Transformation of phosphorus in highly calcareous soils under field capacity and waterlogged conditions

The aim of the present study was to evaluate the transformation of applied inorganic phosphorus (P) in highly calcareous soils under two moisture regimes. The experimental design was a factorial combination of two rates of P (0 and 300 mg P kg(-1) as KH2PO4) and two moisture regimes (field capacity, FC; waterlogged, WL) in a completely randomised design in duplicate with 20 surface soil samples. The fractionation sequence of inorganic P included successive extraction with NaHCO3, NH4 acetate buffer (NH4OAc), MgCl2, NH4F, NaOH-Na2CO3 (HC), Na citrate-bicarbonate-dithionite (CBD), and H2SO4 carried out 80 and 160 days after incubation. Recovery of applied P in each fraction was calculated as the difference between samples treated and untreated with P. Results indicated that NaHCO3-P decreased from 80 to 160 days, and the decrement was higher under WL than FC moisture regime. The NH4OAc-P was lower under WL than FC at 160 days, while P associated with free and crystalline Fe and Al oxides (NH4F-P, HC-P, CBD-P) was higher under WL than FC for both incubation periods. Oxalate-, citrate-, and citrate-ascorbate-extractable iron under FC and in conjunction with oxalate-and CBD-extractable aluminium and quinone-and hydroxylamine-hydrochloride-extractable manganese were the most influential factors regulating all P fractions. Results of the present study revealed that transformation of applied P into Al- and Fe-P fractions is not as low as previously reported in highly calcareous soils and that Al- and Fe-P oxides may be important in P transformation of these soils, especially in waterlogged condition.