Effects Of Nitrogen Deposition On The Migration And Transformation Of Soil Heavy Metals In The Fluctuating Zone Of The Three Gorges Reservoir Area

The water-level of the Three Gorges Reservior(TGR)is periodically fluctuated since the TGR is running.And the water-level fluctuation zone(WLFZ)is formed between the highest water-level(175 m)and the lowest water-level(145 m)on the shore of the TGR.Besides,the basic properties of the WLFZ soil is changed due to the periodically fluctuation of the water-level.And the pollution of heavy metals(HMs)of the WLFZ soil is worsening and there are higher risks under the influence of high nitrogen load and precipitation.We take the WLFZ soil as the research objects in the Tanshao village,Wanzhou District and analysis the morphological distribution characteristics of the HMs(Cd,Cr,Cu,Pb and Zn)in the WLFZ soil through the indoor undisturbed soil column simulation experient and field in situ simulation experient.This can help us to build the relationships between the nitrogen deposition and the releasion,transformation and migration of the HMs in WLFZ soil and clarify the response relationship between the change of the basic properties in WLFZ soil and the soil HMs.Besides,when the nitrogen deposition increased,we can explore the regularity of the HMs in WLFZ soil.This research can provide basical theoretical information to the prevention of HMs pollution in the TGR and the protection of the ecological environment.There are some main conclusions:(1)Study on the morphological transformation of soil HMs in WFLZ by nitrogen deposition flux.When the nitrogen deposition flux ranged from 50 to 100 kg N·hm^-2·yr^-1,Cd,Cu,Pb and Zn in the soil could be significantly transformed from residual,organically bound and Fe-Mn oxide bound states to exchange and carbonate bound states,and the morphological conversion rate of Cd was the highest,with the conversion rate of 269.7%.However,the morphological change of Cr is less affected by nitrogen deposition flux.(2)Study on the morphological transformation of soil HMs by NH₄⁺-N:NO₃^--N ratio under fixed nitrogen deposition flux.When NH₄⁺-N:NO₃^--N were 1.25:1 and 5:1,the conversion of Cr,Pb,Cu and Zn was mainly from carbonate-bound and organically bound states to Fe-Mn oxide bound states,and the conversion rate of Pb was the highest,which was-143.7%.Cd significantly changed from residue states to exchangeable states and carbonate bound states.Among them,the conversion rate of the carbonate bound states in the outdoor simulation is the most significant,which is 235.4%.In the indoor simulation,the exchangeable states significantly increased to 352.3%.(3)Study on the migration and transformation Mechanism of soil HMs by nitrogen deposition.The increase of nitrogen deposition reduces the soil pH and causes soil acidification.When the simulation condition is between 50 and 75 kg N·hm^-2·yr^-1,the soil acidification degree is significant,and the pH decreases by 16%-40%(outdoor)and 4%-9%(indoor),which accelerates the leaching of HMs from the soil and improves the migration characteristics of HMs.The increase of nitrogen deposition flux increases the content of soil organic matter(SOM).The increase of nitrogen deposition flux will inhibit the decomposition of SOM,increase the activation trend of heavy metals in the soil,and increase the risk of heavy metals in the ecological environment.Nitrogen deposition can significantly increase the Cd migration and conversion capacity,and the migration rate is43.0%±0.18.The effects of nitrogen deposition flux on the migration rate of Cr,Cu,Pb and Zn are obviously different.With the increase of nitrogen deposition flux,the desorption rate of Cd,Cr and Zn and the diffusion rate of Cu and Pb in the soil were accelerated,and the migration and release capacity of soil HMs were enhanced.The change of NH₄⁺-N:NO₃^--N ratio had no significant effect on the migration and release of soil HMs.When the nitrogen deposition condition is 50 to 100 kg N·hm^-2·yr^-1,the soil comprehensive ecological pollution damage is the greatest,and the contribution rates of Cd and Cu are higher.