Low molecular weight organic acids activate insoluble soil P through competition adsorption sites and chelating reaction in calcareous soils
[Objectives]Low molecular weight organic acids(LMWOAs)have been noticed effective in mobilizing soil insoluble phosphorus(P).We compared the activating effects of LMWOAs,the possible mechanisms and the proper way of application.[Methods]The LMWOAs used in the research included oxalic acid(OA),citric acid(CA),fulvic acid(FA),oxalic acid plus citric acid(OA + CA),and the three acids together(OA + CA + FA),and KCl was used as control.The low-and high-P soils were collected from Shihezi City,Xinjiang.The P adsorption amount of soils was tested by adsorption balance experiment.The adsorption dynamics of P was tested by isotherm adsorption experiment.The P was fractioned by continuous extraction method and the pH was measured after the unsterilized and sterilized soils were incubated for 30 days.[Results]Langmuir and Elovich equation well fitted soil P adsorptive thermodynamics(R2= 0.852-0.994)and adsorptive dynamics processes(R2= 0.882-0.975)either in low-or high-P soil.The maximum P adsorption(Qmax),maximum buffer capacity(MBC),Langmuir coefficient(KL)and Elovich constant(b)in low-P soil were significantly higher than those in high-P soil,indicating the stronger P adsorption capacity of low-P soil.All the OA,CA,and FA addition notably decreased Qmax,MBC and b values.OA resulted in the largest decrease in Qmax and MBC values,with decrease by 28.5%and 74.9%in low-P soil,and by 14.7%and 73.3%in high-P soil.CA resulted in significant decrease of b value in low-P soil(80.9%),while OA did that by 22.0%in the high-P soil.Compared to CK,the highest Olsen-P content was exhibited in OA treated soil,followed in OA+CA soil,the least effect was in FA treated soil.Under unsterilized condition,OA and OA+CA significantly increased soil Olsen-P by 42.6%and 18.5%in low-P soil,and by 27.3%and 1.01%in high-P soil;increased Resin-P by 80.9%and 77.4%in low-P soil,and 79.5%and 72.8%in high-P soil;decreased Di HCl-P by 8.87%and 5.89%in low-P soil,and 8.83%and 5.54%in high-P soil;decreased Con HCl-P by 25.1%and 12.9%in low-P soil and by 16.9%and 5.30%in high-P soil.CA increased Resin-P by 70.2%and 79.5%in low-and high-P soil,but decreased NaOH-Pi by 14.8%and 26.3%,accordingly,suggesting that OA and OA+CA facilitated soil P transformation from non-labile to labile P fractions,while CA favored to the transformation of NaOH-P to Resin-P.FA did not show significant P mobilizing effect.Besides,the sterilized and unsterilized soils were tested with similar P fraction contents,suggesting not important P-solubilizing effect by microorganisms.The Olsen-P increase was negatively correlated to the decrease of soil pH,implying the negligible effect induced by acidification due to the addition of low molecular organic acids.[Conclusions]Low molecular organic acids could increase soil P bioavailability in calcareous soils mainly through competition on the adsorption sites and chelation reaction with soil P,rather than acidification and microbiological mechanism.The mobilizing efficiency of soil P is in order of oxalic acid>citric acid>fulvic acid,and the single application is more effective than the combined application in mobilizing soil P.
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