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ABB LGR | Application of ABB LGR CCIA and MCIA in the Three Gorges Reservoir of China

Date: 2022-01-18
浏览次数: 16

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


ABB LGR | Application of ABB LGR CCIA and MCIA in the Three Gorges Reservoir of China

Increased CO2 and CH4 emissions are the main causes of global warming (IPCC, 2013), with approximately 44% and 60% of CO2 and CH4 emissions being emitted into the atmosphere by anthropogenic activities. 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.

Based on this, to fill the research gap, in this article, a group of Chinese researchers from Yunnan University and Wuhan Botanical Garden, Chinese Academy of Sciences selected four elevations (i.e., >175 m, 160–175 m, 145–160 m and the water column) at Zhongxian in Chongqing (30°42 N, 108°18 E) (Fig.1) under the riparian zone of the Three Gorges Reservoir, in order to quantify the pattern of CO2 and CH4 emissions and their carbon isotopic signatures, as well as associated controls under the saturated and drained states. They made the following hypothesis: 1) changes in soil conditions (e.g., soil substrate quality, soil moisture and temperature) would alter CO2 emission and the δ13C value of CO2 because of shifts in dominant plant species under flooding; 2) the pattern of CH4 emission and its isotopic signature would be more sensitive to the flooding, reflecting the increased soil anaerobic environment; and 3) the different flooding status (i.e., the saturated and drained states) would result in alterations in enzyme expression and microbial attributes, which would greatly affect CO2 and CH4 emissions.

ABB LGR | Application of ABB LGR CCIA and MCIA in the Three Gorges Reservoir of China

Fig. 1. Location of the study area in Chongqing Zhongxian; (a), satellite imagery of sampling sites and detailed view of the static flux chamber placement along the elevation gradient (b) in the riparian zone of the Three Gorges Reservoir region.

The exchange rates of CO2 and CH4 were measured at the soil/water atmosphere interface via the floating and static chamber method in eleven 1-week campaigns from June 2017 to August 2017, respectively. The concentration and δ13C value of CO2 and CH4 were analyzed with Carbon Dioxide Isotope Analyzer and Methane Carbon Isotope Analyzer, respectively.

ResultsThe 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. They 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.

ABB LGR | Application of ABB LGR CCIA and MCIA in the Three Gorges Reservoir of China

Fig. 2. Soil CO2 emissions of the weekly average (a) and the whole no-flooding period (b) under different elevations (i.e., water column, low, high and upland) in the riparian zone. Abbreviations: W, water column; L, low; H, high; A, upland.

ABB LGR | Application of ABB LGR CCIA and MCIA in the Three Gorges Reservoir of China

Fig. 3. CH4 emissions of the weekly average (a) and the whole no-flooding period (b) under different elevations (i.e., water column, low, high and upland) in the riparian zone.

ABB LGR | Application of ABB LGR CCIA and MCIA in the Three Gorges Reservoir of China


Fig. 4. The δ13C of soil-respired CO2 (a) and δ13C of CH4 (b) during the whole no-flooding period under different elevations (i.e., water column, low, high and upland) in the riparian zone.

ABB LGR | Application of ABB LGR CCIA and MCIA in the Three Gorges Reservoir of China

Fig. 5. Mean CO2 emissions (a) and CH4 emissions (b) in the soil water saturated and drained states at different elevations (i.e., low, high and upland).

ABB LGR | Application of ABB LGR CCIA and MCIA in the Three Gorges Reservoir of China

Fig. 6. Mean δ13C of soil-respired CO2 (a) and δ13C of CH4 (b) at soil water saturated and drained states under different elevations (i.e., low, high and upland).

ConclusionsTo conclude, variations in soil CO2 and CH4 emissions and their carbon isotopic signatures estimated from the riparian zone were strongly affected by periodic flooding, which could determine the strength of the source/sink of CO2 and CH4 in the Three Gorges Reservoir. Compared to upland area, the tolerable soil environments, higher enzyme activities as well as lower substrate quality at riparian zone maintained higher CO2 emissions. The δ13C signatures of soil-respired CO2 further confirmed that the shift in substrate quality and enzyme activities was the main contributor to CO2 emissions. The accumulative CH4 emissions increased from low to high elevations in the riparian zone with CH4 uptake in the upland area. Based on the δ13C value of CH4, we tentatively concluded that the higher CH4 emission during the saturated state was characterized by an acetate cleavage process in stronger anerobic environments. Thus, our results highlighted that dam-triggered periodic flooding resulted in alterations in soil quality (i.e., C:N ratio), enzyme expression and microbial strategies for using C, and methanotrophic processing and potentially changed CO2 and CH4 emissions and their carbon isotopic signatures.


ABB LGR | Application of ABB LGR CCIA and MCIA in the Three Gorges Reservoir of China6377810319642229864855929.pdf


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