| In China,sewage sludge(SS)production is huge,and disposing SS has become a significant challenge.Application of SS as fertilizer or soil amendment in forestland has promising potential.However,heavy metals(HMs)in SS can accumulate in soil or leach into groundwater during this process,leading to the possible contamination of soil or water.At present,there are relatively few studies on the leaching characteristics and environmental effects of HMs during SS forestland application.Based on the possible migration and transformation pathways of HMs(including plant uptake,soil residue,and rain leaching)in the forestland ecosystem and two important factors which affect the leaching and migration of HMs in the forest environment in South China(forest litter and acid rain),this paper studied on:(1)the leaching and migration characteristics and potential ecological risks of HMs(Cu,Zn,Pb,Cd,and Ni)in soil when SS was applied in Eucalyptus urophylla S.T.Blake plantation,Schima superba Gardn.et Champ.plantation and Pinus elliottii plantation,respectively.(2)the migration characteristics of Cu,Zn,Pb,Cd,Ni and their potential impacts on groundwater and environmental risks under different SS application rates through indoor soil column leaching experiments.(3)the effects of different SS application rates on the growth and HMs accumulation in Eucalyptus urophylla,Schima superba and Pinus elliottii seedlings by pot experiments;(4)the effects of litter decomposition on the contents and forms of HMs in lateritic red soil mixed with SS by litter indoor decomposition experiments;(5)the effects of acid rain leaching with different intensities on the migration characteristics and ecological risks of Cu,Zn,Pb,Cd and Ni in lateritic red soil mixed with SS through indoor simulated acid rain leaching experiments.The purpose of this study was to investigate the migration and transformation of HMs in soil-plant-soil water system and the effects of litter decomposition and acid rain leaching on the migration and transformation characteristics of HMs and to provide theoretical basis for the safe forestland application of SS.The main results are as follows:(1)The results of field sludge application in forestland showed that one-time application of sludge at 30 t·ha-1(dry weight)in Eucalyptus urophylla plantation,Schima superba plantation and Pinus elliottii plantation,could significantly improve the physical and chemical properties of soil and increase the contents of HMs,such as Cu,Zn,Pb,Cd and Ni,in surface soil.Compared with the control,there were no significant differences in the contents and forms of HMs in 40?50 cm and 80?90cm layers.After three years of SS application,the content of Cd in the surface soil of Eucalyptus plantation was still significantly higher than that of the control,but the content of Cu,Zn,Pb,and Ni showed no significant differences.The contents of Pb and Ni in the soil of Pinus elliottii plantation and Schima superba plantation had no significant differences with that of the control.For these two plantations,some Zn migrated from surface soil to the 40?50 cm layer,and 50%Cu and 60%Cd remained in the surface soil.No significant change was showed in the contents of HMs in the 80?90 cm layer.The contents of exchangeable HMs in surface soil decreasedover time.The forms of HMs showed no significant change in 40-50 cm and80?90 cm soil layers.Hakanson potential ecological hazard assessment showed that the ecological risk of HMs was mainly appeared in the initial stage of forestland application.With time of SS application going on,the ecological risk gradually decreased.At the end of the third year of SS application,soil showed slight ecological risk in Zn,Cu,Pb and Ni and medium ecological risk in Cd.(2)The results of indoor soil column leaching experiments showed that SS application rate?45%(mass ratio)could lead to a significant increase in HMs concentration in leachate at the initial stage of leaching,but the peak concentration of HMs was lower than the standard III of Groundwater Environmental Quality Standard.The release characteristics of HMs fitted well with the modified Elovich equation(Y=a ln(X)+b).After the rain leaching commensurate with the three-year precipitation in Guangzhou,0.08%?0.2%Cu,0.14%?0.5%Zn,0.06%?0.09%Pb,4.81%?6.11%Cd and 0.08-13%Ni in soil column mixed with SS were leached out.The obvious deposition of Pb,Cu,Cd and Ni could be observed at 40 cm depth of soil column,while Zn could migrate and be deposited to 60 cm depth.When SS application rate was?45%,Zn could migrate to 80 cm depth.As for the studied lateritic red soil,the ecological risk was weak when the SS application rate was less than 30%.With the passage of leaching time,the ecological risk of HMs was gradually shifted from the surface layer to the 20-40 cm layer,and the ecological risk of Cd still exceeded the critical value of safety level when SS application rate was?45%.