Effects Of Different Humic Organic Materials On Soil Cadmium Activity And Cadmium Enrichment In Chinese Cabbage

Dahe Town,Zhongshan District,Liupanshui City,Guizhou Province,is rich in coal resources,and heavy metal pollution is serious in the surrounding agricultural land left over from a long history of mining.Soil organic matter can inhibit the absorption and enrichment of heavy metals by crops,which plays an important role in regulating soil health.In this study,six organic materials with different degrees of humification(straw,cow manure,cow manure organic fertilizer,wood peat,lignite,anthracite)were selected.Through pot experiment,combined with Fourier transform infrared spectroscopy(FTIR)and electron microscopy scanning energy spectrum(SEM-EDS),The effects of adding organic materials with different humification degrees on soil aggregates,granular organic matter and microorganisms were studied,and the effects of changes in soil physical and chemical characteristics on Cd and Cd enrichment and transport coefficient of soil and Chinese cabbage were discussed,providing scientific basis for the popularization and application of this organic material improvement model.The main research results are as follows:(1)The humification degree of six organic materials was as follows: wood peat > cow manure organic fertilizer > lignite > cow manure > anthracite > straw.The yield of Chinese cabbage under wood peat treatment was the highest.The application of straw,cow dung,cow dung organic fertilizer and wood peat could significantly reduce the content of Cd in soil.The content of Cd in the above-ground part of Chinese cabbage was significantly decreased under the four treatments of straw,cow dung,cow dung organic fertilizer and wood peat,while the content of Cd in the above-ground part of Chinese cabbage was increased under the anthracite treatment.Under the treatment of cow manure and cow manure organic fertilizer,the Cd content of underground part of cabbage decreased,while under the treatment of straw,lignite and anthracite,the Cd content of underground part of cabbage increased.The six organic materials treated with cow manure,cow manure organic manure and wood peat had a good effect on reducing the content of Cd in the edible part of Chinese cabbage.(2)The mass percentage of soil macroaggregates(0.5～2mm)was generally increased by organic materials,and the formation of soil macroaggregates was significantly promoted by lignopeat and lignite treatments.The available content of soil Cd was higher in the soil aggregates of > 1mm,and gradually decreased with the decrease of soil particle size,indicating that the organic mineral colloidal particles formed by the soil Cd and the small aggregates formed complexation and chelation,which promoted the adsorption of heavy metals on these large aggregates with larger specific surface area and reduced the migration of Cd.(3)The addition of organic materials promoted the formation of granular organic matter(POM),and the treatment of straw,cow dung,wood peat and lignite increased the proportion of POM of different grain grades;The enrichment characteristics of Cd in soil POM are related to the particle size of POM.Specifically,the enrichment coefficient of Cd in fine particle size(< 0.25mm)POM is generally higher than that in coarse particle size(> 1mm)POM,indicating that with the continuous decomposition process of organic materials,the particle size is smaller,and the specific surface area of POM is increased,which is more conducive to the adsorption and complex of Cd.In this study,the O-H functional group had the strongest complexation effect on Cd,which reduced the migration and enrichment of Cd to Chinese cabbage.Compared with the other treatments,the wood peat showed higher relative strength.The number of bacterial species in soil increased under the four treatments of straw,cow manure,cow manure organic fertilizer and wood peat.Redundancy analysis showed that the effects of microorganisms on Cd enrichment coefficient presented certain differences under different organic material treatments.(4)The correlation results showed that lignin,cellulose and the degree of humification of organic materials were the main factors affecting Cd migration and enrichment.Through PLSPM model fitting,it was found that the addition of different humified organic materials could directly or indirectly affect the aggregates,POM and microorganisms in contaminated soil.The effect values of aggregates,POM and microorganisms on Cd accumulation and migration in soil-cabbage system were-0.65,-0.17 and-0.63,respectively,indicating that the effects of organic materials on aggregates,POM and microorganisms would weaken Cd migration and enrichment in soil-cabbage system.In summary,organic materials have significant differences in lignin,cellulose,humification degree and other characteristic indexes,and different humification characteristics can significantly improve soil physicochemical,biochemical and microbial characteristics and crop absorption.On the one hand,the application of organic materials with different degrees of humification can enhance the combination of soil organic-mineral colloidal particles,promote the formation of soil macroaggregates,and effectively reduce Cd mobility.On the other hand,increasing soil POM can enhance the accumulation and retention of Cd in soil,reduce the available content of Cd in soil,and effectively block the accumulation of Cd into Chinese cabbage.