Benjamin L. Turner

Characterisation of water-extractable soil organic phosphorus by phosphatase hydrolysis

Abstract 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 0.45xa0μg P g −1 in extracts of moist soils to between 1.04 and 1.63xa0μg 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.

Sampling, sample treatment and quality assurance issues for the determination of phosphorus species in natural waters and soils

Phosphorus is an important macronutrient and the accurate determination of phosphorus species in environmental matrices such as natural waters and soils is essential for understanding the biogeochemical cycling of the element, studying its role in ecosystem health and monitoring compliance with legislation. This paper provides a critical review of sample collection, storage and treatment procedures for the determination of phosphorus species in environmental matrices. Issues such as phosphorus speciation, the molybdenum blue method, digestion procedures for organic phosphorus species, choice of model compounds for analytical studies, quality assurance and the availability of environmental CRMs for phosphate are also discussed in detail.

Potential contribution of lysed bacterial cells to phosphorus solubilisation in two rewetted Australian pasture soils

Soil drying renders considerable amounts of phosphorus soluble upon rewetting, which may be partly derived from lysed microbial cells. Using direct bacterial cell counting in water and tetra-sodium pyrophosphate extracts of two Australian pasture soils, we found that almost all extractable cells were lysed following the rewetting of dry soils. The amounts of phosphorus in the lysed cells corresponded closely to the increases in water-extractable phosphorus following soil drying, suggesting that bacterial cell lysis is a major source of the released phosphorus.