The Effect Of Submerged Macrophytes On Migration And Transformation Of Dissolved Inorganic Nitrogen In Shallow Lakes

Excessive introduction of nitrogen has happened in the shallow lakes in the YangtzeRiver region in China,such as Chaohu Lake and Taihu Lake, leading to eutrophication andthe outbreak of water bloom, which may eventually threaten the health and safety ofaquatic ecosystem.Submerged macrophytes are important agents regulating the cycling of nitrogen andphosphorus in the lake ecosystem due to their morphology, community structure andphysiology played in the surrounding environment. They impacted the aquatic productivityand biogeochemical process. Submerged macrophytes, in addition to the direct absorptionof nitrate nitrogen in the lake water, also promote denitrification indirectly in themicrocosm and supply habitat to microorganisms. DNRA and microbial immobilizationare also important pathway of nitrate nitrogen removal. These process all affect themigration and transformation of nitrate.In this study, the Yangtze River of submerged macrophytes were used as test material,the use of laboratory test simulations were carried out two tests, the first test to selectCeratophyllum demersum(C.demersum) and Vallisneria spiralis L.(V. spiralisL.), theyhave a greater morphological differences, Experimental study of the effects of twosubmerged macrophytes on dissolved inorganic nitrogen in overlying water and interstitialwater.The second test choice Myriophyllum spicatum(M. spicatum)to research, usingstable isotope tracer methods to in-depth discussionthe recyclingof dissolved inorganicnitrogen in shallow lakes.From the experiments the results suggested that：(1) The removal effect of DIN in the overlying water is better than that in interstitialwater by submerged macrophytes. The removal efficiency of DIN in overlying water andinterstitial water is NO2--N＞NH4+-N＞NO3--N. The removal rate of DIN by C. demersumis higher than that of V. Spiralis L. in overlying water, while the result is converse ininterstitial water. (2)C. Demersum and V. Spiralis L. decrease the diffusion flux of NH4+-N and NO2--N,and increase the diffusion flux of NO3--N significantly. Consequently, NO3--N replaceNH4+-N and become the main form of DIN, which diffuses from interstitial water tooverlying water. The impact of the diffusion flux of NO3--N between C. Demersum and V.SpiralisL. has no significant difference, and the result is same for NH4+-N.(3)C. Demersum and V. Spiralis L. increased the width of variation of three nitrogenforms to total DIN in overlying water and interstitial water, the influence on the ratio ofDIN by C. demersum is greater than that of V. spiralisL. in overlying water, while theresult is opposite in interstitial water. In general, C. Demersum has more influence in theoverlying water, V. Spiralis L. has more influence in interstitial water, while, the influenceof DIN diffusion flux is not significant.(4) In a twelve-day experiment the percentage of exogenous nitrate nitrogen removedby denitrification, microorganism, submerged plants, dissimilatory nitrate reduction toammonium (DNRA) and conversion to dissolvable organic nitrogen (DON) was47.54%,25.24%,12.76%,0.52%and1.21%, respectively, in the treatment group (planted group),while in the unplanted group, the percentage of exogenous nitrate nitrogen removed bydenitrification, microorganism, DNRA and DON was32.74%30.79%,0.54%and5.83%,respectively.(5) A total of87.24%and69.90%of the exogenous15N was transformed in plantedand unplanted groups, respectively, during the twelve-day experiment. According to ourfinding, denitrification is the main pathway for the removal of nitrate nitrogen, followedby microorganismimmobilization. M. spicatum also plays an important role in the removalof nitrate nitrogen, but the effects of DNRA and DON is relatively weak. M. spicatumpromotes denitrification, accelerates the migration and transformation of nitrate nitrogen,and directly or indirectly accelerates the removal of exogenous nitrate nitrogen in themicrocosm.From the above conclusions, different forms of submerged macrophytes have varyingdegrees of impact on the nitrogen concentration in the vertical direction in shallow lakes,which developed root system of V. Spiralis have more obvious impact on water. Thedifferent morphological and different metabolic approach of Submerged macrophytes willlead to different fluxes of dissolved inorganic nitrogen in the water-sediment interface. The removal of dissolved inorganic nitrogen by submerged macrophytes, In addition to itsown absorption, but also by the impact on the surrounding environment, increasing therate of denitrification to remove nitrogen. The selected test submerged macrophytes arecommon plant of the Yangtze River region, Can provide a reference for the study of thenitrogen cycle in the basin, and restore the ecological environment of the basin.