| The Cd contamination problem is particularly prominent in the main rice producing areas of China,and the frequent occurrence of "Cd rice" incidents in recent years has become a focus of attention in the fields of ecological environment,agricultural safety and human health.In the context of the current low per capita arable land and the demand of soil pollution prevention policy,it is urgent to explore the technical mode of safe utilization while producing to realize the sustainable utilization of the current Cd-contaminated arable land.As a non-essential element,Cd enters rice by competing with essential mineral elements for cation channels,so the accumulation of Cd in rice also affects the uptake of mineral elements by rice.Food-based biofortification by exogenous interventions to enhance crop accumulation of mineral nutrients is an important tool to improve the mineral nutrition status of crops.Therefore,it is important to explore the technical model of combining Cd blockage control and biofortification to reduce Cd in rice while enhancing mineral element accumulation to balance the safety and nutritional requirements of agricultural products.In this study,we explored the ways to achieve the "win-win" goal of low or even zero Cd and high mineral nutrition in rice from three aspects:low accumulation cultivar screening,insitu stabilization and foliar inhibitor technology,and elucidated the mechanism.Firstly,from the perspective of cultivars,we used high-quality japonica rice cultivars as the research object,and screened japonica rice cultivars with different Cd and mineral element uptake through field trials and studied their differential mechanisms from the perspective of rhizosphere soil microorganisms;meanwhile,based on a large amount of data on Cd and 8 mineral elements in brown rice,we identified the key mineral elements affecting Cd uptake in rice.On the basis of the above study,environment-friendly soil conditioner and foliar inhibitor containing one key mineral element were selected to investigate the effects on Cd uptake in rice under three soil conditions of Cd contamination levels(slightly(1 mg kg-1),moderately(2 mg kg-1),and highly(4 mg kg-1))through pot experiments,and comprehensively evaluated their effects and mechanisms from the perspectives of soil effect-rice growth-rice quality-microbial system.The main findings were as follows:(1)Sixty-seven high-quality japonica rice cultivars were applied to investigate the differences in Cd and mineral element absorption in brown rice through field trials.Ultimately,Huaidao9 and Xudao7 were screened as rice cultivars with low-Cd and high-mineral nutrition(Fe,Zn,Mg and Ca,LCHM group);Zhongdaol and Xinkedao31 were rice cultivars with highCd and low-mineral nutrition(HCLM group).The differential accumulation characteristics of Cd and mineral elements between the two groups were mainly attributed to the differences in bacterial community composition.At phylum level,the study showed that the rhizosphere soil of LCHM group was significantly enriched with Acidobacteriota,which survived in high nutrient environment,and sulfate-reducing bacteria MBNT15,which can contribute to the formation of Cd sulfate precipitation and reduce the biological activity of soil Cd.The HCLM group was significantly enriched with Actinobacteriota(phylum level)with metal activation ability,Firmicutes and Actinobacteriota(phylum level)with lignin degradation function and bacteria(Ketobacter,genus level)that can remove nutrients and mineral elements.Based on correlation and stepwise linear regression analysis,Zn,Fe and K were identified as key mineral factors affecting Cd accumulation in brown rice and Zn was the most important factor,accounting for 68.99%.In the present study,mineral elements were innovatively incorporated into the screening system of rice cultivars with low Cd accumulation,and higher quality rice cultivars were selected;the results of the rhizosphere soil microbial angle difference investigation provided an important theoretical basis for the simultaneous achievement of lowCd and high-mineral nutrition in rice through microbial regulation;the identification of key mineral factors affecting Cd accumulation in rice provides an important basis for the subsequent selection of materials in combination with biofortification.