| The "sediment-water" interface is the transformation between the sediment and overlying water, and is a special and important area of water environment. Submerged macrophytes as an important components of this regional ecosystem, play a regulatory role in transformation of phosphorus in the "sediment-water" interface. Meanwhile, the coprecipitation of CaCO3-P in water body is an important mechanism of water purification, it also has a significant impact on phosphorus cycle.In order to study the effect of P. crispus on calcium and phosphorus cycle in lake sediment and overlying water, two experiments were designed. "The effect of P. crispus on calcium and phosphorus cycle in sediment and overlying water of Lake Nanhu under sediment adding calcium" take the system of "water-submerged macrophytes-sediment" as the research object, under simulated conditions of lake ecological environment, we collected sediment and overlying water of Lake Nanhu and submerged macrophytes Potamogeton crispus L. to design a set of pot experiments, including sediment added with calcium, overlying water added phosphorus and planting P. crispus. To research the effect of planting P. crispus and sediment adding calcium on phosphorus cycle between sediment and overlying water, we determined the changes of phosphorus and calcium of sediment, overlying water, pore water and P. crispus at two different growth season of P. crispus (50 d. and 166 d.), at the same time, pH of overlying water, Eh of sediment and content of chlorophyll a were monitored. On this basis, "under different sources of sediments the effect of P. crispus on calcium and phosphorus cycle in lake sediment and overlying water" was carried out, we collected sediments from several lakes at the middle reaches of the Yangtze River, with different calcium activity, which further demonstrated the experiment results. The conclusions are obtained as follows:(1) At 50 d. growth season, TP of sediment was increased by both planting P. crispus and sediment adding calcium, which was more remarkable in higher phosphorus level water; At 166 d. growth season, sediment TP of all treatments had reduced except that of control treatment. This could be accounted for the fact that in different growth period P. crispus affected oxidation-reduction potential of rhizosphere sediment, then changed the status of metal ions, and furtherly influenced the absorption and release capacity of phosphorus on sediment. This is an important mechanism of keeping water at a lower nutrient condition by P. crispus. Compared with 50 d., there has been a tremendous increase in root TP of P. crispus for 166 d., that may be one of the causes for sediment TP decreasing.(2) Planting P. crispus could decrease the Ca2-P and Al-P contents of sediment, which showed P. crispus could effectively absorb Ca2-P and Al-P. In P. crispus seeding growth period, adding sediment with calcium could stimulate P. crispus to absorb Ca2-P. With prolonged culture period the Fe-P and Al-P content of sediment decreased and Fe-O-P and Al-O-P increased.(3) In sediment, there was a highly significant negative correlation between exchanged Ca and Ca2-P, CaCO3-Ca,Ca5(PO4)3OH-Ca and CaSO4-Ca showed highly significant positive correlation with Org-P,Fe-P,Al-O-P,Fe-O-P, there was no correlation between four forms calcium and Ca10-P. This showed in sediment active phosphorus decreased as exchanged Ca increased, and insoluble phosphorus showed a rising trend when insolubale calcium was present, but in a short time Ca10-P was freedom from affectation of calcium forms.(4) Planting P. crispus could remove overlying water TP effectually. Phosphorus was absorbed not only by root but also by stems and leaves. With P. crispus existence, the concentration of soluble phosphorus in overlying water was relatively stable, and the removal phosphorus was general from particle state, in its seeding stage P. crispus had the stronger removal effect of phosphorus, with senescing of P. crispus, the ecological benefit decreased accordingly. There was remarkable removal effect of overlying TP by adding calcium to sediment, the joint action of planting P. crispus and sediment added with calcium had the most removal efficiency. The soluble phosphorus of overlying water was directly impacted by TP of pore water.(5) Under the condition of sediment adding calcium, planting P. crispus could increase the concentration of Ca2+in overlying water, while under the condition of sediment without adding calcium, what happened was opposite, the concentration of Ca2+ in overlying water decreased. It was due to the adsorption of Ca2+ by P. crispus, only when the concentration of calcium came to a certain level, P. crispus could transport Ca+ to overlying water. The concentration of Ca2+in overlying water was affected by the concentration of Ca2+ in pore water, higher the concentration of Ca2+ in pore water was, more slowly the concentration of Ca2+ in overlying water decreased. There was a significant positive correlation between the concentration of overlying water Ca2+ and overlying water TP,TSP and pore water TP.(6) Planting P. crispus could increase the content of total calcium in sediment, with culture period going, the increment was more significant. Experiment indicated at the presence of P. crispus co-precipitation of CaCO3-P was observed, this because planting P. crispus could increase pH of overlying water and stimulate the co-precipitation. Sediment added with calcium could increase the concentration of Ca2+and initial Ca/P, thus speeding up the crystallization process. The co-precipitation rate of CaCO3-P was increased by the joint action of planting P. crispus and sediment added with calcium, furtherly the concentration of overlying water phosphorus was decreased.(7) Both planting P. crispus and sediment added with calcium could decrease the content of chlorophyll a, as time went on, the inhibition of algae began to reduce, while under the condition of sediment adding calcium the algae inhibition of planting P. crispus was still significant. P. crispus inhibited chlorophyll a by reducing phosphorus of overlying water, and sediment adding calcium inhibited chlorophyll a by co-precipitation of CaCO3-P and inhibiting phosphorus of sediment to release to overlying water.
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