Vegetation Nitrogen,Phosphorus And Potassium Nutrient Loss In Water Level Fluctuation Zone Of The Three Gorges Reservoir Based On Submergence-caused Death

After the running of the Three Gorges Dam,the ecosystem of the reservoir changes dramatically in the fluctuation area.The vegetation is an important component of the ecosystem in the water level fluctuation area.The existence of vegetation has positive effects on the ecological environment protection in the water level fluctuation area.In the emergence stage,plants absorb a large number of nitrogen,phosphorus and potassium nutrient from the soil for their development and growth in the water level fluctuation area.However,a lot of plant residues are lost during impounding,so that the nitrogen,phosphorus and potassium nutrient of plants lost in the river,resulting in the loss of lots of nitrogen,phosphorus and potassium nutrient in the soil in this area.In order to explore the vegetation nitrogen,phosphorus and potassium nutrient loss based on flooding death in water level fluctuation zone of the Three Gorges Reservoir,this study investigated the vegetation growth and nitrogen,phosphorus and potassium nutrient content in plants in the typical water level fluctuation area of the Three Gorges Reservoir from August 2017 to May 2018.The main results are as follows:1)Before the impoundment of the Three Gorges Reservoir,there were 50 species belonging to 43 genera and 20 families.The species number increased with increasing of elevation,and there were significant differences in the number of plant species among different altitude regions(p < 0.05).The annual plants dominated the water level fluctuation area.After the emergence from water,there were only 5 families,7 genera and 8 species in this area.There were significant differences of species number before and after impounding(p < 0.05),and perennial plants still survived after impounding,while only some of the dead parts of the annual plants found in the water level fluctuation area.2)Under the mixed influences of impounding stress and allelopathic effect,the distribution pattern of plant communities in the outcropping period showed a single peak growth trend with the increase of elevation in the water level fluctuation area of the three Gorges Reservoir.Perennial plants dominated the 152 m,157 m and 162 m elevation areas,while annual plants dominated the 167 m and 172 m elevation areas.After impounding,the surviving stems and dead stems of perennial plants and the dead stems of annual plants were still retained,and the leaf biomass of plants were totally lost in this area.During the period of submergence,the loss biomass of plants showed a single peak trend with the increase of elevation.The loss biomass of perennial plants was significantly greater than that of annual plants(p < 0.05).3)The nitrogen,potassium and phosphorus contents in plants growing on the fluctuation area of the Three Gorges Reservoir were consistent: K > N > P.The nitrogen content in leaves was significantly higher than that in stems(p < 0.05),while the potassium and phosphorus contents were no significant differences between stems and leaves(p > 0.05).The nitrogen-phosphorus ratio of the plants was less than 10 which grew in the water level fluctuation area,it meant the growth of plants was mainly restricted by nitrogen in soil,moreover the differential demand for nitrogen also affected the distribution of plants in studied area to some extent.During impounding,perennial plants consumed a large amount of carbohydrates stored in stems,while the total amount of nitrogen and phosphorus changed little.All these changes resulted that there were higher content of nitrogen and phosphorus in survival stems than that before impounding.The decrease of potassium content should be caused by the transfer of a large amount of potassium ions to the root system.However,the contents of potassium and phosphorus in the stems of dead annual plants decreased significantly due to the dissolution into surrounding and the decomposition by microorganisms.4)The total nitrogen,phosphorus and potassium nutrient loss of nitrogen,potassium and phosphorus caused by plants flooding death increased firstly and then decreased with the elevation in the water level fluctuation area of the Three Gorges Reservoir.The nitrogen,phosphorus and potassium nutrient loss caused by perennial plants was higher than that of annual plants in the 152 m,157 m,and 162 m elevation areas.The nitrogen,phosphorus and potassium nutrient loss caused by annual plants was higher than that of perennial plants in the 167 m and 172 m elevation areas.In summary,the amount of nitrogen,phosphorus and potassium nutrient loss caused by perennial plants was significantly higher than that by annual plants in the studied area(p < 0.05).The difference in mineral content of the plants led to the total loss of mineral nutrient caused by plant growth in the water level fluctuation area,which was shown as K > N > P.In general,the vegetation growth caused a large loss of nitrogen,phosphorus and potassium nutrient and mainly occurred at 162 m elevation and below in the water level fluctuation area of the Three Gorges Reservoir.The nitrogen,phosphorus and potassium nutrient loss caused by perennial plant growth was higher than that by annual plants in the water level fluctuation area.