An incubation experiment was carried out to study the effect of montmorillonite on pH, the fractions and availability of phosphorus in a sandy yellow soil (SY) and a mineral flavicant soil (MF) applied with biogas residues of swine manure (BSR). The rates of montmorillonite applied were 0%, 5%, 7.5% and 10%, respectively. The results indicated that soil pH was not affected by BSR, but was significantly increased by montmorillonite. In both soils, application of BSR caused an increase in the content and proportion of Al-P during the incubation, and an increase in the content but a slight decrease in the proportion of Fe-P in later period of incubation. The concentration of O-P was not markedly changed, but its proportion was decreased by applying BSR in both soils. The changes of Ca-P varied with soils. BSR had no significant influence on Ca-P content, but decreased the proportion of Ca-P in SY, meanwhile, it increased both Ca-P content and proportion in MF. Addition of montmorillonite in soils applied with BSR decreased the contents and proportions of Al-P and O-P, and the proportion of Fe-P did not change the contents of Fe-P, but increased the content and proportion of Ca-P in SY. In MF, montmorillonite could decrease the contents and proportions of Fe-P and O-P, and the proportion of Al-P did not affect the content of Al-P, but increased the contents and proportions of Ca-P greatly. The contents of available phosphorus (A-P) were enhanced by 17.9%-38.0% and 17.1%-33.7% in SY and MF respectively, and the phosphorus activation coefficient (PVC) was significantly improved in both soils by applying BSR. The contents of A-P were reduced by 8.8%-35.5% and 1.1%-11.6% in SY and MF respectively. Correlation analysis showed that montmorillonite reduced the availability of phosphorus through increasing pH and contents of Ca-P, decreasing the content of Al-P in SY, and through increasing pH and decreasing content of Fe-P in MF. The availability of phosphorus in SY was influenced more obviously than that in MF by montmorillonite.
Keywords: available phosphorus; montmorillonite; organic fertilizer; phosphorus fractions; soil.