Dynamics of the C and O isotopic signatures of chamber respiration varying with wetting and drying cycles in semi-arid grassland

Our semi-arid grassland research site had coexisting C3 (cool-season) and C4 plant (warm-season) species, with climate characterized by brief periods of water availability with pulsed rainfall events. Understanding respiration dynamics in this ecosystem type is demanded for improving global C cycle models since the majority of stable isotope biogeochemistry research has been focused on C3-ecosystems. We measured CO2 respiration and corresponding δ13C and δ18O values from four different vegetation cover types, C3, C4, mixed C3 and C4 and bare ground patches over growing seasons with wetting and drying cycles, three times during several diurnal campaigns. We evaluated how plant community specific δ13C and δ18O from chamber respiration (δ13CR and δ18OR) varied with moist versus drying conditions or pulse event-driven episodes. Contrasting influences of soil moisture availability on C3 and C4 grasses were obvious for both δ13C and δ18O"nmeasurements. During moist conditions, recently fixed carbon assimilates contributed to the apparent different δ13CR values; -22 permil from C3 patches, -16 permil from C4 patches, and -19 permil from mixed C3 and C4 patches. Significant differences in δ18OR between C3 and C4 patches also occurred during moist condition. However, during dry conditions, the apparent vegetation type differences were not distinguishable, suggesting reduced autotrophic activity, soon after the onset of dry conditions for δ13CR. Reduced variations during dry conditions for δ18OR may be explained by three counteracting factors such as root distributions, physiological traits in terms of internal CO2 concentration and CA activity between two species, and a marked gradient in soil water δ18O profile. Small rainfall pulses initiated respiration of labile C and improved leaf gas exchange, greatly influencing δ18O of foliar respiration from patches dominated by C4 plants, which have shallow roots. Incorporating dynamic aspects of labile/stable C sources that vary with wetting and drying cycles should improve regional and global scale C cycle models.