(3)Pot experiments lasting 12 months showed that SS application could significantly improve the physical properties and fertility as well as the contents of Cu,Zn,Pb,Cd and Ni in the substrate.Appropriate SS application could significantly increase plant height,ground diameter and biomass of Eucalyptus urophylla seedlings and Pinus elliottii seedlings.The optimal SS application rate was 45%and 75%(mass ratio)for Eucalyptus urophylla and Pinus elliottii,respectively.SS application significantly inhibited the growth of Schima superba seedlings,with plant height,ground diameter and biomass significantly lower than those of the control.HMs contents in seedlings increased significantly under higher proportion of SS application,but they had no significant change in<30%SS application.At the end of pot experiments,0.02%?0.87%Cu,0.01%?0.80%Zn,0.04%?0.21%PB,0.09%?1.46%Cd and 0.08%?0.60%Ni were absorbed by plants.The contents of HMs in the matrix of planting Eucalyptus urophylla or Pinus elliottii decreased differently,showing Eucalyptus urophylla or Pinus elliottii planting could mitigate the ecological risk of HMs at a certain level.Considering plant growth and HM ecological risk,the appropriate sludge application rate for Eucalyptus urophylla or Pinus elliottii planting is about 30%.(4)Results from litter decomposition experiments showed that decomposition of Eucalyptus urophylla,Schima superba and Pinus elliottii litters could decrease the p H value of lateritic red soil.Compared with treatment without litter being added,decomposition of Eucalyptus urophylla and Schima superba litters increased Cu and Zn contents in soil,and Pinus elliottii litters decomposition increased Ni and Zn contents.Moreover,these litters decomposition increased the contents of exchangeable Cu,Zn,Cd,and Ni,and decreased their residual form contents.The distribution exponent of the forms of Cu,Cd and Ni in the differently treated lateritic red soil slightly changed with the decomposition time of litter in a positive way,indicating the overall forms of these three elements were shifted towards more stable directions.The order of the overall stability of these HMs was Ni>Cu>Pb>Zn>Cd.(5)The leaching experiments of simulated acid rain in laboratory showed that the intensive acid rain of p H 2.0 significantly promoted the leaching and migration of HMs in sludge.After the simulated acid rain leaching commensurate with the three-year precipitation in Guangzhou,0.14%?0.27%Cu,0.15%?0.31%Zn,0.10%?0.21%Pb,6.27%?12.49%Cd and 0.12%?0.21%Ni in soil column with SS application were leached out.The highest concentrations of these HMs did not exceed the Groundwater Environmental Quality Standard,indicating HMs leached from SS may not cause a pollution risk to the groundwater in the short term.Lower p H in simulated acid rain caused higher proportion of exchangeable HMs in SS.The migration distance of HMs in the soil column was not affected by p H value in simulated acid rain.Most of HMs in SS remained in surface soil.Taken together,when the amount of SS application is strictly controlled,no ecological risk to groundwater caused by SS forestland application was found during the study period.The potential ecological risk to soil was mainly occurred in the early stage of application,and Cd was the major pollutant.Both plant species and application rate are important factors for safe forestland application of SS.It is therefore suggested these two factors should be screened before a large area of forestland application of SS.In addition,the decomposition of forest litter increased the solubility of some HMs,and high-intensity acid rain increased the leaching and migration of HMs from sludge,but no ecological risk was found.This paper has some limitations in terms of research cycle,research system,soil types and tree species.In the future,more in-depth studies should be carried out in combination with indoor and field experiments to better explore the chemical behavior and ecological risk of HMs in the process of forestland application of SS. |
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