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

Carbon isotopic composition of soil respired CO2 (soil delta C-13(R)) has been regarded as a good indicator of the linkages between aboveground processes and soil respiration. However, whether delta C-13(R) of autotrophic or heterotrophic component of soil respiration dominates the temporal variability of total soil delta C-13(R) was rarely examined by previous studies. In this study, carbon isotopic composition of atmospheric CO2 (delta C-13(air)) and soil delta C-13(R) in control (with roots) and trenched (without roots) plots were measured in a temperated mixed forest. A C-13 isotopic profile systemand an automated soil respiration systemwere used for delta C-13(air) and soil d(13)C(R) measurements, respectively. Wefound that soil delta C-13(R) in the control plots changed substantially in the growing season and itwas more negative (by similar to 0.6%) than that in the trenched plots, while soil delta C-13(R) in the trenched plots showed a minor temporal variability. This suggests that delta C-13(R) from the autotrophic respiration is the key decider of the seasonal variation pattern of the soil delta C-13(R). Moreover, the seasonal variation of soil delta C-13(R) in the control plots showed a similar pattern with the seasonal variation of delta C-13(air). A significant time-lag was found between delta C-13(air) and soil delta C-13(R), showing that soil delta C-13(R) generally lagged behind delta C-13(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 delta C-13(air) in shaping soil delta C-13(R). These findings are highly valuable to develop the process-based models of the carbon cycle of forest ecosystems. (C) 2021 Elsevier B.V. All rights reserved.