SHORT-TERM CHANGES IN THE MOLYBDATE REACTIVE PHOSPHORUS OF STORED SOIL WATERS

Precise determination of molybdate reactive phosphorus (MRP) in waters is needed to accurately quantify sources of bioavailable phosphorus (P) entering into fresh water systems. Leachate waters from soils were collected from lysimeters to study the short-term stability of MRP during sample storage from 2 h up to 32 d. The effects of different chemical preservatives (iodine, sulfuric acid, and mercury) were different in different soil water types, sometimes causing an increase in MRP a few days after storage. The effects of storage environment (refrigeration, freezing, outside, darkness, and in ambient laboratory conditions) also varied between soil types, with the least changes noted in samples stored in the ambient laboratory conditions and under refrigeration. Larger vessels (25 L) retained MRP more effectively than smaller vessels (30 mt) and teflon-polytetrafluoroethylene (PTFE) was the vessel material preferred to polyethylene and polythene. The mechanisms responsible for the changes are thought to be a complex interaction between biological (microbial incorporation/release), chemical (adsorption/desorption and hydrolysis), and physical (presence/absence of particle material) factors. The conclusions were that different waters behave differently, that losses are rapid (hours and days) and that rapid analysis is essential to provide a correct assessment of water quality.