| After the completion of the Three Gorges Dam,a hydro-fluctuation belt with a vertical drop of30 m and an area of about 350 km~2 has been formed.Because of the water level regulation mode of the�Winter Storage Summer Drainage�,the original vegetation community in the reservoir area has gradually disappeared due to the inability to endure high-intensity,long-term and off-season flooding,and a large number of annual herbs has become the most important plant types in hydro-fluctuation belt.Typical annual herbs,e.g.,Digitaria sanguinalis,Echinochloa crusgali,Bidens tripartita and Xanthium sibiricum will be transformed into litter which is easy to be decomposed at the end of growing season.Meantime,during the water-level and fall period of the Three Gorges Reservoir,a large amount of sediment carried by the river will directly bury these herbaceous litter,and the average thickness of sediment deposition and burial is about 10 cm.During the flooding period,the decomposition and nutrient content of litter of four annual herbs will be affected.The inorganic nutrients released by the litter during decomposition may enter the water and soil,and thus change the nutritional status of the water and soil.Therefore,the effect of different sediment depths on ground litter decomposition and nutrient dynamic characteristics of four annual dominant herbaceous plants in the water-level fluctuation zone of the Three Gorges Reservoir was investigated using the litter-bag method.Three sediment depths,namely 0 cm,5 cm and 10 cm,were treated with an experimental period of 180 days.The changes of nutrient contents in the litter,water and soil relevant to four annual herbs during decomposition were studied.The results showed that:(1)Different sediment depth treatments can significantly affect the decomposition of four annual herbaceous species litter.During the experiment period,the decomposition rate of four annual herbaceous species litter was fast at first and then became slow.Compared with that in 0 cm sediment depth treatment,the decomposition rate significantly decreased in 5 cm and 10 cm,indicating that the dry mass loss of litter can be significantly inhibited by sediment burial.(2)When the decomposition ended,the carbon content of four annual herb species litter remained stable,the phosphorus element released,and the nitrogen element was enriched under sediment burial.Except for changes in phosphorus content of D.sanguinalis and E.crusgali litter,the content of carbon,nitrogen and phosphorus of four annual herb species litter in 0 cm was significantly lower than in 5 cm and 10 cm treatment groups,respectively.On the contrary,the potassium content of four annual herbs species litter in 0 cm was significantly higher than in 5 cm and 10 cm treatment groups,respectively.(3)During the decomposition of litter of four annual herb species,the oxidation-reduction potential and the pH value of soil in 0 cm was significantly higher than in 5 cm and 10 cm treatment groups.With the extension of decomposition time,the pH value of the soil in 5 cm and 10 cm treatment group changed from alkaline to acid.There were no significant differences the content of total carbon,total nitrogen,total phosphorus and total potassium in the soil of four annual herb species litter among all the groups,but by the end of the experiment,the total carbon content of the soil was higher than the initial value.However,there was no significant difference between the content of total nitrogen,total phosphorus and total potassium in the soil and their corresponding initial value,respectively.D.sanguinalis and E.crusgali in each buried depth treatment group the soil nitrate nitrogen content was no significantly,and B.tripartita and X.sibiricum in same buried depth treatment the soil nitrate nitrogen content decreased with the increase of decomposition time.Four annual herb species at the end of decomposition,the contents of ammonium nitrogen in the soil was higher than their corresponding initial value.On the contrary,the contents of available phosphorus and available potassium in the soil were lower than their corresponding initial value,respectively.(4)The decomposition of litter of four annual herb species significantly reduced the oxidation-reduction potential and dissolved oxygen of water,but increased the conductivity.Four annual herb species decomposition the initial stage,the content of nitrogen and phosphorus in the water body reached the maximum.With the extension of decomposition time,the content of total phosphorus in the water body gradually decreased and tended to reach the water initial content of total phosphorus,but the content of total nitrogen first decreased and then increased.By the end of experiment,four annual herb species the total nitrogen content of water body was higher than its initial value,and the total nitrogen reached the eutrophication level of water body.Therefore,it is suggested that litter decomposition of four annual herb species will have negative effect on the water environment.In summary,a large amount of sediment deposition inhibits the decomposition rate and the release of nutrients of four annual herbaceous species litter,which is beneficial to reduce the contribution of litter decomposition to water eutrophication and nutrient suspension in the Three Gorges Reservoir,and to a certain extent,it has a positive effect on water quality protection.The oxidation-reduction potential and the pH value of surface soil are significantly different from that of the soil with buried litter in the water-level fluctuation zone of Three Gorges Reservoir,and thus the decomposition of litter has a greater impact on available nutrients of the soil.However,when the experiment ended,the total nitrogen content in the water body increased,while the total phosphorus content in the water body was close to its initial value.It can be inferred that the nitrogen released by the decomposition of surface litter enters the water body through leaching,which has a negative effect on the water environment.In order to reduce the potential risk of annual herb species litter decomposition to water eutrophication in the Three Gorges Reservoir area,the aboveground litter should be harvested and/or buried before impoundment. |
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