Migration Characteristics Of Cd And Regulation Effect Of Fertilization In Soil-Rice System In Karst Area

The northern part of Luodian County has a large area of typical karst landform and belongs to the high cadmium?Cd?area of Guizhou Province.Planting crops in this soil with high Cd geological background is very likely to cause the enrichment of Cd in crops,seriously affect the safety of regional agricultural products and threaten the health of the human body.This paper aims at the environmental problems that the pollution of heavy metal Cd in farmland soil in karst area.The rice planting areas of villages and towns in the north of Luodian County of Guizhou Province were investigated,through field investigation and sample analysis,to explore the spatial distribution characteristics of paddy soil and the characteristics of Cd uptake by rice in karst area.At the same time,a field experiment was conducted on a high Cd farmland in the study area to study the effects of chemical fertilizer,rapeseed cake organic fertilizer,Si-Ca-Mg fertilizer and cow manure on the absorption of Cd in rice,and to explore the migration and accumulation characteristics of Cd in soil-rice system.The main research results are as follows:?1?The soil-enriched Cd in karst areas is generally higher than that in non-karst areas,with average Cd values of 1.1 mg·kg^-1and 0.6 mg·kg^-1,respectively.The characteristics of soil Cd content under different land use methods were forest land soil>paddy soil>dryland soil,and the Cd excess rate in soil in Karst area was94%,75%and 64%in turn.In addition,according to the spatial distribution map of Cd content in the study area,the samples in the paddy soil with Cd content above 1.17mg·kg^-1are mainly distributed in the Karst area,and the samples below 1.17 mg·kg^-1are mainly distributed in the non-Karst area.There is a strong risk of Cd pollution in the paddy soil in Karst area,with the northwest of Limu Township and the southwest of Muyin Town having the highest risk.?2?The distribution characteristics of soil Cd morphology and content under different utilization patterns in Karst area are reducible Cd>exchangeable Cd>residual Cd>oxidizable Cd;among them,exchangeable Cd accounts for the total Cd in forest land,paddy soil and dryland soil The proportions of content are 37.75%,19.54%and 43.70%,respectively.In addition,the average Cd content in rice grains was 0.102 mg·kg^-1,and the over-standard rate was 26%.The corresponding average Cd content in paddy soil was 4.29 mg·kg^-1.The areas with high Cd content in rice grains and high exchangeable Cd content in paddy soil are both in the west of Dongdang Township and southeast of Limu Township,and their content distribution is not consistent with the distribution of total Cd content in paddy soil.The cumulative amount of Cd in grains showed a very significant positive correlation with the exchangeable Cd content in the corresponding paddy soil?P<0.01?,and a positive correlation with the total amount of Cd,but none of them reached a significant level?P>0.05?.This indicated that the rice grain Cd content was not directly related to the total Cd,but was related to the exchangeable Cd.?3?In the study area,the soil DTPA-Cd showed a tendency to decrease first and then increase after the rice was grown from maturity.The variation range was9.01?15.62%.Contrary to the p H change trend,the soil DTPA-Cd was directly affected by the p H.The Cd content of each part of rice satisfies root>stem and leaf>brown rice,and the Cd content of each part was significantly positively correlated with soil available Cd and exchangeable Cd?P<0.05?.The accumulation of Cd in rice roots is far greater than the Cd content in stems,leaves and grains.?4?The paddy soil with high Cd content was treated with different fertilization methods.Overall,the soil Cd content at the jointing stage and mature stage was higher than that at the tillering stage and filling stage.Compared with the control CK,the six fertilization methods all increased the soil p H and reduced the effective Cd content in the soil,but the performance was not significant?P>0.05?.Among them,the application of silicon calcium magnesium fertilizer and cow dung reduced the effective Cd in the soil relatively well.After applying cow dung,the Cd content of brown rice was 0.186 mg·kg^-1,which was lower than the food safety limit(0.2 mg·kg^-1);although the fertilization method of adding silicon calcium magnesium fertilizer can significantly reduce the Cd content in brown rice?P<0.05?,Decreased to 0.246 mg·kg^-1,but still exceeded the standard;while the application of organic fertilizer and chemical fertilizer of rapeseed cake significantly increased the Cd content of brown rice?P<0.05?to 0.546 mg·kg^-1.In general,the effect of different fertilization methods on rice in order to reduce Cd is MN>MOS>MS>O>M>MO,and the control effect on Cd during rice maturation period is better than other growth periods.