Characterisation of water-extractable soil organic phosphorus by phosphatase hydrolysis

Information on the chemical forms of organic phosphorus (P) in soil waters is fundamental to understanding the dynamics of soil organic P and its potential for transfer from soils to watercourses. Phosphatase enzymes were used to classify water-extractable molybdate-unreactive P (MUP) from five Australian pasture soils into compounds that could be hydrolysed by (i) alkaline phosphomonoesterase (comprising labile orthophosphate monoesters, such as sugar phosphates), (ii) a combination of phosphodiesterase and alkaline phosphomonoesterase (comprising labile orthophosphate monoesters and orthophosphate diesters, such as nucleic acids and phospholipids), and (iii) phytase (including inositol hexakisphosphate). The phosphomonoesterase and phosphodiesterase preparations were specific to the target substrates, but the phytase preparation hydrolysed all ester-P bonds. Air drying of soils increased the amounts of water-extractable MUP from between 0.15 and 0.45 μg P g−1 in extracts of moist soils to between 1.04 and 1.63 μg P g−1 in extracts of dry soils. Only small amounts of the MUP were hydrolysed by phosphomonoesterase alone (mean 5.6%), whilst a combination of phosphomonoesterase and phosphodiesterase hydrolysed much greater proportions (6–63%). This suggested the dominance of orthophosphate diesters in grassland soil solutions. The phytase preparation hydrolysed large proportions of MUP in extracts of dry soils (33–49%), suggesting the release of enzyme-hydrolysable inositol hexakisphosphate to water following the rapid rewetting of dry soils. The large proportions of MUP that remained unhydrolysed in all extracts probably consisted of microbial cell debris and high molecular weight P-containing compounds. The phosphatase technique is a simple and accurate method for determining functional classes of MUP in soil waters.