Soil CO2 and CH4 emissions and their carbon isotopic signatures linked to saturated and drained states of the Three Gorges Reservoir of China

Abstract

Human activities such as dams disturb the structure and function of wetlands, triggering large soil CO2 and CH4 emissions. However, controls over field CO2 and CH4 emissions and their carbon isotopic signatures in reservoir wetlands are not yet fully understood. We investigated in situ CO2 and CH4 emissions, the δ13C values of CO2 and CH4, and associated environments in the saturated and drained states under four elevations (i.e., the water column, <147 m, permanent inundation area without plants; the low, 145–160 m, frequently flooded area with revegetation; the high, 160–175 m, rarely flooded area with revegetation; and the upland area as the control, >175 m, nonflooded area with original plants) in the Three Gorges Reservoir area. The CO2 emissions was significantly higher in high elevation, and they also significantly differed between the saturated and drained states. In contrast, the CH4 emissions on average (41.97 μg CH4 m−2 h−1) were higher at high elevations than at low elevations (22.73 μg CH4 m−2 h−1) during the whole observation period. CH4 emissions decreased by 90% at low elevations and increased by 153% at high elevations from the saturated to drained states. The δ13C of CH4 was more enriched at high elevations than in the low and upland areas, with a more depleted level under the saturated state than under the drained state. We found that soil CO2 and CH4 emissions were closely related to soil substrate quality (e.g., C: N ratio) and enzyme activities, whereas the δ13C values of CO2 and CH4 were primarily associated with root respiration and methanogenic bacteria, respectively. Specifically, the effects of the saturated and drained states on soil CO2 and CH4 emissions were stronger than the effect of reservoir elevation, thereby providing an important basis for assessing carbon neutrality in response to anthropogenic activities.