Flooding increases plant-derived carbon accumulation in soils of aquatic-terrestrial ecotone

Soils in the aquatic-terrestrial zone undergo periodic flooding and act as significant carbon sinks. However, the mechanisms governing soil organic carbon (SOC) formation in these zones are not well understood. This study elucidates the effects of periodic flooding on SOC accumulation at the water level drawdown zone of the Three Gorges Reservoir, using lignin phenols and amino sugars as indicators of plant- and microbial-derived carbon. Results showed that SOC content averaged at 7.52, 8.31, and 8.76 g kg^-1 in 0-20 cm soils at low, intermediate, and high flooding levels, respectively, compared to 5.87 g kg^-1 in control soils. Total lignin phenols and amino sugars averaged at 0.351, 0.377, 0.337 g kg^-1 and 0.697, 0.718, 0.756 g kg^-1 in 0-20 cm soils at high, intermediate, and low flooding levels, respectively, compared to 0.161 and 0.624 g kg^-1 in control soils. Similar patterns were observed in 20-40 cm soils. Periodic flooding significantly enhanced the accumulation of plant-derived carbon and its contribution to SOC accumulation by decreasing lignin phenol oxidation, while microbial-derived carbon contribution remained unaffected. Ratios of cinnamyl to vanillyl (1.13 in flooded soils vs. 1.08 in control) and syringyl to vanillyl (0.20 in flooded soils vs. 0.17 in control) indicated that lignin phenols originated primarily from woody angiosperms and remained stable. Flooding also modified edaphic variables, such as clay mineral and particle feature, enhancing organic compound accumulation. Clay minerals, particularly chlorite and kaolinite, played more pivotal roles than illite in regulating SOC accumulation. These findings underscore the potential for managing flooding regimes as a strategy to enhance carbon sequestration and improve ecosystem resilience in aquatic-terrestrial zones.