Carbon dioxide isotopic compositions during tundra ecosystem respiration and photosynthesis in relation to environmental variables in maritime Antarctica

Stable carbon isotope ratios ( 13 C/ 12 C) have been extensively used to trace carbon flow, and partition net ecosystem CO 2 exchange. However, the CO 2 isotopic data are still very limited in terrestrial ecosystems of maritime Antarctica. Here, we chose six tundra sites to investigate the isotopic compositions of CO 2 during ecosystem respiration and photosynthesis, and the relationships with environmental variables using the static chamber method. For all tundra sites, ecosystem respirated-CO 2 was 13 C-depleted while vegetation photosynthesis resulted in the enrichment of 13 C in the chamber headspace, compared with local atmospheric CO 2 . The δ 13 C-CO 2 showed a strong negative correlation with CO 2 concentrations in the chamber headspace both under dark condition ( r 2 = 0.70, p < 0.01) and under light condition ( r 2 = 0.29, p < 0.05), whereas δ 18 O-CO 2 was almost stable, only with a small fluctuation from − 7.7 to − 8.3‰. The δ 13 C of ecosystem-respired ( δ r ) and photoassimilated-CO 2 ( δ p ) fluctuated from − 12.4 to − 15.1‰ and − 10.5 to − 12.6‰, respectively, indicating that lower C isotopic discrimination occurred during plant photosynthesis and tundra ecosystem respiration, compared with reported data from other global ecosystems. Overall there was a small but distinct negative carbon isotopic disequilibrium ( D : − 1 to − 3‰) between δ r and δ p at the spatial or summertime scale. The δ p and δ r showed consistent summertime variation patterns with significant positive correlation ( r 2 = 0.95, p < 0.01). The δ r and δ p significantly negatively correlated with respiration rates ( r 2 = 0.91, p < 0.01) and photosynthesis rates ( r 2 = 0.86, p < 0.01), respectively, and the δ r , instead of δ p , significantly correlated with net ecosystem exchange fluxes ( r 2 = 0.75, p = 0.012). The summertime patterns of δ r , δ p and D sensitively corresponded to changing temperature, precipitation and sunlight intensity, supporting the links between CO 2 isotope fractionation and environmental variables during ecosystem respiration and plant photosynthesis. It was found that penguin activities had no significant effect on δ r and δ p although their activities significantly increased tundra ecosystem respiration and photosynthesis fluxes. The investigation of CO 2 isotopic compositions contributes to better understanding of carbon cycling in maritime Antarctic tundra ecosystems.