Evaluation Of Critical Load Of Heavy Metals In Farmland Ecosystem Based On Steady-state Model Of Soil Mineral Weatherin

The environmental capacity of heavy metals in agroecosystems is the basis for sustainable agricultural development,but because the input and output processes of heavy metals are complex and involve physicochemical processes in some contaminated soils,quantitative assessment can effectively understand the input and output ratios of heavy metals.At the same time,understanding the input threshold of heavy metals to soil,which will not cause problems such as soil degradation and threatening food safety levels,is also important for soil pollution control,agricultural soil management and sustainable use of land resources.Based on the PROFILE model,we quantified the weathering rates of soil and the releasing rates of heavy metals of weathering leaching in Yangtze River Delta.And evaluated the input and output fluxes of heavy metals and the potential ecological hazard index of heavy metals.The long-term changes of critical loads,concentrations and potential ecological hazard indices of heavy metals were predicted in soils of the study area.The following conclusions were obtained:（1）The mean values of weathering leaching rates（R_W）and based cations(Ca²⁺,Mg²⁺,Na⁺and K⁺)releasing rates(R_BC)of soils in the Yangtze River Delta were 1.11and 0.97 keq/（ha·a）,respectively.The mineral abundance,soil physicochemical properties was determined the R_W and R_BC strength.The average release rates of As,Cd,Cr,Cu,Hg,Pb and Zn from soils in the study area were 4.87,0.20,31.33,19.99,0.08,18.7 and 42.77 g/（ha·a）,respectively.The release rates of heavy metals were significantly correlated not only with the salt-based cation concentration,heavy metal concentration,and R_BC,but also with the pH,CaO,C_org.and TFe₂O₃of the soil.（2）the input pathways of Cd,Cu,Pb,Zn,Cr,and Hg from high to low were as follows:atmospheric deposition（49.82%～94.72%）>irrigation（5.06～49.09%）>fertilizer（0.22～1.75%）in the agricultural ecosystem of Yangtze River Delta;The input pathways of As from high to low were:irrigation（62.37%）>atmospheric sedimentation（33.47%）>fertilizer（4.15%）.The output pathways of Hg,As,Cd,Cu,and Zn from high to low are as follows:straw absorption（78.08-68.69%）>weathering leaching（11.46-20.53%）>seed absorption（1.37-17.41%）;The output pathways of Pb and Cr from high to low are as follows:weathering leaching（63.58%,90.02%）>straw absorption（35.21%,8.61%）>seed absorption（1.20%,1.37%）.Atmospheric sedimentation was the main input pathway for heavy metals in agricultural soil,and the main output pathway for straw absorption and weathering leaching of heavy metals in soil.（3）The static critical loads（SCL）of heavy metals in paddy soils of the Yangtze River Delta were 0.84,20.52,3.21,273.22,156.74,61.40 and 179.13 kg/ha,respectively.The steady-state critical loads（SSCL）of heavy metals after 40 years,with Hg,As,Cd,Cr,Pb,Cu and Zn Cd,Cr,Pb,Cu,and Zn were 0.02,0.53,0.08,6.77,3.82,1.38,and 4.42 kg/ha,respectively.Comparing with the results of SCL,the critical loads of heavy metals were significantly lower after 40 years.The spatial distribution of the critical load of heavy metals was mainly influenced by the metal concentration（C_t）,bulk density（ρ）and specific surface area of minerals（A_w）of the surface soil.（4）Based on steady-state critical load（SSCL）model,metal concentration（C_t）increased with time in paddy soils and SSCL decreased exponentially with time.The mean value of potential ecological risk index of heavy metals of in Yangtze River Delta after 40 years was 42.26,which was at a mild ecological risk level.Cd of moderate ecological hazard level increased from 6.25%to 11.72%,and the range of high value areas was expanding in paddy soils.Overall,the potential ecological risk of heavy metals to farmland ecosystems has increased after 40 years.（5）The SSCL model was mainly influenced by the standard limits of heavy metals（C_l）,the release rate of salt-based cations(R_BC),and soil bulk density（ρ）.The R_BC had a significant linear correlation with pH.The critical load of heavy metals was significantly correlated with the release rate of salt-based cations and the standard limit value of heavy metals.Due to the effect of based cation release on pH and SSCL,it was shown by Monte Carlo simulation that appropriate increase of the standard limit value of heavy metals could effectively cope with the contradiction between pollutants and limited environmental capacity caused by economic development.