Large water conservancy project can strongly alter the plant community composition, however, how these changes can potentially affect ecosystem carbon (C) and nitrogen (N) dynamics is not fully understood. Here, we investigated natural 13C and 15N abundance of C3 and C4 plants and soil in different fractions [labile C (LC) and N (LN), recalcitrant C (RC) and N (RN)]from 6 sites with two elevations (flooding zone, 145-175m, area with revegetation due to flooding, N=6); and unflooding zone, >175m with original plant as a control, N=3) in riparian zones of the Three Gorges Reservoir, China. The dominant species were the C4 plants in the upstream including Changshou (CS), Fuling (FL) and Zhongxian (ZX) and the C3 plants in the downstream in unflooding zone including Wanzhou (WZ) Badong (BD), and Zigui (ZG). C4 plant in flooding zone was significant decreased by mean 25% compared with unflooding zone in the upstream but significantly increased the by mean 59% in the downstream. The 13C isotopic differences between soil and plant (Deltadelta13C) was lower than zero in both flooding and unflooding in the upstream, but was only lower than zero in flooding zone in the downstream. The proportion of C3-derived C in soil organic carbon pool (average 74.64%) was lower for the flooding zone compared to the unflooding zone (average 87.26%) in most sites, while the proportion of C3-derived C in LC (average 44.38%) was decreased in the flooding zone compared to the unflooding zone (69.52%) in the downstream. Additionally, the delta15N values of soil were higher than plant community in most sites, and were strongly associated with soil C and N pool content, as well as soil pH. Overall, our results revealed that soil C accumulation was primarily determined by C3 plant in situ and new C input by existing dominant C4 plant, whereas soil N dynamics was predictably dependent on soil relative C and N availability in response to flooding at regional scale.

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