Effects Of Long-term Fertilization On Heavy Metals And Their Binding Mechanism In Soils

Fertilization practices play an important role in agricultural production and the improvement of agricultural production,which mainly depends on the supply of soil fertility.However,the improper fertilization practices can lead to an imbalanced soil structure,accumulation of heavy metals and soil pollution.Therefore,to study the effect of long-term fertilization on agriculture soil health has great significance on guiding agricultural production,protecting the safety of agricultural products and controlling soil pollution.In this study,four fertilization treatments in several long-term fertilization experiments?>20 years?were selected,including control?no fertilizer?,NPK?chemical fertilizer?,NPKM?chemical fertilizer plus organic fertilizer?and M?organic fertilizer?.The total content and bioavailability content of five elements Cu,Zn,Cd,Ni and Pb were determined.Based on the results of total content and bioavaibility,the complexing mechanism was further explored with the titration of Cu,Zn and Cd to soil dissolved organic matter from the NPK and M treatments,using Fourier transform infrared spectrometer?FTIR?,²⁷Al and ²⁹Si NMR combined with two dimensional correlation analysis?2D COS?.The main results are as follows.1.As for commercial organic fertilizers?97 samples?collected from different sites throughout China,most of total content of Cu,Zn,Cd,Pb and Ni were in their normal range.Compared to the National Organic Fertilizer Standard,these commercial organic fertilizers contained several trace elements and most of them were meeting national security standard.2.Soil samples from six long-term fertilization experiments were wet-digested with a concentrated acid mixture?HNO₃-HClO4-HF?.The results showed that except for Xinjiang experiment,the total content of Cu,Zn and Cd in soils from the M treatment were significantly increased than those from control and NPK treatments.Specifically,the total content of Cu,Zn,Cd in soil increased 21.32mg/kg,35.13mg/kg and 0.95mg/kg,respectively at Shenyang Agricultural University experiment;59.15mg/kg,71.40mg/kg and 0.74mg/kg at Hunan Qiyang experiment;14.81mg/kg,106.01mg/kgand 0.28mg/kg at Jiangxi Jinxian experiment;9.76mg/kg,20.93mg/kg and 0.22mg/kg at Jiangxi Yingtan experiment;16.17mg/kg,30.65mg/kg and 0.96mg/kg at Jilin Gongzhuling experiment.The total content of Pb and Ni were almost unaffected by fertilization treatment.In addition,the total content of Cu in soil from the M treatment at both Hunan and Jiangxi Jinxian experiments exceeded the national standard.Moreover,the total content of Cd exceeded the national standard in the M treatment at five long-term fertilization experiments,except for Xinjiang experiment.3.The bioavailability of heavy metals in soil from long-term fertilization experiments was detected by diffusive gradients in thin films?DGT?.The result showed that the bioavailability of Zn,Pb and Ni in the soil from NPK treatment was significantly increased when compared to M treatment at Shenyang,Hunan and Jiangxi jinxian experiments.The bioavailability of Zn,Ni and Pb in the soil from NPK treatment was higher than that from NPKM treatment.Meanwhile,the bioavailability of Cd in the soil from NPKM treatment was higher than NPK treatment.In addition to Pb and Ni in Jiangxi yingtan experiment,other bioavailability elements were significantly higher in the soil from M treatment than those from NPK treatment.Cu,Zn and Cd in the soils from M treatment increased 1.42?g/L,41.71?g/L and 2.46?g/L,respectively.Availability elements in Xinjiang were lower than other long-term fertilization site.4.The results of FTIR,²⁷Al,and ²⁹Si NMR combined with 2D COS analysis showed that heavy metals were mainly bound with the functional group of nano minerals and almost did not bind with soluble proteins in the soil DOM.With the titration of heavy metals to soil DOM from the NPK and M treatments,Si-O(1050^-1000cm^-1)and Si-O-Al(915cm^-1)functional groups played a predominant role in the binding process.Moreover,²⁷Al and ²⁹Si NMR also proved direct evidence that the IR range at 1050^-1000cm^-1 belonged to Si-O functional group;however,due to without corresponding IR absorption peak at the region of 915cm^-1,the Al-containing functional groups did not exist in this region.