MEASUREMENT OF SOIL MICROBIAL BIOMASS PHOSPHORUS BY AN ANION-EXCHANGE MEMBRANE METHOD

A method to measure soil microbial biomass phosphorus (biomass P) in granitic soils and andosols of high P retention capacity is described. Strips of anion exchange membrane (AEM) were shaken with suspensions of soil in (1) distilled water, (2) a mixture of distilled water and alcohol free chloroform (CHCl3) liquid, and (3) a standard inorganic P solution (2.5 mg KH2PO4-Pl(-1)), to estimate the recovery of CHCl3-released P. After shaking, the AEM strips were rinsed with distilled water to remove the soils from the AEM strips. Phosphorus adsorbed by the AEM strips was then eluted by 0.5 M HCl, and determined colorimetrically. The amount of CHCl3-released P was calculated from the difference between the amount of inorganic P adsorbed by AEM in unfumigated and fumigated soils. The high P retention capacity of soils, and the dark color of the extracts solution due to humic substances did not interfere with the measurement of P by this method. Biomass P was calculated from: biomass P = Ep/Kp x 100/R, where Ep = (inorganic P released by CHCl3 and extracted by distilled water) minus (inorganic P released from unfumigated soil), Kp = Biomass P released by CHCl3 and extracted as inorganic P by distilled water (Kp = 0.40 at 25 degrees C), R = percentage recovery (82.0-91.2%) of added P. Compared to the original CHCl3-fumigation extraction (FE)-method, the AEM-method is simpler, more accurate, and more flexible, allowing assay of biomass P in soils of high P retention capacity, e.g. andosols. The use of AEM allows quick and efficient adsorption of P in the soil suspensions, and is analytically very convenient.