| Water level fluctuation zone is an unstable special wetland ecosystem formed by alternative change of aquatic ecosystem and terrestrial ecosystem. Difference between Three Gorges reservoir region and natural river in terms of water level fluctuating speed, amplitude and frequency due to the artificial scheduling of the Three Gorges reservoir, the instability of fluctuating zone there has been intensified. Since the fluctuating zone only exposes for a short term, submergence tolerant herbs are the most important plants in the drawdown area. Submerged decomposition of herbaceous plants is one key component element in material cycling and energy flow in the reservoir ecosystem, which makes a significant contribution to the trophic structure of reservoir benthic community. On the other hand, the decomposition of plants might cause water eutrophication. There will be great effects of water conditions and plant species in the study area on plant nutrient releasing rate and the quality of overlying water. Therefore, through examining the decomposition rate dynamics of five different submergence tolerant plants(Phragmites 3#, Miscanthus 1#, Graminoid 5#, Graminoid 3#, and Graminoid 1#) under water, our research aims to offer early data and theoretical foundation for determining their decomposition could cause secondary pollution to water body or not.Three aspects were included in our research. First, differences of decomposition rate of isolated part and non-isolated part of these five species under different flooding elevation were studied in field, as well as the differences among flooding elevations at Ru-xi catchment in Zhong County in the middle area of the Three Gorges reservoir region. Second, to simulate flooding situation in the Three Gorges reservoir region, impacts of different flooding depths on the decomposition rate of the experimental species, and the impacts of plant stem’s integrity on experimental species decomposition rate were researched in a six-month’s submergence. Last, the effects of flowing water and stagnant water on the decomposition rate of five experimental species were also explored in simulating the water flow rate change in the Three Gorges reservoir region.Through the above studies, results indicated that:i) Different flooding depth had no significant effects on decomposition rate of the five species in the river bank of Re-xi catchment. In the same elevation, decomposition rate of isolated samples was significantly higher than that of non-isolated parts. Thus, it was death and survival of plants in flooded conditions that have affected the decomposition rate of plants in hydro-fluctuation zone, ii) The monthly decomposition rate of the five species samples in the first month was higher than the other five months. Therefore, water quality should be closely monitored for the first month when plants are submerged, in order to prevent secondary pollution. In addition, flooding depth had little effect on the decomposition rate of plant. The effects of plant fragmentation on decomposition rate were differed with species. Specifically, the decomposition rate of broken stem of Graminoid 3# and Graminoid 5# were significantly higher than their complete stems respectively, whereas there was no significant difference between the decomposition rate of broken and complete stem, broken and complete leaf in terms of Miscanthus 1# and Phragmites 3# respectively, iii) No significant difference was found in the decomposition rate of these five submergence tolerant plants between regularly water change and no change of water, including the leaves and stems of Graminoid 5#, Graminoid 3#, Miscanthus 1# and Phragmites 3#, and the whole plant body of Graminoid 1#. |
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