Periodic inundation accelerates the release process of organic carbon from plant litter

The total organic carbon (OC) from plant litter in riparian zones is an important nutrient source for aquatic organisms and plays a crucial role in the nutrient cycling of river ecosystems. Nevertheless, the total amount of OC in dammed rivers gradually decreases, and the restoration methods are rarely researched. A hypothesis was proposed that the periodic inundation altered the process of OC release from plant litter. To explore the impact of periodic inundation on OC release from litter in the riparian zone, litter bags in situ tests were conducted in the Yalong River. Three inundation treatments were conducted for the test samples, which were NS (never submerged by water), PIS (periodic submerged), and PMS (permanent submerged). Results indicated that the amount of OC released from litters in PIS treatment was about 1.1 times that in PMS treatment, and about 2.1 times that in NS treatment. The average release rate coefficient k of PIS treatment (at mean water level) was the highest (12.8 × 10^-4 d^-1), followed by PMS treatment (11.0 × 10^-4 d^-1), and NS treatment (5.6 × 10^-4 d^-1), which demonstrated that the periodic inundation was critical for OC release. The mean water level was a demarcation line where there was a significant difference in the release of OC in the riparian zone (p < 0.05). Flow velocity alone could account for 84% of the variation in OC release rate, while the flow velocity and inundation duration together could achieve an explanatory degree of 86%. This research can provide a valuable scientific basis for the protection and restoration of river ecosystems, especially for the recovery of OC concentration in dammed rivers.