Components of organic phosphorus in soil extracts that are hydrolysed by phytase and acid phosphatase

Extracts were prepared from soil using water, 50 mM citric acid (pH similar to2.3) or 0.5 M NaHCO3 (pH 8.5), and wart: incubated with excess phytase from Aspergillus niger to determine the amounts of labile P. Two A. niger phytase preparations were used: (1) a purified form which exhibited a narrow substrate specificity and high specific activity against phytate; and (2) a commercial preparation (Sigma) with activity against a broad range of P compounds. A comparatively large proportion (up to 79%, or 5.7 mug g(-1) soil) of the organic P (P-o) extracted with citric acid was hydrolysed by the commercial phytase, while between 28% and 40% (up to 3.1 mug g(-1) soil) was hydrolysed using purified phytase By comparison, only small quantities of the P-o in water and NaHCO3 soil extracts were enzyme labile. While extractable P-o was increased both with increasing concentrations of citric acid (up to 50 mM) and increasing pH (pH 2.3-6.0), enzyme-labile P increased only with citric acid concentration. The labile component of P-o in citric acid extracts from soils with contrasting fertiliser histories indicated that enzyme-labile P-o is a relatively large soil P pool and is potentially an important source of P for plants.