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

Soil CO₂ and CH₄ emissions and their carbon isotopic signatures linked to saturated and drained states of 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.

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.

【Results】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. 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.

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.

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.

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.

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).

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).

【Conclusions】To 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.

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