Phosphorus release kinetics and availability in calcareous soils of selected arid and semiarid toposequences

Rate of phosphorus (P) release from solid to solution phase may control the lability of P in soil environments. Kinetics of P release from Iranian calcareous soils at upper-, mid-, and lower-slope positions of two arid and two semiarid landscapes were determined and relations between the P release rate parameters and P uptake by wheat were investigated. The kinetic data was best described by simple Elovich equation as evidenced by the relatively higher values of determination coefficient (r2) and the relatively lower values of the standard error of estimate. Power function, parabolic diffusion and first order equations also well fitted the time-dependent P release data. The initial and final P release rates (i.e., DRin and DRf) that were estimated at reaction times of 0.25 and 72 h and the P release rate parameters derived from the Elovich (1 / β), power function (ab) and parabolic diffusion (R) equations were well-correlated with the wheat shoot dry matter yield production and the amount and/or the concentration of P in wheat tops both at an early (4 weeks) and a later (10 weeks) stages of growth confirming the importance of P release rate in plant P nutrition in the soils. The rate parameters showed positive significant relationships with clay, active calcium carbonate and citrate–bicarbonate–dithionate (CBD)-extractable Al contents of the soils. The soil properties promoting the P release rate and the P release kinetic parameters increased from the upper-slope to the lower-slope positions of the arid toposequences.