Release Of Nitrogen And Phosphorus From Cynodon Dactylon (L.) Pers.and Its Effect On Cadmium Migration In Sediments During Submergence In Water-level Fluctuation Zone

Periodic water storage in the Three Gorges Reservoir area affects the soil environment in the water-fluctuating zone,and also leads to plant decay and decomposition,increasing the risk of releasing nitrogen,phosphorus,heavy metals and other substances to the water body.At the same time,the change of sediment environment may affect the form of Cd in sediments,thus affecting the migration ability of Cd in sediments to water environment.Therefore,studying the characteristics of nitrogen,phosphorus and heavy metals released by plants in the water-fluctuating zone and clarifying the material migration and exchange process between water,plants and soil can provide scientific basis for the environmental effects of the water-fluctuating zone and formulate suitable management system of the water-fluctuating zone in the reservoir area.In this paper,the ecological environment of Xiangxi Bay was investigated,and the dominant plant Cynodon dactylon in the water-level-fluctuating zone was selected for flooding simulation experiment.The release characteristics of nitrogen,phosphorus and Cd during Cynodon dactylon decay were discussed.The influence of Cynodon dactylon decay on the overlying water and sediment was analyzed.The influencing factors of Cd morphological change were analyzed,and the contribution of N and P release from Cynodon dactylon decay to water body was calculated.This paper mainly obtains the following conclusions:(1)The results of soil monitoring at 165 m elevation in 5 years show that:From 2017to 2021,the TN content in soil did not change significantly after the impoundment period,and the fluctuation range of TN content was 334.08～936.99 mg/kg.TP content was significantly different in different years and seasons.The phosphorus element in the soil of the water-level-fluctuating zone was easily affected by the change of the surrounding environment,and the main fluctuation range was 248.92～938.03 mg/kg.Submerged-desiccation had a significant impact on the TP content in the soil.The variation of TN and TP contents in different spaces in the same season is mainly moderate variability,which indicates that the distribution of N and phosphorus contents in the water-level fluctuation zone is relatively concentrated and is greatly affected by human activities.The mean Cd content varied from 0.120 mg/kg to 0.405mg/kg.In the same season,the variation of Cd content in the soil of different spatial water-level fluctuation zones was weak,indicating that the distribution of Cd in the soil of the water-level fluctuation zone was relatively balanced.The annual or perennial herbaceous plants growing in the Xiangxi River water-level-fluctuating zone are mainly,among which Cynodon dactylon(L.)Pers.,Xanthium sibiricum Patrin ex Widder,Cyperus rotundus L.and Bidens pilosa L.are widely distributed.Cynodon dactylon(L.)Pers.has the largest biomass and density,which is the largest dominant species in the Xiangxi River.It has the strongest enrichment ability for Cd,the weakest enrichment ability for Cr,and the strongest migration ability for Cr in plants.(2)During the decay process of Cynodon dactylon(L.)Pers.,the release of nitrogen from the overlying water was obvious,mainly in the insoluble state.The TN content in the overlying water fluctuated greatly in the early stage,and decreased and tended to be stable in the late stage.The release rate of TP from Cynodon dactylon(L.)Pers.decay at the early stage of flooding was higher than that at the late stage,Phosphorus released by TDP primarily.Soil has obvious adsorption to TDP released from decay of Cynodon dactylon(L.)Pers.into water.(3)In the study area of 668571.61～1692882.75 m~2of the fluctuating zone,The amount of nitrogen and phosphorus released by Cynodon dactylon(L.)Pers.were 94.59～239.52 kg and 89.69～227.09 kg,respectively.The amount of nitrogen entering water was 60.61～153.47 kg,and the amount of phosphorus was 58.85～149.01 kg.The amount of nitrogen adsorbed by sediment was 33.98-86.05 kg,and the amount of phosphorus was 30.84-78.09 kg.(4)DOM released from Cynodon dactylon(L.)Pers.as a carrier of heavy metals,to a certain extent,reduces the adsorption of Cd in soil on water,and strengthens the precipitation of Cd in soil.The Fe-Mn oxide bound Cd in soil will transform to the exchangeable Cd,and the amount of transformation increases with the increase of flooding time.The risk of Cd release from soil to water increased with the increase of flooding time.The soil phosphorus content is less than 557 mg/kg,and the passivation effect is the most obvious with the increase of soil phosphorus content.