Process Of Direct Seeding Affecting The Accumulation Of Heavy Metal In Rice(oryza Sativa L.)and The Regulatory Mechanism Of Alkaline Combined Fertilizer

Rice（Oryza sativa L.）is an important worldwide staple food crop for more than three billion people.And rice is also an important principal food for nearly 60%population in China,heavy metals-contaminated rice is the primary route of heavy metals exposure to humans and thus poses a health risk.At present,the increasingly serious heavy metal pollution in rice,especially in the main rice-producing areas of southern China,has seriously threatened people’s health.Therefore,it is of great significance to ensure the safe production of rice.Global rice production is undergoing a shift from traditional seedling transplanting to direct seeding.The direct seeding method has the advantages of saving labor and irrigation water consumption,higher economic benefits,reducing methane emissions,shorter growth period and more suitable for mechanization,etc.It is generally the case that direct seeding rice performs better than transplanted rice and therefore should set the scene for the global production of rice.To our knowledge,however,little information is currently available regarding the effects of seedling establishment methods on heavy metals uptake and translocation in rice.In this study,field experiments were conducted in a typical rice production area in Hunan,with three seedling establishment methods（direct seeding（DS）,seedling throwing（ST）,and manual transplanting（MT））,multiple sites（Zhuzhou,Liuyang）,multiple seasons and multiple varieties（high-Cd variety Tianyouhuazhan,low-Cd variety Xiangwanxian 13）,and measuring these indicators,including iron plaque,root morphology and distribution,soil physical and chemical properties,the variation of soil heavy metals availability,and the accumulation and translocation of heavy metals at each growth stage of rice,to investigate the influence of three seedling establishment methods on the translocation and accumulation of heavy metals in soil-iron plaque-root-rice system.In addition,a series of field experiments were conducted to evaluate the effects of different proportions of nitrogen-phosphorus-potassium（N-P-K）fertilizer（urea,calcium magnesium phosphate,and potassium carbonate,respectively）alone or coupled with a topdressing of manganese（Mn）fertilizer at the tillering stage on reducing heavy metals bioavailability in soil and uptake in rice.The main results are as follows:（1）Data revealed that the DS method enhances cadmium（Cd）accumulation in roots,straw,and brown rice.In particular,the Cd concentration in DS brown rice was21.8%-43.3%higher than in MT and ST methods,respectively.In each growth stage,DS rice was more apt to harbor Cd distributed in the plant’s aerial parts.During the vegetative stages,the Cd uptake percentage was higher in DS than in MT rice,and those plants arising from the DS method were capable of absorbing more Cd earlier in their growth and development.And at the maturity stage,substantial Cd accumulated in straw could be remobilized within rice plants,with this remobilized Cd specifically transported to reproductive growth tissues.（2）Using DS decreased the amount of iron plaque covering the root surface in every growth stage,and there was a significant difference in some growth periods（p<0.05）.At the tillering stage,the amount of iron plaque on the root surface was significantly positively correlated with the Cd content in rice,which showed a promoting effect;But from the heading stage,the negative correlation was observed,especially in the mature stage,the iron plaque and the Cd content in the brown rice showed a significant negative correlation（p<0.05）,which showed a barrier effect;The date of u-XRF also confirmed the same results,especially in the critical period of Cd accumulation,such that the roots’middle areas were distinguished by a near-complete absence of iron plaque,thus weakening its role as an effective barrier to Cd uptake from soil.So DS mode of planting will increase Cd’s distribution in the aboveground parts of rice,and heightening the risk of Cd contamination in grain.（3）The specific surface area,number of root tips,and specific root length of direct seeding rice are significantly higher than those of transplanted rice.At the same time,its diameter is thinner and its volume is smaller.A multiple regression model was developed with root morphological indices and the total As concentration in brown rice,and the R²value of the model was 0.819,which was significant at the 1%level.Compared with the other establishment methods,the thinner diameters,smaller volumes,larger specific surface areas,and greater numbers of root tips in DS rice roots across growth stages promoted Cd uptake.（4）The rational application of N-P-K fertilizer not only provided the basic nutrients to promote the normal growth of rice but also increased soil p H and thereby reduced the Cd bioavailability in soil.The Mg（NO₃）₂-extracted Cd concentrations in the three soils were reduced by 26.46-56.53%,while TCLP-extracted Cd was reduced by 19.87-45.41%.The application of Mn fertilizer at the tillering stage increased Mn and Cd sequestration in the iron plaque.The Mn content in iron plaque increased by 15.71-58.67%.Collectively,this combined method of fertilization significantly reduced Cd accumulation in rice tissues,particularly to brown rice,in which accumulation decreased by 25.19-44.70%,37.35-47.84%,and 38.00-60.88%in three typical paddy fields.Thus,the basal application of combined urea and alkaline inorganic fertilizers followed by topdressing of Mn fertilizer may be a promising and cost-effective tactic for the remediation of Cd-contaminated paddy soils.（5）DS method resulted in the greatest accumulation of As in the two rice cultivars and led to more As distributed in aboveground tissues during the entire growth period.Especially in DS brown rice,the concentration of total As increased by 24.0%to 40.8%,and that of inorganic As increased by 24.4%to 40.0%,compared with the concentrations in MT and ST rice.Compared with the other establishment methods,the thinner diameters,smaller volumes,larger specific surface areas,and greater numbers of root tips in DS rice roots across growth stages promoted As uptake.The concentrations of As in root tips were approximately five times greater than those in the root base,and root tips were the key factor determining the difference in As accumulation in rice roots under the different seedling establishment methods.On the other hand,the accumulation of As in various tissues of rice showed an upward trend after the application of combined fertilizers.Especially in brown rice,the As content increased by 36.4%at the highest,and the difference was significant（p<0.05）.In areas contaminated by both Cd and As,although the combination of fertilizers can effectively reduce the accumulation of Cd in rice,it will increase the risk of As pollution.