Dissolving Phosphorus, Releasing Heavy Metals From Phosphate Rock And Alleviating Wheat Salt-Stress By Phosphate Solubilizing Bacteria

Phosphate-solubilizing microbes are one kind of important soil microbes in agricultural ecosystem,which exist extensively in the agricultural and natural soil. Grasping the microbes' distribution and their phosphate-solubilizing function characteristics are the preconditions for their application in practice because there is significant difference among different conditions in phosphate-solubilizing function for their different adaptations to certain environment.Their special distribution,the effect of salinity and heavy metals on their quantity and population structure and the relationship between their resistance to heavy metals and p-solubilizing ability,etc.were studied in this study.The results are as follows:12 families of P-solubilizing microbes were isolated from the soil samples collected respectively from the saline area in XinJiang,ShanDong and JiangSu,the polluted area by heavy metal in HuNan,JiangSu,AnHui,the Phosphorus Mineral Region in ZhongFang, ShiMen,LiuYang of HuNan.18 pure strains were isolated from the P-solubilizing bacteria. Analysis of the diversity and the effect of P-solubilizing microbes under different soil conditions showed that Pseudomonas sp.and Bacillus sp.were found high-frequency in the area of heavy metal and salt.The influence degree of organic matter content in the area of salt on popular abundance of P-solubilizing bacteria is lager than number abundance.In the soils polluted by heavy metals,the integrated pollution index and organic matter content are the main influence factors of the number of P-solubilizing bacteria.The popular abundance in the soils heavy polluted by single heavy metal is smaller than in the soils light polluted by combined heavy metals.In the soils of phosphorus mineral region,the distribution of P-solubilizing bacteria was influenced by the content of soil available phosphor,the level of soil organic matter,and the soil background value of heavy metals.There are large quantities of P-solubilizing bacteria and abundant population in the phosphorus mineral region with the proper level of available phosphor and some organic matter content,on the contrary there are few quantities of P-solubilizing bacteria and simple population. Therefore,to a certain extent,the diversity of P-solubilizing microbes can reflect the environmental conditions of this ecological area.The main screening standards of high efficiency P-solubilizing strains were based on the degree of discoloration of BPB cultured in the liquid of NBRI-BPB and the absorbance OD680nm.5 high efficiency P-solubilizing strains named HH-1,HH-2,EnHy-401, ArHy-505 and AzHy-510 were screened from 18 isolated strains.Those bacteria were all isolated from Phosphorus Mineral Region in ZhongFang of HuNan and belong to the Enterobacter,Pantoea agglomerans,Arthobacter globiformis,Enterbacter,and Dexira gumos separately,which were identified primarily according to their physiological and biochemical characteristics and sequence analysis of 16SrDNA.The results indicated that P-solubilizing microbes isolated from low-phosphorus environment may not have high efficiency P-solubilizing activity.Studies on the solubilization of phosphate rock by P-solubilizing bacteria and the effects of soluble phosphate of continuous batch fermentation of phosphate rock show that associated heavy metal in phosphate rock will leach out with the phosphorus release.The degree of difficulty of leach is different according to the association of associated heavy metal in phosphate rock.Test on the 5 associated heavy metals,Zinc is significantly related to the phosphorus release.The amount of released Zinc is about 90%of the total amount of Zinc in phosphate rock.The Leach rate of Zinc could indirectly reflect the degree of the phosphorus release in phosphate rock.Among the associated heavy metals,Pb shows high stability.The leach of those associated heavy metals will inhibit the growth of P-solubilizing bacteria and the metabolism of soluble phosphate,which leads to affect the advantage of phosphate-solubilization of bacteria.Generally,the ability of P-solubilizing of bacteria in the second fermentation is higher than the first fermentation.Under low-phosphorus conditions,the P-solubilizing bacteria is more sensitive to Zinc,the difference of the ability of the phosphate solubilization in different batch fermentation is significantly more;on the contrary,the difference of the ability of the phosphate solubilization in different batch fermentation is less in the phosphate rock with the stable associated Zinc.Therefore,the ability of the P-solubilizing and the ability of the resistance of Zinc in low-phosphorus conditions should be taken full consideration during selection of P-solubilizing bacteria to improve the effect of solubilization of phosphate rock.Results of P-solubilization by different P-solubilizing bacteria on different insoluble phosphates showed that P-solubilizing bacteria producing polysaccharide seem to be more efficient in solubilization of the calcium phosphates,but less effective in solubilizing AlPO₄ and FePO₄ compared to Bacillus megaterium P17.Analyzing the relativity between the amount of soluble phosphate in cultures and the total exopolysaccaride production,and the free water-soluble exopolysaccaride yield,respectively,it was found that the phosphate solubilization of P-solubilizing bacteria producing polysaccharide was high related to the production of the exopolysaccaride.Basing on the high relativity between the phosphorus content in exopolysaccaride and the dispersion of phosphorus concentration in superuatant between before and after EPS extracted,we could presume that the exopolysaccaride produced by P-solubilizing bacteria had ability for phosphorus-holding,which influenced the synergistic effects of exopolysaccaride and organic acid on Ca₃(PO₄)₂ solubilization. The difference existed in the solubilization kinetics of Ca₃(PO₄)₂ in NBRIP-citric acid medium with or without exopolysaccaride showed that the addition of exopolysaccaride changed the balance in homeostasis of by P-solubilization and resulted in greater phosphorus release from insoluble phosphate.The P-solubilizing activity of EnHy-401 was affected by the rate of carbon,nitrogen resource,and KH₂PO₄ content in culture.The results of spot experiment showed that the strain EnHy-401 had the ability to activate the insoluble accumulated phosphorus in saline soil and enhanced nutrient uptake efficiency by wheat plants,then conferred resistance in wheat plants to salt stress and resulted in a significant growth increase.In saline soil inoculated with Enterobacteria sp EnHy-401,available phosphorus and exchangeable calcium was increased from 6.4 mg kg-1 and 1162 mg kg-1 to 10.3 mg kg-1 and 1214 mg kg-1,respectively.Wheat seedling grown in soil inoculated with the EnHy-401 strain increased shoot weight by 28.1%and root weight by 14.6%compared to the control.Phosphorus,calcium,potassium,and magnesium contents in shoots increased 34.4%,36.3%,31.5%,and 6.3%compared to the control,respectively.In our experiments,the fact that the increases in available P,biomass P, and Ca2+ concentration in saline soil treated with PSB Enterobacter sp EnHy-401 inocula (Table 2),and high relativity between the P,Ca,K,and Mg content in wheat tissue and dry matter(Table 4) indicated that PSB Enterobacter sp EnHy-401 suppressed the adverse effect of salinity stress in plants through nutrient(P and Ca) supply and nutrient(K and Mg) uptake enhancement.The phosphate solubilizing activity of Enterobacteria sp EnHy-401 and the amelioration of salt stress on wheat plants by the strain varied with the salinity levels and content of organic matter in the saline soil.Inoculation of the strain in low salinity soil had a greater increase in available phosphorus and greatly alleviated the negative effects of salt in wheat plants compared to plants cultivated in high saline soil.Combined inoculation of Enterobacter sp EnHy-401 and organic fertilizer application brought about maximum suppression of salt stress to wheat and resulted in the maximum increase of wheat biomass (65.4%) under high salinity condition.