Effect of incubation time on transformation rate and chemical forms of phosphorous in calcareous soils along a climotoposequence

Soil phosphorous fertility can be significantly affected by the P transformation rate in soils. The present study was designed to evaluate the application of time-dependent P fertilizer in terms of its availability and chemical forms in 23 soils of a climotoposequence. The collected soil samples were incubated with 200 µg g-1 P as KH2PO4 for 1, 20, 60, 100, 150, and 200 days at 25 °C and after that, the available and chemical forms of P were determined for each incubation time. A rapid decrease in available P was observed within 20 days after application of P fertilizer. A 200-day application of P led to a decrease in the available P by 15% compared to the one-day application. In addition, the biphasic pattern of transformation rate of available P fitted well to parabolic and power function models. The results of sequential extraction methods showed that in treated and untreated soils, Ca-bound P, residual P, Fe- and Al-bound P were the highest fractions, while the exchangeable form was the lowest. At the end of incubation, on average, only 15% of total P (200 μg g-1) remained as exchangeable-P, while this amount was 40.6% and 28.5% at days 1 and 20, respectively. In addition, among all soil characteristics, mineralogical properties showed the most obvious effects on controlling the chemical forms of P over time.