Discussion On Technology Patterns Of Safety Utilization Of Paddy Soils Lightly Polluted By Cadmium In Western Chongqing

The Western Chongqing region is the main grain producing area in Chongqing.The regional soil pollution is characterized by slight to light cadmium（Cd）pollution.Safety utilization is the main strategy for controlling the risk of heavy metal pollution in agricultural soils in the region.The local low-Cd accumulation rice screening is still in progress,however,and there are few stereotypes.In addition,the lack of large-scale practical application of the passivators resulting in a technology model for remediation and safe use of heavy metal pollution in paddy fields that adapts to local characteristics has not established.Two types of typical paddy soils（acidic and calcareous purple soils）were selected in this area for the establishment of safety rice production technics by in situ field remediation experiments,in which a low-Cd accumulation rice variety（Changliangyou 772）alone or in combination with heavy metal passivators（silicon-calcium fertilizer,iron powder,biomass char and straw organic fertilizer）and two foliar inhibitors（foliar silicon fertilizer and selenium（Se）-containing silicon fertilizer）were compared for the purposes.The effects of the four types of passivators on the changes of basic properties of soil and the transformation of Cd form were compared by laboratory passivation simulation experiments.A regional paddy field safety utilization technology model was initially established to provide technical support for rice safety production.The results showed that:（1）All the passivators except for Fe powder could increase the rice yields,and straw organic fertilizer showed the best effects in the acidic purple paddy soil with a rice yield increase of 47.43%,while biomass char was the best in calcareous paddy soil with a yield increase of 23.95%.Foliar silicon fertilizer and selenium（Se）-containing silicon fertilizer increased the rice yield by 19.35 and25.46%in calcareous paddy soil with foliar spray Se-containing silicon fertilizer being the best,while no significant yield increase was found in acidic paddy soil.（2）The low-accumulation rice variety alone could not meet the requirements of rice safety production in the acid purple paddy soil（pH=4.75）,its combination with passivators except for the silicon-calcium fertilizer could significantly reduce the Cd contents in rice grains by14.81%-54.88%,which matched the national safe food standard for rice(0.2 mg·kg^-1,GB2762-2017).The Cd contents in rice grains varied between 0.012-0.030 mg·kg^-1in the calcareous purple paddy soil（pH=7.77）under various treatments,which were all far lower than criteria of the safety standard,however,passivators（except for biomass charcoal）still reduced the Cd contents of rice by 26.67%-59.00%.（3）The passivators altered the Cd transportation and distribution in the rice plant.Taking acidic soil as an example,silicon-calcium fertilizer inhibited the transport of Cd to from stem to rice grains,Fe powder and biomass char reduced the enrichment of Cd in roots and inhibited the transport of Cd from stem to rice grains,and straw organic fertilizer inhibited the transport of Cd from roots to stems.The results indicated that both foliar fertilizer inhibited the transport of Cd from rice stalks to brown rice,and reduced Cd contents in brown rice by 39.73%-51.85%in acidic purple soil and 40.00%-43.33% in calcareous paddy soil with similar Cd-polluting levels.Foliar spray treatments showed effective effects that meets the safety threshold standard.（4）Application of passivators promoted the transformation of soil Cd from labile forms into residual form,reduced the Cd bioavailability in the soil,and thus reduced the accumulation of Cd in rice plants.In acidic soil,biochar char showed the best effect,while straw organic fertilizer was the best in calcareous soils.Silicon-calcium fertilizer and straw organic fertilizer significantly increased the pH value and organic matter content for acid soils,consequently the soil available-Cd contents decreased by 39.45%and 34.69%respectively,while no such effects were observed on calcareous soil.In addition,the application of passivators has no adverse effect on soil enzyme activities（urease,phosphatase,catalase）in acidic soils.（5）The four passivators and two foliar foliar inhibitors did not adversely affect rice growth while reducing the Cd content of brown rice,and had no significant adverse effect on the nutritional quality of rice.Passivators improved the nutritional quality of rice grain soil with a Zn content of brown rice increases by silicon-calcium fertilizer,biomass charcoal,and Fe powder in acidic soil,and the increases of Fe content and Zn content with use of silicon-calcium fertilizer in calcareous soil.Both foliar fertilizer improved the nutritional quality of rice grain with the increases of Se,Si and protein contents and the significant decreases of Cd contents in the rice grains.The comprehensive effects in improving the rice quality follows the order of foliar spray of Se-containing Si fertilizer>foliar spray of Si alone.（6）The indoor simulation of contaminated soil experiments showed that acidity regulation of acid soils is of great significance to the effectiveness of Cd control.The content of Ca Cl₂-Cd is mainly affected by the contents of AE-Cd,OXD-Cd and RES-Cd.RES-Cd content has a significant negative contribution,and the other three forms of Cd content have a positive contribution to Ca Cl₂-Cd content.The application of passivators changed the soil acidity and affected the complexation of soil dissolved organic matter（DOC）and Cd²⁺to promote Cd form converted to the other three forms with lower activity,thereby reducing the effectiveness of Cd.