Saline-alkali Soil Remediation By The Combined Application Of Halotolerant Phosphate Solubilizing Microorganism And Rock Phosphate

Saline-alkali soil is a serious global environmental problem,the salts and metal ions have an adverse effect on the growth of crop plants and are responsible for soil degradation.Saline-alkali soil improvement mainly adopts measures such as applying chemical modifiers,improving irrigation and drainage,and growing salt-tolerant plants.In recent years,using biological methods instead of chemical additives have been employed to improve crop production.Phosphate-solubilizing microorganisms?PSMs?are a group of plant growth-promoting rhizobacteria,which can be beneficial bioinoculants to alleviate salinity stress and further improve the quantitative and qualitative factors related to plant and soil productivity.Application of rock phosphate along with PSMs has been suggested to improve plant growth and soil quality.Soil samples were collected from sites located in the Dongying area and Xinjiang area.Soil physical and chemical properties were determined.The screening strategy used during this research enabled the identification of PSM colonies both on TPM and YM media containing Ca₃?PO₄?₂ and egg yolk,respectively,as the sole source of P.All the purified strains were amplified using16S rDNA genes as the target.Results showed,40 bacterias which mainly distributed in the genus Bacillus,Pseudomonas,Arthrobacte,Acinetobacter,Providencia rettgeri,2 actinomycetes and 10 fungi belong to Aspergillus and Penicillium were isolated from soil located in Dongying area.The amount of soluble-P ranged between 2.25⁴42,2.25⁴42,0.2¹4.6 and 0.3¹9.72 mg/L in the presence of Ca₃?PO₄?₂,AlPO₄,FePO₄ and lecithin.A total of 23 phosphate solublizing bacterias which belong to Bacillus,Acinetobacter,Brevibacillus,Pseudomonas,Paemibacillus,Gordonia,Ensifer adhaerenss and Chryseobacterium were isolated and purified from Xinjiang.There were 6 phosphate solublizing fungi,belonging to Aspergillus,Penicillium and Talaromyces flavus.The amount of soluble-P ranged between 65⁴96,0.4¹1.4,0.1²8.5,2.1¹0.3 mg/L in the presence of Ca₃?PO₄?₂,AlPO₄,FePO₄ and lecithin.In addition,the Providencia rettgeri and Gordonia sp.strain were confirmed,for the first time,as PSMs.Isolates were also checked with respect to their saline-alkali tolerance and secretion of organic acid.Results showed that Bacillus?YM-9,YM-16,TPM-26,YMX5,TPMX18,TPMX19?,Providencia retteri?TPM-23?,Enterobacter sp.?YM-14?,Acinetobacter sp.?YMX11?,Ensifer adhaerens?TPMX5?,Gordonia sp.?TPMX16?could grow under the condition of 1.5 mol/L NaCl,the highest Na₂CO₃tolerance was 50 mmol/L,and the highest pH tolerance was 10.The higher solubilization of Ca₃?PO₄?₂ was related to the secretion of propionic and oxalic acid.Pot experiments were carried out to evaluate the promoting effects of Enterobacter sp.,Bacillus megaterium,Providencia retteri and Ensifer adhaerens.Results showed that all four strains shown strong resistance to NaCl and high phosphate-solubilizing ability even an NaCl concentration of 1.4 mol/L,In addition,all four strains demonstrated diverse levels of phosphate solubilization activity in the presence of various carbon and nitrogen sources,which indicated their high phosphate solubilizing efficacy.Saline conditions reduced peanut growth,however,the adverse effects were amended by PSB inoculation.Providencia rettgeri?TPM23?showed the best effect among the four strains.The underlying mechanism is likely related to plant hormones[i.e.,indole-3-acetic acid?IAA?,abscisic acid?ABA?and gibberellic acid?GA3?]and successful root-colonization by bacterial inoculants.We performed a pot experiment to investigate the effect of Providencia retteri inoculation combined with rock phosphate and corn straw on peanut growth and soil quality.The results of soil physico-chemical properties showed that these amendments reduced soil pH with the number of 0.62.The contents of Na⁺and Cl^-decreased and the contents of Ca²⁺and SO₄^2-increased.The contents of available N,P,K and organic matter in soil increased by 189.71 mg/kg,19.57 mg/kg,873.34mg/kg and 23.06 g/kg,respectively.Also,significant improvement in activities of phosphatase,urease and dehydrogenase were observed.The application of PSM suspension significantly increased soil microbial population and promoted the accumulation of bacteria which related to soil nutrients cycling.This study isolated and identified the PSMs naturally colonize in the saline-alkali soil,which provides sources for halotolerant PSMs that can be used as bioinoculants to protect plants against salt stress.Pot experiments showed that the halotolerant PSMs can promote plant growth,and improve the soil quality combined with rock phosphate and corn straw.Thus,the inoculation of halotolerant PSM was an economical and sustainable strategy for improving the plant growth and repairing the saline-alkali soil.