Intense wet-dry cycles weakened the carbon sequestration of soil aggregates in the riparian zone

Increasing the frequency and magnitude of wet-dry cycles can fundamentally affect soil carbon (C) turnover. However, the response of soil respiration to intense wet-dry cycles in the riparian zone is unclear. Hence, this study aimed to identify whether soil respiration and soil aggregates in different land use types were affected by different flooding intensities in the riparian zone of the Three Gorges Reservoir, China, and determine the responses of soil organic carbon (SOC) sequestration by soil aggregates to wet-dry cycles in the riparian zone. At the Wuyangwan of the Pengxi River, a tributary of the Yangtze River in the Three Gorges Reservoir, soil respiration and soil aggregates were investigated at different elevations of the riparian zone with representative land use types, namely a cornfield (173C) and a paddy field (173P), both at an elevation of 173 m above sea level, and two paddy fields, 168 m and 163 m above sea level (168P and 163P). The results showed that soil aggregate stability increased (e.g., the mean weight diameter increased by 38% and 18% for 173C and 173P, respectively) in the low flooding intensity sites and decreased (e.g., the mean weight diameter decreased by 44% and 33% for 168P and 163P, respectively) in strong flooding intensity sites after four wet-dry cycles in the riparian zone of the Three Gorges Reservoir. Moreover, the wet-dry cycles increased soil respiration (i.e., the soil respiration increased by 36%, 218%, 129%, and 31% for 173C, 173P, 168P, and 163P, respectively). In addition, soil aggregate stability had a direct negative effect on soil respiration (namely, the mitigation of CO2 emissions), and the negative effect weakened after four intense wet-dry cycles (pathway coefficient was −0.76 in 2014 and −0.28 in 2018). Thus, the carbon sequestration of soil aggregates was weakened in the riparian zone. The size of the soil aggregate fractions and their organic carbon contents directly or indirectly affected soil respiration. Most notably, the proportion of microaggregates and their associated nitrogen and organic carbon contents strongly influenced soil respiration. Finally, from the perspective of maintaining soil aggregate stability and SOC sequestration, we suggest that agricultural activities should be prohibited in areas with high flooding intensity in the riparian zones.