000 growing season (79.7 mm of precipitation from April up to and including July) than in 2001 (189 mm). During moist conditions, δ13C averaged -22‰ from C3 patches, -16‰ from C4 patches, and -19‰ from mixed C3 and C4 patches. However, during dry conditions the apparent spatial differences were not obvious, suggesting reduced autotrophic activity in C4 grasses with shallow rooting depth, soon after the onset of dry conditions. Air and soil temperatures were negatively correlated with δ13CR; vapor pressure deficit was a poor predictor of δ13CR, in contrast to more mesic ecosystems. Responses of respiration components to precipitation pulses were explained by differences in soil moisture thresholds between C3 and C4 species. Stable isotopic composition of respiration in semi-arid ecosystems is more temporally and spatially variable than in mesic ecosystems owing to dynamic aspects of pulse precipitation episodes and biological drivers