Interception, degradation and contributions of terrestrial organic carbon obtained from lignin analysis in Wujiang River, southwest China

Understanding the fate of terrestrial organic carbon (OC) in a cascade impoundment system is critical for recognizing the role of carbon sink for reservoirs. Surface sediments collected from eight cascade reservoirs across the Wujiang River, southwestern China, were analyzed for elemental and stable carbon isotopic (δ13C) composition, and lignin phenols (Σ8 and Λ8) to investigate the spatial distribution, contribution, origin and degradation of sedimentary terrestrial OC. The values of total organic carbon (TOC) and Σ8 exhibited a remarkable reduction along the upstream–downstream transect suggesting the trapping effect of cascade-damming. A relatively broad range of δ13C (−26.61 to −25.54‰, 95% CI) and C/N (6.80–18.20) indicated mixed allochthonous/autochthonous OC sources in surface sediments. The quantitative simulation indicates that the OC of the sediments mainly was derived from terrestrial organic matter. Soil-derived OC rather than C3 vascular plant-derived OC makes a major contribution to sedimentary terrestrial OC in reservoirs on karst terrain. As evidenced by lignin compositions and δ13C, the predominant vascular plant origins of terrestrial OC along the Wujiang River are non-woody angiosperm C3 plants. The aged reservoirs showed a trend of increasing contribution of autochthonous OC, which potentially weaken the role of carbon sink for reservoirs. The relationship between runoff inputs, watershed area/water surface area ratios, and water residence time and Λ8 were explored, indicating the natural and anthropogenic influences on terrestrial OC remains very complex in a cascade-damming river.