(2)Based on the fact that K is one of the key mineral elements affecting the accumulation of Cd in brown rice,the mineral based potassium humate was selected to carry out the in-situ stabilization test,and the rice cultivar "Zhongdao 1",a rice cultivar with high-Cd and lowmineral nutrients,was used for the test.The results showed that the application of 0.25%mineral based potassium humate(MBPH)significantly reduced the Cd content in brown rice under slightly(46.8%)and moderately(44.0%)Cd-contaminated soil conditions,and significantly increased the contents of Fe,Mn,Zn and K in brown rice under slightly Cdcontaminated soil conditions and Fe and Zn in brown rice under moderately Cd-contaminated soil conditions,respectively.The application of 0.5%MBPH in highly Cd-contaminated soil was sufficient to significantly reduce the Cd concentration in brown rice(37.7%)to within the food safety standard while significantly increasing the concentrations of Fe,Mn,Zn and K in brown rice.MBPH reduced the accumulation of Cd in rice by reducing the concentration of Cd in soil solution and the content of soil available Cd(DTPA-Cd),increasing the pH of soil solution,the concentration of total K and DOC,the adsorption,complexation and precipitation of Cd by its own rich microporous structure and oxygen-containing functional groups and regulating the transport of Cd in various organs of rice.The increase in photosynthetic intensity and antioxidant enzyme activity of rice leaves enhanced Cd resistance and promoted rice growth.The present study verified that MBPH has the potential to reduce Cd accumulation in brown rice under Cd contamination stress while increasing mineral element accumulation,and clarified the optimal application dose and Cd contamination remediation mechanism under slight,moderate and high Cd contamination degree soil conditions.(3)Based on MBPH in-situ stabilization remediation experiments,we investigated the changes of rhizosphere soil chemical properties,metal available state and soil microbial community of rice.The study showed that the application of MBPH significantly increased rhizosphere soil pH,EC,SOM,NO3--N and Ava-K,and decreased soil NH4+-N,and significantly increased available Fe,available Mn,available Cu and available Zn,and significantly decreased soil available Cd under various soil conditions of Cd pollution.The application of MBPH increased the microbial richness and changed the microbial community composition structure,function and co-occurrence network characteristics.The relative abundance of Chloroflexi(phylum level)was significantly increased in all Cd-contaminated soils after MBPH application;the relative abundance of Proteobacteria(phylum level)was significantly reduced in both moderately and highly Cd-contaminated soils.both with a significant dose effect;the relative abundance of nitrogen cycle-related functional groups of soil microorganisms was significantly increased.The enhanced complexity of soil microbial co-occurrence network characteristics indicated that the interconnectedness of microorganisms was enhanced,and the increased number of key species in the co-occurrence network represented the enhanced exchange efficiency and community stability of the microbial community,and this enhanced effect increased with the increase of MBPH application.NH4+N,Ava-K potassium and available Zn were the most important environmental factors affecting soil microbial community formation characteristics on light,moderate and heavy Cdcontaminated soils,respectively;available Fe was the second driving factor under each level of contamination soil conditions.The present study revealed the changes in the rhizosphere soil microbial of rice under the action of MBPH,indicating that MBPH application could improve the soil nutrient status,focusing on revealing that the enhancement of mineral elements in brown rice was the result of the increase in the effective state of soil mineral elements under the action of MBPH and the enhanced biochemical activity of rhizosphere soil microorganisms contributing to the enhancement of the soluble fraction of mineral elements in the rhizosphere soil.(4)Based on the fact that Fe and Zn are the key mineral elements affecting the accumulation of Cd in brown rice,the sugar alcohol chelated Fe and sugar alcohol chelated Zn was selected to carry out the foliar inhibitor test,and the rice cultivar "Zhongdao 1",a rice cultivar with high-Cd and low-mineral nutrients,was used for the test.The results showed that the foliar application of both types of inhibitors improved rice growth performance and rice biomass by increasing photosynthetic intensity,chlorophyll,chlorophyll fluorescence parameters and antioxidant enzyme activities,and mitigated Cd toxicity,and sugar alcohol Fe was more beneficial to root and stem biomass accumulation,and sugar alcohol Zn was more beneficial to leaf biomass accumulation.On slightly and moderately Cd-contaminated soils,spraying low