| The Three Gorges Reservoir(TGR) area is the extra-large and anti-season running reservoir, with lots of function such as power generation, flood control, navigation, and irrigation, etc., providing enormous economic and social benefits, however, having an important impact on the ecology environment. Being a collection zone of the runoff, water-level fluctuating zone(WLFZ) becomes a collection area of nutrients and pollutants undoubtedly. Meanwhile, the vegetation and soil in WLFZ are the ecological barrier of the reservoir, so nutrients absorbed by vegetation and soil will affect the quality of water environment. In dry period, the vigorous restoration of vegetation can protect water system by intercepting contaminants and reducing the carbon, phosphorus and other nutrients. In storage period, most of vegetation are flooded and start to decompose. In processes of decomposition, vegetation will release carbon, phosphorus and other nutrients to the ecosystem of reservoir, actively or passively, thus exerting exacerbate pressure on the environment. Meanwhile, the mass transport and energy flow are complex between soil and overlying water. Therefore, it is actually important to understand the environment change mechanism of TGR and management of WLFZ by exploring the degradation dynamics and release characteristic of flooded vegetation. In this paper, we study the degradation dynamics and release characteristics of nutrients for traditional flooded vegetation in WLFZ, including the structure characteristics of vegetation, correlations between vegetation and soil nutrients, and flooding simulation in laboratory, on the basis of methodology of degradation in lake, wetland, and grassland. The sampling sites are in Zhenxi Town, Shibao Zhai, Zhuxi River, and Hanfeng Lake. The main results are as follows:(1) Vegetation in TGR exhibited periodical change with the increase or decrease of water level: emerge-growth-submerge, and further substituted by the herbaceous plants to adapt to the environment change. We totally collected 76 herbaceous species which belonged to 26 families. Grasses species were the most(19 species) followed by asteraceae species(13 species). There were respectively 45 and 31 types of biennial and perennial herbs, in which Setaria viridis, Cynodon dactylon, Xanthium sibiricum, Alternanthera Philoxeroides, Hemarthria altissima, Bidens pilosa, Paspalum distichum were in the advantage.(2) There were significant differences between the ground and underground biomass of the vegetation(P<0.01). The aboveground biomass ranged from 0.21 to 1.57 kg·m-2, with a mean value of(0.64±0.21) kg·m-2, and the underground biomass ranged from 0.14 to 0.81 kg·m-2, with a mean value of(0.39±0.16) kg·m-2. The aboveground biomass of plants was significantly higher than that of the underground biomass. The ground and underground biomass followed an order as "less- more- less" with the elevation of altitude. Plant biomass was closely related to its environment, and the ground biomass was positively related to soil SOM and TN, while negatively related to soil AP, indicating that soil nitrogen nutrient was the key nutrients of herbaceous plants’ growth.(3) The contents of carbon, nitrogen and phosphorus of the typical plants were different significantly(P<0.01). Hemarthria compressa had the maximal TC, Alternanthera philoxeroides had the minimum. Xanthium sibiricum had the minimal TN, while Hemarthria altissima had the minimum. Wormwood had the maximal TP, while TP of Cynodon dactylon was minimal. The initial N/P values of typical plants were all <14, which indicated that the growth of plants was limited to nitrogen. The correlation between plant and soil nutrient contents showed that plant TN was significantly negatively related to soil p H, and positively correlated with soil SOM and TN. Plant TP was significantly negatively correlated with soil p H and AP.(4) The decomposition of plants under inundation resulted in the increase of p H value and the release of carbon and phosphorus under the overlying water. The release processes of TP, DTP, DIP and DOC fitted the kinetic equations in the form of, k>0. In the early stage, nutrients were released rapidly, and tended to be gentle, but slightly decreased in the later period. The DOC and P reached the peak at 11th~ 20 th days after flooding. The largest release amounts of TP and DOC were Wormwood. And DTP and DIP release amounts of Xanthium sibiricum were the largest. Whereas Cynodon dactylon TP, DTP and DIP release amounts were the smallest. DOC release amount of Paspalum distichum was minimal. TP, DTP and DOC release rates of Wormwood and DIP release rate of Xanthium sibiricum were the largest. TP, DTP and DIP of Cynodon dactylon and DOC release rate of Alternanthera philoxeroides were minimal. The forn of phosphorus released from soaked plants was dominated by DIP, and the mean ratio of DIP and TP was 80.1%. Importantly, P release amounts had significant positive linear correlations with initial N and P contents of plants, while it correlated negatively significantly to C/N and C/P in linear equations. Whereas DOC release was weakly correlated with initial contents of vegetation. After half a month of inunadation, the average release loads of TP, DTP, DIP and DOC were 864, 7.50, 6.89 and 105.23 kg·hm-2, respectively, and based on this estimate the fluctuating, the release total amounts of TP, DTP, DIP and DOC were 301.48, 261.70, 240.41 and 3671.79 t/a, respectively. Therefore, seasonal plants’ growth-submerged would have important effects on nutrient cycling and water quality.(5) The higher the p H was, the easier DOC was released, while acidic conditions were propitious to P release. On the same light condition, plants without the addition of microbial inhibitors demonstrated substantial release of carbon and phosphorus compared to that proceeded by microbial inhibitors; while on the same microbial condition, both shading and lighting groups demonstrated no obvious difference for DOC and phosphorus release, indicating that microbial in water was a main factor affecting nutrients’ release of plants. The high nutrient contents in the overlying water could promote the release of carbon and phosphorus.(6) During inunadation, p H value of the overlying water firstly decreased rapidly, then increased and tended to be stable in the later period in plant & soil & water, plant & water and soil & water treatments. At the end of inunadation, p H value was higher than the intial p H value of the overlying water. The trends of DOC and phosphorus contents of the overlying water were similar, which showed firstly a rapid increase and then a decrease, at last tended to be stable. DOC and phosphorus contents in plant & soil & water and plant & water treatments were much higher than that in soil & water treatment, indicating that plants had a greater contribution to DOC and phosphorus contents of the overlying water. At the early stage of inunadation, DOC and phosphorus contents of overlying water in plant & soil treatment were higher than that in plant & water and soil & water treatments, suggesting that nutrients absorbed by vegetation from soils promoted the release of nutrients in soils. However, during the mid-late period, they were less than that in plant & water treatment. This was mainly caused by the microbial uptake and soil adsorption. Therefore the seasonal growth of plants had a significant effect on the transport and circulation of carbon and phosphorus. |
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