Binding Mechanisms Of Typical Heavy Metals In Red Soils After Long-term Fertilization Mangement

Fertilization practices play an essential role in the improvement of agricultural production,which provides an important guarantee for increasing agricultural production and incomes.However long-term improper fertilization practices can lead to a series of soil health problems.Excessive heavy metals in the red soil of south China can have a potential impact on human health.Therefore,it is significant to study the effect of long-term fertilization on the behavior mechanism of heavy metals in red soils to ensure the increase of agricultural production and also for food security.To ensure food security and the increase of agricultural production,recently,it has been discovered that the behavior of heavy metals in soil is mainly influenced by the soil ecosystem,rather than the characteristics of heavy metal themselves.Among them,the compound effect of mineral-organic matter in the soil is one of the primary mechanisms in the binding process of heavy metals.Yet the regulation of mineral components,organic matter and microorganisms in soil with heavy metals and the change of the behavior mechanism of heavy metals in soil due to fertilization remain unclear.In this paper,soils from a long-term fertilization site were used to explore the behavior mechanism of heavy metals in soils,combined with the traditional physical chemistry methods,the advanced synchrotron radiation technologies,electron probe micro-analysis(EPMA)and two dimensional correlation spectroscopy(2D-COS),to study the behavior mechanism of typical heavy metals(Cd and Cu)in soil after long-term different fertilization treatment.Meanwhile,the distribution and binding mechanism of typical heavy metals,mineral components,and various functional groups were investigated in situ by using the synchrotron radiation techniques and EPMA based on in situ culture experiments and microcosm studies.The main results were listed as followed:(1)The basic physical and chemical properties of soil were significantly changed after 23 years different fertilization treatments.Long-term different fertilization treatments significantly(P≤0.05)increased the concentration of SOC and SON compared with the control,and their order followed:M>NPKM>NPK>Control.Otherwise,compared with the control,the addition of manure(NPKM and M)significantly(P ≤ 0.05)increased soil pH,while long-term NPK application led to soil acidification.(2)The concentration of total Cd and Cu in soils were significantly(P ≤ 0.05)increased after 23 years of manure application treatments(NPKM and M).Specifically,the M treatment had the highest concentration of Cd(1.00 ± 0.05 mg/kg)and Cu(95±1.4 mg/kg)in the soil,about 3.3 times and 1.9 times higher than the national second standard.The DGT results indicated that,long-term M treatment significantly(P ≤ 0.05)increased the bioavailability of Cu(26±1.7 μg/L),while long-term NPK fertilization did not significantly(P ≤ 0.05)increased the total amount Cd and Cu in the soil,and also did not significantly(P ≤ 0.05)increased the bioavailability of Cu.Furthermore,long-term different fertilization can dramatically influence the concentration of Cd and Cu,and its biological accumulation coefficient(BAC)in crops.Long-term manure(NPKM and M)treatments significantly(P ≤ 0.05)decreased the BAC of crops,while the long-term NPK treatment significantly(P ≤ 0.05)increased the BAC.The results showed that organic fertilizer treatment had the potential to inhibit the absorption and enrichment of heavy metals from the soil by plants.(3)Different long-term fertilization treatments significantly influenced the concentration of DOC,mineral elements and anions in the soils.Compared with control,long-term manure(NPKM and M)application significantly(P ≤ 0.05)increased the concentration of DOC in the soil,and the sequence of the content changes was:M≈NPKM>NPK≈Control.The concentration of Fe,Al,and Si in the soil DOM solution was significantly increased after long-term manure(NPKM and M)application,and the organic fertilizer treatment significantly increased the concentration of anions NO3-and PO43-in the DOM solution,while the long-term NPK treatment significantly increased the concentration of Cl-and SO42-.