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 (Q_max), maximum buffer capacity (MBC), Langmuir coefficient (K_L) 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 Q_max, MBC and b values. OA resulted in the largest decrease in Q_max 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.