dose(0.1%)of either sugar alcohol Fe or sugar alcohol Zn was sufficient to significantly reduce Cd concentration in brown rice,and the reduction effect of sugar alcohol Zn was significantly better than that of sugar alcohol Fe,while significantly increasing the concentration of Zn or Fe in brown rice;the reduction effect of high dose(0.5%)spraying did not reach significant difference.On highly Cd-contaminated soil,spraying low and medium doses(0.3%)of sugar alcohol Fe or sugar alcohol Zn significantly reduced the Cd concentration in brown rice,but still did not reach the safe consumption standard;high doses of sugar alcohol Fe or sugar alcohol Zn significantly reduced the Cd concentration in brown rice and reached the safe consumption standard,while the concentration of Fe or Zn in brown rice was significantly increased.Therefore,it is suitable to spray low dose of sugar alcohol Fe or sugar alcohol Zn on slightly and moderately Cd-contaminated soils,and the Cd reduction effect of sugar alcohol Zn is better than that of sugar alcohol Fe;it is suitable to spray high dose of sugar alcohol Fe or sugar alcohol Zn on highly Cd-contaminated soil,and there is no significant difference in Cd reduction effect between the two inhibitors.The present study verified that both sugar alcohol Fe and sugar alcohol Zn have the potential to reduce Cd accumulation in brown rice under Cd contamination stress while increasing the accumulation of mineral elements,and clarified the optimal type and dose of inhibitors to be applied under the soil conditions of slight,moderate and high Cd contamination.(5)Based on the foliar inhibitor test,the more effective sugar alcohol Zn chelate was selected to investigate the changes of osmoregulatory substances,organic functional groups and phyllosphere microbial communities in rice leaves.The results showed that the concentrations of soluble sugars,soluble proteins and free proline and the peak areas of five types of organic functional groups(C-O,C=O/C=N,C=O/C=C,C-H and O-H/N-H)in leaves were increased to different degrees after spraying with sugar alcohol Zn.Spraying low doses of sugar alcohol Zn did not affect phyllosphere microbial abundance and diversity;medium and high doses significantly reduced phyllosphere microbial abundance under slightly Cd-contaminated conditions and significantly increased phyllosphere microbial diversity under moderately and highly Cd-contaminated soil conditions,and both showed a significant dose effect.At phylum level,the relative abundance of Actinobacteriota and Acidobacteriota decreased after spraying sugar alcohol Zn on slightly Cd-contaminated soil;the relative abundance of Bacteroidota increased and that of Myxococcota decreased after spraying sugar alcohol Zn on moderately Cd-contaminated soil,and both showed a significant dose effect.The relative abundance of phyllosphere microbial groups related to material degradation and nitrogen cycling was increased after sugar alcohol Zn spraying.The negative correlation between leaf physiological indicators and functional groups related to organic matter(cellulose,aromatic compounds,lignin)degradation and positive correlation with functional groups related to nitrogen cycle(nitrate respiration,nitrogen respiration,nitrate reduction)increased with increasing dose of sugar alcohol Zn.The present study revealed changes in the phyllosphere microbial under the effect of sugar alcohol Zn,highlighting that sugar alcohol Zn can enhance the accumulation of organic matter such as lignin,cellulose and protein in leaves by increasing the relative abundance of phyllosphere microbial functional flora associated with the decomposition of lignin and cellulose-like substances,improving the nitrate reduction function,and enhancing the chelation,complexation and immobilization of organic matter to Cd to reduce the mobility of Cd.To sum up,this study combined the concept of Cd control and biofortification,explored the ways to achieve low Cd and high mineral nutrition in japonica rice from three perspectives:cultivar selection,in-situ stabilization and foliar inhibitor test,identified japonica rice cultivars with high quality characteristics of low-Cd and high-mineral nutrition,determined the optimal solution to achieve low-Cd and high-mineral nutrition in rice under different soil conditions with different levels of Cd pollution,and revealed the response mechanisms of rhizosphere/phyllosphere microbial from different perspectives.This study contributes to the safe use of contaminated cropland to ensure the safety of agricultural products while safeguarding the supply of nutrients. |
PDF Full Text Request