Effect Of Sulfur Fertilizer On The Bioavailability Of Cadmium In Rice Rhizosphere And The Characteristics Of Rhizosphere Microbial Community

In recent years,the issue of food safety caused by excessive cadmium in soils of cultivated land in China has attracted much attention.How to control heavy metal pollution effectively in farmland and ensure food security has become one of the difficulties and hot spots in environmental management.Sulfur is one of the essential elements for the growth and development of plants,and soils in many provinces in China are in a state of general sulfur deficiency.The morphological transformation process of sulfur in soil is closely related to the morphological transformation and bioavailability of heavy metals such as cadmium.Therefore,it is of great practical significance to regulate the nutrition of sulfur in the soil to protect the agricultural production and to prevent the excessive accumulation of heavy metals in agricultural products.The mechanism of the effect of sulfur nutrition on the uptake of cadmium in the soil is not clear.At the same time,soil and rhizosphere microorganisms play a crucial role in regulating the transformation of soil sulfur and the root system of heavy metals.However,it is still rare to report the application of sulfur fertilizer to regulate the uptake of cadmium by influencing the composition of rhizosphere microorganisms in the international community.Hence,it is important to make researches on sulfur biological effectiveness of cadmium and the relationship of rhizosphere microorganisms,guiding the food security of cadmium pollution in farmland production has important guiding significance.In this paper,Rice which is the main grain crop in China was used as the research object to carry out indoor simulation incubation experiments.Traditional chemical analysis methods,inductively coupled plasma mass spectrometry(ICP-MS),scanning electron microscope X-ray energy spectroscopy(SEM-EDAX)and 16S rRNA gene sequencing are used to study the bioavailability of cadmium(Cd)in soil-crop systems and effects of rhizosphere microbial community characteristics using two forms of sulfur fertilizers(S0 and Na2SO4)under different water managements(Flooded and Int-F3D5).The main findings are as follows:(1)With soil culture experiments,the distribution characteristics of sulfur and cadmium and the effect of sulfur fertilizer application were analyzed by traditional chemical analysis methods,such as ICP-MS.The application of soil sulfur fertilizer can significantly increase the effective sulfur of rhizosphere and non-rhizosphere soil,and the Int-F3D5 is better.The application of sulfur fertilizer to soil can promote the increase of rice plant biomass and yield.The growth promotion efficiency of rice by applying S0 fertilizer is greater than that by applying Na2SO4 fertilizer.Under the condition of Flooded,the growth and yield of rice were best by applying S0 fertilizer.Under the two water managements,two forms of sulfur can reduce effective state of cadmium content in rice rhizosphere soil,thus reduce the cadmium content in rice grains,and under the management of flooded that the accumulation of cadmium in rice grain is relatively small.(2)The SEM-EDAX and ICP-MS were used to analyze the effect of sulfur fertilizer on the formation of rice root surface iron plaque.Applying sulfur fertilizer(100 mg/kg S0 and 100 mg/kg Na2SO4)in paddy soil can promote the formation of the iron plaque on the root surface.S0 is faster than Na2SO4 to form iron plaque in the same sulfur content.In the same treatment,the iron plaque content of rice roots cultured under Flooded was much higher than that of rice roots cultured under Int-F3D5.The adhesive iron plaque of rice root surface mainly adheres to the outer root epidermis and consists of elements such as Fe,O,A1 and Si,absorbing a certain amount of cadmium.The decrease of the bioavailability of cadmium in the soil and the increase of the iron plaque of root surface lead to the decrease of cadmium content in the root,stem,leaf and grain of rice.Therefore,it is one of the mechanisms of effective resistance to the absorption and accumulation of cadmium in the system of rice by adding sulfur fertilizer to promote the formation of the root surface iron plaque of rice.(3)Based on 16S rRNA gene amplicon sequencing,the structural differences of rice rhizosphere bacterial communities were analyzed.Under Flooded,the application of sulfur fertilizer increased the a-diversity of the bacterial community and changed the bacterial community structure.Under the different water managements and the different forms of sulfur treatment,rice rhizosphere specific community composition is not the same.However,the abundance of Gammaproteobacteria is found the highest in all treatments.Exogenously added 100 mg/kg of Na2SO4 under Int-F3D5,the Syntrophobacteraceae was especially abundant in the rhizosphere.This indicated that the Syntrophbacteraceae may play an important role in the application of sulfur fertilizer to alleviate cadmium stress.Under Flooded,the rhizosphere-specific bacteria of the S0 and Na2SO4 treatment groups were mostly Bacillus,Paenibacillus,and Rhodospirillaceae of Firmicutes,which are all metal-tolerant and have growth-promoting effects.The research results show that the flooded management and adding sulfur will significantly affect the rice rhizosphere microbes community composition,form rich specific rhizosphere microorganisms(such as Syntrophobacteraceae,Paenibacillus,Bacillus,etc.),which affects in rhizosphere soil sulfur nutrient and cadmium biological effectiveness,is the impact of a potential cause of cadmium accumulation in rice grain.