| The accumulation of heavy metals in paddy soil and the excessive cadmium(Cd)in brown rice have caused widespread concern in the whole society because of their influence on human health through the food chain.The factors affecting effective cadmium and brown rice Cd have become hot and difficult problems in soil environment research.Many studies have shown that environmental factors affect the occurrence of heavy metals in soils and the uptake of cadmium by rice and the migration of cadmium in soil-rice systems,but many phenomena are still difficult to explain,such as the investigation found that even if the soil Cd content is low,its brown rice production also has the possibility of exceeding Cd,and many researchers can obtain the effect of reducing rice Cd in the case of high Cd.Therefore,it is of great theoretical and practical value to study the relationship between soil heavy metal cadmium and soil environmental factors in contaminated areas.In this study,359 soils of 6 kinds of soil types in the cadmium pollution monitoring area of Changsha,Zhuzhou and Xiangtan were collected,and 359 samples of brown rice of early rice and late rice were collected.Soil pH value,organic matter,total cadmium,effective cadmium distribution frequency;soil effective cadmium content and its influencing factors;cadmium content and its distribution frequency in cadmium pollution monitoring area;brown rice cadmium content and its influencing factors in different parent material soils;A field trial was carried out to add the effect of quicklime and improver on typical soil pH and in-situ passivation of effective cadmium in soil.Research indicates:1.The soil of the paddy field in the cadmium pollution monitoring area ofChangsha,Zhuzhou and Xiangtan is more acidic.The average pH value of the paddy field in the cadmium pollution monitoring area was 5.61,and the paddy soil developed by the shale parent was the highest(5.93),followed by the river and lake sediments(5.74)and the glutenite(5.73),red clay(5.54),purple sand shale(5.36),and granite(5.34)are lower.The soil pH of the cadmium pollution monitoring area is mainly distributed between 4.5 and 6,the distribution frequency is 74.7%,and the distribution frequency of the strongly acidic soil(pH<5)is 19.2%.The distribution of soil organic matter in the monitoring area is relatively uniform.The distribution frequency of organic matter level above 30 g/kg is 75.8%,while that of less than 30 g/kg is only 24.2%.Paddy soils with organic matter content shale parent material(38.79 g/kg)were the highest,followed by granite(38.36 g/kg)and glutenite(37.63),and again quaternary red clay(35.81 g/kg),purple sand shale(35.35 g/kg),while river and lake sediments are the lowest(33.62 g/kg).2.The average soil cadmium in the paddy field of the cadmium pollution monitoring area in Changsha,Zhuzhou and Xiangtan was 0.458 mg/kg,of which the distribution frequency was greater than 0.30 mg/kg and the distribution frequency was 71.1%,while the lower than 0.30 mg/kg was 29.9%.The average effective cadmium in soil is 0.187 mg/kg,mainly distributed between 0.05 and 0.2 mg/kg,the distribution frequency is 66.8%,followed by more than 0.2 mg/kg,the distribution frequency is 27.6%,and the distribution frequency is less than 0.05 mg/kg.Only 5.6%.There were significant differences in cadmium in different types of soils.The paddy soil(0.669 mg/kg)and quaternary red clay(0.580 mg/kg)developed by the shale parent material were the highest,which was significantly higher than other types of soil,while the effective cadmium was the tetrahed red clay(0.265 mg/kg)was the highest and the glutenite was the lowest(0.131 mg/kg).The ratio of effective cadmium to total cadmium in paddy soil in cadmium pollution monitoring area is between 32.7-53.2%.The effective cadmium content of paddy soil in the sediments of rivers and lakes is the highest(53.2%),followed by quaternary red clay(45.7%),the lowest is shale(32.7%).The analysis results of effective cadmium in soil showed that the correlation between soil effective cadmium and total cadmium and organic matter reached a very significant correlation level(n=359,0.791**;0.295**),among which the effective cadmium and total cadmium and organic matter of granite reached extremely significant levels(n=59,0.921**;0.953*),effective cadmium and total cadmium in river and lake sediments were significantly correlated(n=60,0.526**),effective cadmium and pH of quaternary red clay the value is significantly negatively correlated(n=60,-0.308*).3.The average brown rice cadmium content of early rice in the paddy field of the cadmium pollution monitoring area is 0.365 mg/kg,and the distribution frequency of more than 0.20 mg/kg is 67.3%.The average brown rice cadmium content of late rice was 0.471 mg/kg,and the distribution frequency of greater than 0.20 mg/kg was 78.6%.The cadmium exceeding rate of brown rice in late rice is higher than that of early rice.The content of rice cadmium in early rice in paddy soil developed by purple sand shale parent material was the highest(0.540 mg/kg),followed by the quaternary red clay(0.484 mg/kg),and the lowest with the glutenite(0.182 mg/kg);the late rice cadmium purple sand shale is the highest(0.639 mg/kg),followed by shale(0.611mg/kg)and granite(0.277 mg/kg).4.The influencing factors of brown rice cadmium are complex.The correlation degree between brown rice cadmium and total cadmium,effective cadmium,pH and organic matter in different soils is different.The correlation between brown rice cadmium in early and late rice and soil total cadmium in the quaternary red clay parent material reaching a extremely significant level(n=60,0.733**)、(n=60,0.473**),correlation coefficient was the highest among the six parent materials,r=0.6448.r=0.5992;the correlation between brown rice cadmium in early rice and effective cadmium to total cadmium in quaternary red clay was also reaching a extremely significant level(n=60,0.353**),correlation coefficient was the highest,r=0.3306;the correlation between the ratio of brown rice cadmium in early and late rice to total cadmium and pH value was reaching a extremely significant level in purple shale(n=60,-0.435**)、(n=60,-0.513**),correlation coefficient was the highest,r=-0.4181.r=-0.5006;study on the correlation between brown rice cadmium and organic matter was found,the correlation coefficient between brown rice cadmium in late rice and organic matter was the highest in quaternary red clay,r=0.5592,reaching a extremely significant correlation level(n=60,0.353**).5.Under the same application conditions of application of LG improver 1125 kg/hm2,1350 kg/hm2 and 1575 kg/hm2,the comparison of the application of lime 1500 kg/hm2 with non-lime lime showed that the soil pH increased by 5.59,4.39%and 3.80%after 20 days of treatment.The effective cadmium passivation rate of soil increased by 2.21%,2.59%and 0.32%.The pH value of soil during maturity increased by 6.31%5 6.19%and 6.67%,and the effective cadmium passivation rate of soil increased by 0.82%,4.21%and 3.24%.Therefore,the application of LG improver soil conditioner together with the application of lime(1500 kg/hm2)can effectively increase the soil pH value and enhance the effective in situ passivation effect on soil.The soil in the cadmium pollution monitoring area of Changsha,Zhuzhou and Xiangtan is highly acidic,and the proportion of effective cadmium in total cadmium is high.It is an important cause of excessive cadmium in rice.There are differences between different kinds of soils.Therefore,according to soil type and characteristics,folly considering soil self-purification ability,improving soil pH,reducing soil effective cadmium,reducing brown rice cadmium content,and achieving the goal of using the excess soil safely should be an important research direction. |
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