(4)The 2D-COS combined with Fourier Transform Infrared(FTIR),27Al,29Si Nuclear Magnetic Resonance(NMR)and Fe K-edge XANES spectra can be used to assess the complexes of different ligands and their binding mechanism with Cd and Cu in soil DOM.The results showed that different fertilization modified the heavy metal binding properties of different functional groups,especially in both NPK and M treatments.Specially,aliphatic C-OH,polysaccharides(C-O)and silicates(Si-O)were bound to Cu and Cd in the NPK and M treatments,respectively.Otherwise,the Al-O and Fe-O functional groups also play an important role in the binding process of Cd and Cu in the M treated soil.(5)The analysis results of FTIR,27 Al,29Si NMR and Fe K-edge XANES spectra combined with 2DCOS showed that the FTIR band at 1030 cm-1(Si-O)was positively correlated with the NMR band at-91 ppm and the Si-O(1030 cm-1)stretching vibration of silicate had the same origin as silicon at-91 ppm.Al-O(900 and 570 cm-1)stretching vibration was positively correlated with the NMR band at 0 ppm.Also,Si-O(1030 cm-1)and Fe-O(470 cm-1)had the same origin with ferric oxalate or ferrihydrite.The results also showed that long-term organic fertilizer treatment could promote the formation of active Fe,Al and Si minerals in the soil and the complexation with Cd and Cu.(6)The in situ analysis techniques showed that mineral elements(Fe and Al),different functional groups,and heavy metals had a similar distribution at the microscale in the soil particles,while Si also had a similar distribution with heavy metals,but not as homogeneous as Fe and Al.This indicated that both Fe and Al of mineral elements had the effect on the complexation of these two typical heavy metals,and in situ distribution of Fe and Cd at the microscale is relatively strong,and the correlation coefficient is 0.605.When compared with the NPK treatment,NPKM and M treatment could increase the concentration of active minerals in the soil,especially the content of ferrihydrite,while long-term NPK treatment decreased the content of ferrihydrite in the soil and even eliminated it.The effects of different fertilization treatment on the total amount of active minerals in the soil were as followed:M≈NPKM>Control>NPK,the increase of the active minerals improved the adsorption of heavy metals and organic matter.(7)Based on the microcosm experiment results,the content of H2O2,HO and Fe(Ⅱ)in the soil were significantly changed under long-term application of NPK and M,and the content of H2O2,HO and Fe(Ⅱ)in the soil through different diameters were also significantly changed.Compared with NPK treatment,long-term M treatment significantly decreased the content of H2O2,HO and Fe(Ⅱ)in the soil.The content of HO’ in the soil through the 1000 μm diameter is generally lower than the other soils through different pore sizes,and the content of Fe(Ⅱ)in the soil is unchanged after through 1000 μm and 20 μm pore size.16s rRNA high-throughput sequencing results further indicate the soil bacterial community structure and diversity index with the M fertilization was significantly higher than that of NPK treated soil,the microbial abundance,and diversity index in the soil through 1000 μm pore size was also significantly higher than that through other pore size soils.It indicated that long-term M fertilization and root secretion would decrease the Fenton reaction;increasing the soil bacterial community structure and diversity index,and playing an important role in reducing the bioavailability of heavy metals in the soil.Synchrotron radiation based Fe-STXM-NEXAFS showed that the Fe redox state in the soil particles changed significantly after different fertilization treatment.The result of Fe K-edge XANES showed that the content of active minerals(especially ferrihydrite)was significantly increased by M fertilization treated,which can increase the adsorption and sequestration ability of heavy metals.It also demonstrated that the M treated soil has a higher potential for the sequestration of heavy metals.In summary,the results from long-term fertilization experiment and microcosm culture experiment showed that the behavior mechanisms of heavy metals were significantly affected by different long-term fertilization.Long-term different fertilization treatment could affect the SOC,mineral elements,and microbial community characteristics and further affected the bioavailability and ultimate fate of heavy metals in the soils.Organic fertilization treatment could increase the content of SOC and active minerals increased the sequestration of Cd and Cu,reduced the soil Fenton reaction,promoted the microbial community and diversity index,and then further mitigated the migration rate of heavy metals and decreased the bioavailability of them.