Effects Of Exogenous Substances On Growth And Phosphate Dissolution Of Phosphorus Solubilizing Bacteria

Phosphorus?P?is a second important nutrient element essential for plant after nitrogen.P-deficient soil is over than two third of total arable lands in our country due to low available P.Moreover,P use efficiency is quite low?10-35%?.P solubilizing bacteria?PSB?are important beneficial microorganisms in soils.They are able to convert insoluble inorganic P into soluble forms and increase P bioavailability for plants.It is necessary to evaluate the influence of exogenous compounds,including heavy metals,pesticides,and plant allelochemicals in soils,on the growth and functions of PSB.Therefore,liquid and solid incubations were carried out to study the growth and P solubilization by three strains of Burkholderia Yabunchi with ability to dissolve inorganic P in response to Cadmium?Cd?,imidacloprid,extract of Alternanthera philoxeroides roots?EAR?and extracts from fibrous roots of Coptis chinensis?ERC?.The main aim is to understand the influence of exogenous compounds on growth and P solubilizing role of PSB,and to provide us with some basic information for PSB use in improvement of crop P nutrition in agricultural practices.Following are the main results obtained.?1?Effects of Cd on growth and P dissolving of PSBThe abilities of P solubilization of three tested PSB strains changed in the sequence:B07>B07>B05,with P dissolving index and rate showing 2.37 and 2.89 fold differences amoung the bacterial strainns,respectively.After agitation incubation for120 hours,culture solution pH showed an order of B05>B09>B07,and a negative correlation was found between soluble inorganic P and pH value in culture solutions?soluble inorganic P=-83.148 pH+581.96,R²=0.9052,n=25?.The diameters of bacterial colonies and P-dissolving haloes on solid culture mediums were decreased with Cd²⁺concentrations increased.In liquid culture mediums with 0.5-2.0 mg Cd²⁺/L,the number of PSB was reduced by 22.09-68.18%compared with CK(without Cd²⁺).Simultaneously,the efflux of oxalate,acetate,citrate,and proton by PSB was inhibited in presence of this heavy metal,and the amount of inorganic P dissolved from insoluble phosphates was reduced by 5.27-38.45%.Therefore,PSB might grow poorly and dissolve inorganic P slowly in cadmium polluted soils,being unbeneficial to soil P supply.?2?Negative allelopathic effects of Alternanthera Philoxeroides root extract on growth and P dissolution of PSBAlternanthera philoxeroides Griseb,a kind of ill weed hard to eliminate in the world,is able to grow in both soils and waters low in available phosphorus?P?and now widely distributed in over twenty provinces?regions or municipalities?in this country,causing enormous hazard to agriculture,forestry,animal husbandry and aquiculture.Bacterial colonies,P-dissolving rings and P-dissolving indexes in the solid culture media decreased with increasing EAR concentration.The effect on was found the highest with Strain B09,which was followed by Strain B05 and B07.In the solid culture media amended with EAR 0.050 g/mL of the root exudate,bacterial colonies decreased by 40.32%⁵0.00%in diameter,by 49.09%⁶7.98%in diameter of P-dissolving rings,and by 9.86%²7.38%in P-dissolving index.Growth of the PSB in the liquid culture media was also inhibited by EAR in varying degrees,and hence their populations reduced by 48.13%⁷3.03%.In the liquid culture media amended with EAR 0.050 g/mL,the amount of P dissolved by Phosphate-solubilizing bacteria?PSB?decreased by 47.32%?B05?,11.43%?B07?and 36.00%?B09?as compared with the control.In addition,the content of inorganic P dissolved by the bacteria was positively related to the efflux of proton?r=0.836,p?0.05,n=12?and total organic acids?r=0.947,p?0.05,n=12?.EAR inhibited significantly the release of protons and organic acids from PSB and hence the dissolution of P remarkably.?2?Allelopathic effects of extract of Coptis chinensis fibrous roots on PSBCoptis chinensis is a medicinal herb traditionallycultivated and used for many centuries in China.As it is now cultivated extensively in large areas and often continuously for years in a row,sustainable development of the industry has come across serious obstacles caused by mono-cropping.it is,therefore,essential to explore allelopathic effects of this medicinal plant on soil microorganisms,in order to understand mechanisms of the occurrence of obstacles in the mono-cropping and provide certain scientific basis for lifting the obstacles.Results show that the higher the concentration of the ERC,the smaller the size of bacterial colonies,the diameter of P-dissolving halos and the lower the P-dissolving index on the solid culture medium.ERC also inhibited the growth and reproduction of PSB in variable degrees,reducing the population of PSB by 35.98%⁵2.98%in the liquid culture medium as compared with the blank control?without ERC?.All the three strains of bacteria tested in the liquid culture released protons,oxalate,and citrate into the solution.Besides,acetate was found in the culture solution with B05,malate with B09,and acetate and succinate with B07.In addition,PSB varied greatly in tricalcium phosphate dissolving capacity in the culture solution.The mean amount of P dissolved was 228.31 mg/L for B05,311.64 mg/L for B07,and 174.89 mg/L for B09.Inorganic P dissolution by these bacteria was positively related to efflux of proton?r=0.877,p?0.05,n=12?and organic acids?r=0.604,p?0.05,n=12?.When ERC inhibited efflux of protons and organic acids in culture solutions,it reduced P dissolution by 21.68%⁶0.55%.During the growing period,C.chinensis roots released allelopathic chemicals,which affected adversely community structure and physiological and ecological functions of the PSB,thus reducing bio-availability of inorganic P in the soil,which could be one of the possible reasons why obstacles occur in mono-cropping of C.chinensis.?4?Effects of imidacloprid on growth and phosphate dissolution of PSBImidacloprid inhibited the growth of tree PSB strains in solid culture,with maximal diameters of colonies decreased by 26.67-55.45%.However,PSB strains varied greatly in P solubilization in the presence of imidacloprid,with B09 showing the reduction of P dissolving halos and indexes in contrast to B05 and B07 which showed the increment of maximal diameter of colonies by 3.57-16.28%and P-dissolving indexes by 28.89-51.68%.The growth of PSB was also inhibited in liquid culture.P dissolved from insoluble phosphate by B05 increased with imidacloprid in culture solution.High concentration of imidacloprid promoted the P solubilization by B07 although there was no significant changes at low and middle concentrations.In contrast,high and middle concentration of imidacloprid decreased P dissolution B09,while P solubilization changed little at low imidacloprid.All three bacterial strains were able to effuse protons,oxalate,and citrate,with these three organic acids accounting for 67.00%-83.00%of the total.In addition,acetate was detected in culture solution of B05 and B07,malate in B09,and succinate in B09.The amount of P dissolved from tricalcium phosphate by bacterial positively correlated with total organic acids?r=0.823,p<0.05,n=12?and proton efflux?0.876,p<0.05,n=12?.Therefore,imidacloprid inhibited the growth and reproduction of PSB in variable degrees in both liquid and solid incubations,and promoted or inhibited P solubilization of PSB as bacterial strains and imidacloprid concentrations varied.The protons and organic acids released from PSB could promote P solubilization by PSB and imidacloprid resulted in a great change in P release from tricalcium phosphate by PSBvia the efflux of protons and organic acids.PSB strains varied in activities,numbers,and functions aftera large application of imidacloprid in pest control practices.