Stabilization Of Soil Organic Carbon By Iron Oxides And Calcium Ions And The Effect Of Organic Matter On The Fraction Of Pb

Soil organic matter?SOM?contributes the largest organic carbon?OC?pool of the global terrestrial ecosystems,and it closely correlates with soil fertility and global carbon cycles.The ubiquitously iron oxides and calcium ions play an important role in regulating the accumulation and stability of OC in soils.In addition,the transformation of SOM has a significant effect on the migration and conversion of contamination elements in the soil.Therefore,typically agricultural soil samples at the east and central China were collected,traditional chemical extraction,stable isotope,and synchrotron radiation-based Fourier transform infrared?SR-FTIR?spectromicroscopy methods were adopted to analyze the regularities of distribution and the molecular composition of iron oxide-bound OC?Fe-bound OC?in arable land.Meanwhile,chemical analysis,Fourier transform infrared?FTIR?spectroscopy,ultraviolet spectrum,and near-edge X-ray absorption fine structure?NEXAFS?were used to investigate the contribution of varying crystallinity iron-bearing mineral and calcium ions on the accumulation of soil organic carbon?SOC?,and the effect of long-term fertilization on the stabilization of SOC by iron oxides and calcium ions.In addition,a Pb-contaminated soil was incubated with rice straw for two-years.A sequential chemical extraction and X-ray absorption fine structure?XAFS?spectroscopy were employed to illuminate the responses of the distribution of Pb in the soil to the interaction of soil components,and reveal the dynamic change of chemical and molecular forms of Pb under long time scale.The main results were represented as follows:1)Our study provided a systematic evaluation for the Fe-bound OC in China arable lands,the effects of ecological factors on the occurrence of Fe-bound OC,and the molecular composition and biological sources of Fe-bound OC.The Fe-bound OC contributed to 6.2³1.2%of SOC in agricultural soil,and is an important component of C cycle in the soil system.Surface adsorption is the dominant mechanism of binding OC with iron oxides in agricultural soils,and coprecipitation also plays an important role in soils with a large SOC content.The distribution of Fe-bound OC showed no clear variation in relation to the site,but Fe-bound OC reached a peak in soils with an annual mean temperature of 16.4°C.Correlation analysis demonstrated that the binding of Fe-bound OC was influenced by the total organic carbon?TOC?and the active Fe ratio.Carbon-nitrogen ratio analyze combined with ¹³C isotope analyze and SR-FTIR suggested that iron oxides preferentially protected plant-derived polysaccharides and aliphatic compounds in agricultural soil.2)The contribution of varying crystallinity iron-bearing mineral on the accumulation of soil organic carbon?SOC?,and the effect of long-term fertilization on the stabilization of SOC by iron oxides were investigated.The concentrations of iron oxide-bound OC in the Red soil followed the order of noncrystalline colloidal Fe bound OC(OC_PP)>crystalline Fe oxides bound OC(OC_DH)>noncrystalline hydrated Fe oxides bound OC(OC_HH).Surface adsorption is the dominant mechanism of binding OC with noncrystalline hydrated Fe oxides and crystalline Fe oxides,but the association of OC_PPP was through coprecipitation or chelate.In this soil,OC_PP and OC_HHH were sensitive to long-term fertilization,but the long-term fertilization has little influence on OC_DH.The chemical fertilizer?NPK?treatment significantly increased the binding of OC with noncrystalline colloidal Fe and noncrystalline hydrated Fe oxides,and the organic fertilizer?M?treatment notably increased the association of OC with noncrystalline hydrated Fe oxides.The average molecular weight and degree of aromatization of OC_HHH were higher than OC_DH.The NPK treatment significantly increased the hydrophobic and aromatic of OC_PP,and the average molecular weight of OC_DH.The M treatment remarkably reduced the average molecular weight of OC_PP,and the hydrophobic and aromatic of OC_PP and OC_HH.3)The effects of calcium ions on the accumulation of SOC were elucidated.Approximately 4.55¹9.28%and 9.00²5.15%of SOC was represented as Ca-bonded OC in the Red soil and fluvo-aquic soil,respectively.The NPK treatment significantly decreased the amounts of Ca-bonded OC.The M treatment notably increased the concentrations of Ca-bonded OC in the fluvo-aquic soil but has an insignificant effect on the amounts of Ca-bonded OC in the Red soil.Comprehensive studies of FTIR and NEXAFS showed that calcium selectively protected aromatic C and carboxylic C.The M treatment increased the binding of O-alkyl C with calcium in the soil.In all the soils examined except for the M treated fluvo-aquic soil,¹³C was relatively enrichment in Ca-bonded OC compared with the TOC.The NPK treatment promoted the enrichment of¹³C in Ca-bonded OC,but M treatment facilitated the relative depletion of ¹³C in Ca-bonded OC.4)The dynamic changes for the chemical fractions of Pb caused by the interaction of soil components on a long-term scale were obtained.In our examinated Pb-contaminated soil,Pb was mainly presented as Fe oxide-associated,surface adsorbed and residual fraction.The incorporation of rice straw can effectively reduce the labile species?sum of exchangeable and surface adsorbed fractions?of Pb in a subtropical contaminated soil by increasing the Fe oxide-bound Pb.We demonstrate that long-term aging enhances the transfer of Pb with Fe oxide components and this transformation is promoted by the presence of organic matter.Organic matter input and soil aging tend to shift Pb to amorphous Fe oxides than crystalline Fe oxides.The incubation of rice straw increased the Fe oxide-bound Pb may be attributed to the form of Fe-OC-Pb and Fe-Pb-OC.5)The responses of the molecular forms of Pb in the soil to the interaction of soil components were revealed.The XAFS result shown that Pb mainly consisted of Fe oxide-bound Pb,hydroxypyromorphite and montmorillonite-sorbed Pb in our sample soil.The incubation of rice straw and soil aging increased the transformation of hydroxypyromorphite and montmorillonite-sorbed Pb to Fe oxide-bound Pb.Iron oxide-bound OC plays an important role in the forming of Fe oxide-associated Pb.