| The production mode of renewable rice(rice first-regeneration season)is loved by farmers and consumers because of its high input-output ratio and excellent quality.However,in some renewable rice planting areas in southern China,due to the acidic soil,the availability and migration of soil heavy metal cadmium(Cd)are strong.The Cd content and enrichment characteristics of rice in firstregeneration season and regeneration season in the planting process of renewable rice are worthy of attention.Therefore,this paper selected two typical areas where the cultivation of renewable rice was relatively common in China,and analyzed the Cd enrichment characteristics and influencing factors of rice in the first and second seasons through two consecutive years of field sampling survey.Pot experiments were conducted to study the differences in Cd uptake and accumulation by rice at different planting time(early rice-regenerative rice and middle rice-regenerative rice).Through the biochar treatment experiment,the remediation effect of biochar on soil Cd pollution in the first and regeneration seasons of rice was discussed.This study aims to provide data support for the safe utilization and remediation of Cd-contaminated cultivated land and the protection of Cd safety in the first and second season rice during the planting of renewable rice.The main results of this study are as follows:(1)Through synchronous sampling of soil and rice samples in the first and regeneration seasons,field sampling survey in the regeneration rice survey area.The results showed that the soil Cd pollution in the survey area belonged to mild pollution of safe use.In 2020,the Cd content of different varieties of rice samples was at the medium variation level,with exceeding rates of 12.5%and 37.5% in the first season and regeneration season,respectively.The Cd content of rice was generally expressed as that in the first season,contrary to the conclusion of 2020.The correlation coefficient of Cd content in the first-season rice of the same variety in the two years was 0.99,showing a very significant positive correlation(p<0.05).The Cd content in the first-season rice was significantly positively correlated with soil p H value,Cd available content,total Cd content in soil and mineral elements Fe,Ca,Mg and Zn content in soil(p<0.01).The Cd content of regenerated rice was extremely significantly positively correlated with organic matter content and Cd available content(p<0.01),and was extremely significantly positively correlated with soil Se,Fe,Ca,Mg and Cd content(p<0.01).The principal component analysis showed that Mg,Ca,Zn and Fe in the first season rice showed strong load in PC1,and Cd and Se showed strong load in PC2.The absorption of the same grouped element in rice had similar characteristic properties.The Cd/Zn ratio in the first season was higher than that in the regeneration season,and the Cd/Se ratio was lower than that in the regeneration season,the decrease of nutrient element content or affect the Cd food safety of rice.(2)The results of pot experiments in different planting periods of ratoon rice showed that the Cd content in the first season rice of group A(early rice-ratoon rice)ranged from 0.012 mg/kg to0.035 mg/kg,and the coefficient of variation was 36.05%;the Cd content in reclaimed rice ranged from 0.002 to 0.012 mg/kg,and the coefficient of variation was 53.82%.The Cd content of the first season rice in group B(middle rice-regeneration season)ranged from 0.018 to 0.049 mg/kg,and the coefficient of variation was 29.53%.The Cd content in reclaimed rice ranged from 0.014 mg/kg to0.163 mg/kg,and the coefficient of variation was 105.88%.The Cd content of first season rice was negatively correlated with soil p H and soil organic matter content,and positively correlated with soil available Cd content(p<0.05).Cd content in regenerated rice was negatively correlated with soil p H value,significantly positively correlated with organic matter content(p<0.05),and positively correlated with available Cd content in soil.In group A,the plant height of rice in the regeneration season was 27.0% lower than that in the first season,and the yield was 71.2% lower.In group B,the plant height in the regeneration season was 37.0% lower than that in the first season,and the yield was 91.2% lower.The Cd accumulation ratios in different organs of different rice varieties were different in the two groups.The Cd accumulation coefficients of panicle axis and stem and leaf in the regeneration season were lower than those in the first season,accounting for60.7% and 66.7%,respectively.The order of Cd content in different parts of rice was root > stem and leaf ≈ panicle axis > rice husk > rice,and it was rice > rice husk in the first season of group B;the Cd transfer coefficients between each part of rice and rice were mostly shown as the first season of group B > the first season of group A > the regeneration season of group B > the regeneration season of group A.The Cd transfer rates of rice plants in the first season were higher than those in the regeneration season.(3)The results of biochar passivation test showed that after biochar with different mass ratios(1%,3%,5%)was applied to the soil,compared with the control group,the p H value,organic matter content,and cation exchange capacity(CEC)of the soil in the first and regeneration seasons of rice were significantly increased(p<0.05),and the improvement effect was more obvious with the increase of biochar application ratio.The soil available Cd content in the first season decreased by2.64%,4.29% and 7.92%,respectively.The soil available Cd content in the regeneration season decreased by 3.29%,7.00% and 9.05%,respectively.Compared with the control group,after 1%,3% and 5% biochar was applied to the soil,the Cd content in the first season decreased by 7.3%,5.8% and 9.1%,respectively,while the Cd content in the regenerated season did not change significantly after 1%,3% and 5% biochar was applied.The application of biochar in soil significantly increased plant height and yield of rice.Compared with the control group,the plant height and yield of rice in the first season increased by 15% and 37.2% respectively after adding 5%biochar.In the regeneration season,the plant height increased by 15.9% and the yield increased by79.9%.Comprehensive analysis showed that 5% biochar had the best remediation effect on soil Cd availability and rice Cd content. |
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