Measurement of microbial biomass phosphorus in rhizosphere soil

³²P-labelled monocalcium phosphate solution was supplied by point injection to the root system of wheat plants grown in soil cores in a controlled environment. There was no detectable incorporation of³²P into organic P fractions in the soil remaining after roots were removed, confirming field observations. The techniques used to measure organic P (including biomass P) could detect an incorporation of³²P into soil microbial biomass equivalent to 0.3 μgP.g^?1 soil, compared to a total soil biomass P content estimated to be ca. 6.5 μgP.g^?1 soil. The limited incorporation of the added P into microbial biomass in the root-free soil may be due partly to a limited diffusion of³²P into the non-rhizosphere soil and partly to the removal of³²P-labelled microbial biomass adhering to or in very close association with the root surface. it is proposed that in studies of soil nutrient status, total soil biomass P (roots + soil flora + microfauna) should be measured, rather than attempting an estimate of microbial P. A sequential extraction procedure using a single soil sample, where a biocide is added to the extracting solution, is proposed as an alternative to the conventional procedure for measuring soil biomass P where two soil samples, one treated with a biocide, are extracted simultaneously.