Effect Of Particulate Organic Matter On Bioavailability Of Cadmium In Purple Paddy Soil

Rice,as a major staple food crop in China,is liable to accumulate cadmium?Cd?in soil.Purple paddy soil is playing an important role in rice production in China.Cd is the most common heavy metal element in agricultural soil pollution.Soil organic matter has been reported as one of the most important factors influencing the transformation,mobility and bioavailability of cadmium in soils.The impact of organic matter on the migration and conversion of Cd is related to its quantity and composition.Soil particulate organic matter?POM?,referring to the soil organic matter that has a particle size of 0.053-2 mm combined with soil mineral particles.It is an organic component with high activity,a low degree of humification,and fast turnover of animal and plant residues into soil humus.It plays an important role in microbial growth,nutrient supply,soil carbon,and nutrient cycling.POM can effectively reflect the characteristics of organic matter and is sensitive to soil environment and management measures.The study found that soil POM has a significant enrichment effect on heavy metals.The relationship between the effectiveness of POM on soil available Cd and Cd uptake by rice remains to be studied.In this study,organic carbon mineralization experiments were used to study the decomposition dynamics of POM in paddy soils under aerobic and submerged conditions.The effect of POM on the decomposition of soil POM and the conversion and migration of Cd under the same conditions was studied by soil culture experiments.A pot experiment was utilized to study the effects of POM on rice Cd uptake and its relationship with soil condition.A long-term location experiment was conducted to study the effects of tillage methods on the enrichment characteristics of Cd in POM,the content of available Cd,Cd uptake by rapeseed rice.The response of POM to different agronomic measures and Cd uptake by crops,and their relationship were discussed.The main conclusions obtained are as follows:?1?Soil pH increased by addition POM,and soil Eh decreased by addition POM.The rate of soil organic carbon mineralization gradually decreased with the increase of culture time.The cumulative mineralization of organic carbon in POM soils increased with addition of POM to soil.The content of soil dissolved organic carbon?DOC?,POM increased by addition POM under flooding condition.The content of POM-C,the amount of POM-C and its contribution to soil total organic carbon increased by addition POM under aerobic condition.The addition of POM increased the content of soil POM-Cd,the amount of POM-Cd,and its contribution to soil total Cd content under flooding condition.The content of available Cd was increased by addition POM aerobic condition,while decreased under flooding condition.The results of the main factor analysis showed that the total amount of organic carbon and Cd in the soil POM component was the key factor affecting the available Cd content in soil.?2?When POM was applied with 2.5g·kg^-1,the proportation of POM-C and POM-Cd in soil was significantly increased.Application of POM increased rice plant biomass and total Cd accumulation in rice.The content of Cd was reduced by 24%-42%in rice root,and increased by 9%-30%in rice shoots with application POM.The content of Cd in rice grain was reduced by 17%,and 36%respectively when the application of POM was 0.5 and 1.0 g·kg^-1,and increased by 39%when the application of POM was2.5 g·kg^-1.Application of POM had no significant effect on the distribution of Cd in rice root and shoot,but significantly affected the distribution of Cd in rice grain.The transport of Cd from shoot to grain was inhibited when the amount of POM was less.It promoted the Cd transport from shoot to grain,and increased the Cd content in rice grain when the amount of POM reached a certain amount.Correlation analysis showed that the content of available Cd was the main factor affecting Cd accumulation in rice shoot.The total content of POM-Cd in soil was the main factor affecting Cd accumulation in rice grain.Therefore,the addition POM in soil could affect the Cd uptake of rice by changing the content of SOC,DOC,POM-C,POM-Cd,and available Cd.?3?Different tillage and rotation methods had no significant effect on the content of Cd in soil and rape tissues.The enrichment factor for Cd was in range of 2.2-5.5.And Cd in POM fraction accounted for 11.47%-33.16%.Rotation method is the main factor affecting the POM content and the amount of POM-C in soil.The main influencing factors of the amount of POM-Cd were rotation method.Tillage method was the main factor influenced the available Cd in soil.The rotation of paddy and dry land increased Cd content in rice stems and leaves,pods,and grains.Conventional rice cultivation reduced the Cd content in rice stems and leaves,and grains.The main factor influenced Cd content in rice stems and hulls was the amount of POM-C in soil.The amount of POM-Cd in soil was the main factor influencing the Cd content in rice grain.