Aging effects on phosphorus transformation rate and fractionation in some calcareous soils

Rate of phosphorus (P) transformation in soils can significantly influence P fertility of soils. The transformation rate and fractionations of P in 20 calcareous soils of varying properties were investigated. Phosphorus was added to the samples at rates of 200 mg kg− 1 soil. The samples were incubated for 3, 24, 168, 336, 504, 720, 1440, and 2160 h at 25 °C and Olsen-P was determined after each incubation period. The P release in the studied soils was initially rapid followed by a slower release that lasted up to 2160 h. The transformation rate of Olsen-P for soils was estimated by best fitted kinetic equation (parabolic) for above incubation periods. There were differences in the rates at which redistribution took place between soils and P. Phosphorus in control and amended soils were fractionated before and after 2160 h incubation by sequential extraction procedure, in which the P fractions were experimentally defined as exchangeable (KCl–P), Fe- and Al-bound (NaOH–P), Ca-bound (HCl–P), and residual P (Res-P) fractions. The results showed a sharp decrease in Olsen-P within 3 h after P addition. Relative NaOH–P in amended soils ranged from 6.6 to 13.5%. Relative HCl–P, Res-P and KCl–P ranged from 49.9 to 71.2, 13.5 to 26.7, and 7.4 to 13.3%, respectively. There were changes in the proportional distribution of P in all the soils during 2160 h of incubation with amended P. In general the proportions of P associated with the most weakly bound fraction (KCl–P) tended to decrease, with corresponding increases in the NaOH–P and HCl–P fractions during the incubation. The principal component analysis showed that the first four components explained 77.1% of the overall variation.