Autotrophic respiration modulates the carbon isotope composition of soil respiration in a mixed forest

Carbon isotopic composition of soil respired CO₂ (soil δ¹³C_R) has been regarded as a good indicator of the linkages between aboveground processes and soil respiration. However, whether δ¹³C_R of autotrophic or heterotrophic component of soil respiration dominates the temporal variability of total soil δ¹³C_R was rarely examined by previous studies. In this study, carbon isotopic composition of atmospheric CO₂ (δ¹³C_air) and soil δ¹³C_R in control (with roots) and trenched (without roots) plots were measured in a temperated mixed forest. A ¹³C isotopic profile system and an automated soil respiration system were used for δ ¹³C_air and soil δ¹³C_R measurements, respectively. We found that soil δ¹³C_R in the control plots changed substantially in the growing season and it was more negative (by ~0.6‰) than that in the trenched plots, while soil δ¹³C_R in the trenched plots showed a minor temporal variability. This suggests that δ¹³C_R from the autotrophic respiration is the key decider of the seasonal variation pattern of the soil δ¹³C_R. Moreover, the seasonal variation of soil δ¹³C_R in the control plots showed a similar pattern with the seasonal variation of δ¹³C_air. A significant time-lag was found between δ¹³C_air and soil δ¹³C_R, showing that soil δ¹³C_R generally lagged behind δ¹³C_air 15 days. This result supports the hypothesis that soil respiration is closely related to carbon assimilation at the leaf-level and also stressed the importance of δ¹³C_air in shaping soil δ¹³C_R. These findings are highly valuable to develop the process-based models of the carbon cycle of forest ecosystems.