Effects Of Organic Materials On Dissolved Organic Matter In Purple Soil And Cadmium Accumulation In Rice

Organic materials are widely used in soil improvement and pollution remediation.The application of organic materials can change the organic matter components of the soil,while dissolved organic matter?DOM?is the most active part of soil organic carbon.Soil DOM has a small molecular weight and large migration,which plays an important role in soil carbon cycle,migration and transformation of pollutants and bioavailability.During the growth process of rice,most of them are under flooding conditions,and DOM is also the most active in the migration and availability of soil Cd.In this paper,the pot experiment was combined with laboratory test and analysis to investigate the effects of organic materials on soil dissolved organic matter and rice Cd accumulation.Firstly,the basic properties of organic materials were analyzed and characterized,and their effects on the adsorption properties of purple soil Cd were studied.The rice pot experiment was carried out in greenhouses from May 2017 to October 2017,and treated at different exogenous Cd concentrations.Under the influence of the control group?CK?and the application of pig manure compost?PM?,humus soil?HM?,sludge compost?CS?on the transport and accumulation of Cd in rice,using UV-visible and fluorescence spectra,combined with fluorescence The regional integration method was used to analyze and discuss the effects of organic materials on soil DOM composition and properties,and to explore the relationship between the active components of organic materials and the composition and properties of soil organic carbon and Cd content in rice.The results show:?1?The types of active functional groups of the three organic materials are similar,but their contents and proportions vary greatly depending on the type of organic materials.There are similar characteristic absorption peaks between the three organic materials,but there are significant differences in the absorption intensities of different absorption peaks.The order of oxygen content and polarity of organic materials is HM>CS>PM,and the order of humification is PM>HM>CS.?2?Organic materials will affect the adsorption and adsorption strength of purple soil to Cd.The Langmuir equation can better fit the adsorption isotherm of purple soil to cadmium in the presence of PM,HM and CS.The maximum adsorption amount qmax of purple soil to Cd after adding PM is 539.40 mg?kg^-1,which is 21.50 mg?kg^-1lower than the control group.the maximum adsorption amount qmax of purple soil to Cd after adding CS was 576.46mg?kg^-1,which increased by 15.56 mg?kg^-11 compared with the control group;and the addition of PM and HM increased the purple soil to Cd.The adsorption strength reduces the desorption amount of Cd,while the addition of CS decreases the adsorption strength of purple soil to Cd and increases the desorption amount of Cd.The Elovich equation?logqt=log a+b log t?can be better fitted.The adsorption kinetics curve of purple soil on Cd after adding organic materials.The instantaneous adsorption amount a of the purple soil on Cd after adding CS is 350.51mg/kg,increased by 107.67 mg/kg compared with the control group.After adding three organic materials,the adsorption rate of Cd adsorption increased with time in purple soil,and the purple soil with CS added had the highest reduction rate.The b value was 31.73 mg/?kg?min?,compared with the control group,the decrease was 21.27 mg/?kg?min?,the treatment of adding PM first reached the adsorption equilibrium,and the treatment with the addition of HM reached the slowest adsorption equilibrium.?3?The three organic materials tested were beneficial to alleviate the growth toxic effect of Cd-contaminated soil on rice.The plant height,effective panicle,leaf area,1000-grain weight and yield per plant increased significantly.Organic materials inhibited Cd accumulation in rice grains.The pathway includes reducing the absorption of Cd by roots,inhibiting the transport of roots Cd to stems and leaves,and transporting stems and leaves to kernels.Under the same Cd pollution level,the Cd enrichment factor and transport coefficient of the treated organic materials were significantly lower than those of the untreated organic materials.And the cumulative effect on rice Cd is related to the nature of organic materials and the degree of soil Cd pollution.When the soil Cd concentration exceeds the risk screening value of GB15618-2018,about 2mg?kg^-1,the humus soil rich in oxygen-containing functional groups has the best effect,and the Cd content of rice grains can reach GB 2762-2017 food.National standards for safety.When the soil Cd concentration exceeds the risk control value,about 10mg?kg^-1,the rice grain Cd content decreases after only the pig manure composting,but it still fails to meet the national food safety standards.It is recommended to combine other control measures to achieve rice cadmium pollution control.?4?Adding organic materials will increase the exogenous organic matter and DOC content of the soil.The molecular weight of DOC is small and the activity is strong.It is easy to combine Cd adsorption in soil and increase the content of Cd in soil exchange,while soil organic matter can form stability with Cd.A higher complex promotes the formation of Cd in the residual state.Correlation analysis showed that there was a significant positive correlation between soil exchangeable Cd and soil DOC,and a significant negative correlation with soil organic matter.The residual Cd was significantly positively correlated with soil organic matter.The reduction of soil exchange state and iron-manganese oxide-bound Cd content by organic materials is a key factor affecting the accumulation of Cd in rice grains.The application of organic materials can increase the soil organic matter content.The soil organic matter will reduce the soil exchange state and the iron-manganese oxide-bound Cd content,which will significantly reduce the Cd accumulation in rice roots,while the Cd content in rice roots is significantly positively correlated with the Cd content in rice grains and leaves.?5?Soil Cd pollution will reduce the aromaticity of soil DOM(SUVA₂₅₄).Adding organic materials can significantly increase the aromaticity,hydrophobicity(SUVA₂₆₀)and humification degree?HIX?of DOM,and the humification index of DOM?HIX?showed a significant positive correlation with soil total Cd and effective Cd content,indicating that when soil exogenous Cd concentration is high,it will inhibit crop and microbial activities and promote DOM to exhibit humification characteristics.After the addition of organic materials,the humus-like fluorescence peaks A and C,as well as the tyrosine-like peak B and the tryptine-like peak T were detected in the soil DOM.Among them,the tryptophan-like peak?peak T?in soil DOM can be used as a sensitive indicator to reflect the soil Cd contamination status after application of organic materials.The increase of exogenous Cd concentration will stimulate the preferential degradation of tryptophan peak T during microbial degradation.Promoting degradation of DOM peak T and peak B release significantly reduced the tryptophan-like peak?peak T?in DOM.?6?The exchangeable cadmium of purple soil was significantly negatively correlated with Peak T of DOM,and positively correlated with Fn?355?and HIX.Carbonate-bound cadmium was also significantly negatively correlated with Peak T of DOM.The relationship was significantly positively correlated with HIX;iron-manganese iron oxide cadmium was significantly negatively correlated with DOM's Peak T and SUVA_260,and positively correlated with Fn?355?and HIX;residual cadmium and DOM's Peak T were positively correlated.There was a significant positive correlation,which was significantly negatively correlated with HIX,and there was a significant positive correlation between soil effective Cd content and FDOM content in DOM.The soil DOM plays an important role in the transformation of soil cadmium.When the content of tryptophan in soil DOM is high and the degree of humification is high,it will promote the high active cadmium in the soil?exchange state,carbonate bound state and Iron-manganese iron oxide cadmium?is converted to low-active state?residual cadmium?,inhibiting the activity of cadmium in soil,thereby affecting the absorption of cadmium by rice roots and changing the cadmium uptake and transport capacity of rice.