Shift in functional plant groups under flooding impacted ecosystem C and N dynamics across riparian zones in the Three Gorges of China.

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 C and N abundance of C and C 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, 145-175 m, area with revegetation due to flooding, N = 6); and unflooding zone, >175 m with original plant as a control, N = 3) in riparian zones of the Three Gorges Reservoir, China. The dominant species were the C plants in the upstream including Changshou (CS), Fuling (FL) and Zhongxian (ZX) and the C plants in the downstream in unflooding zone including Wanzhou (WZ) Badong (BD), and Zigui (ZG). C 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 C isotopic differences between soil and plant (ΔδC) 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 C-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 C-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 δN 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 C plant in situ and new C input by existing dominant C plant, whereas soil N dynamics was predictably dependent on soil relative C and N availability in response to flooding at regional scale.