Measurement of microbial biomass phosphorus in soil

A method for measuring the amount of P held in soil micro-organisms (biomass P) is described and the assumptions on which it is based are discussed. Biomass P is calculated from the difference between the amount of inorganic P (Pi) extracted by 0.5 (Spm) NaHCO3 (pH 8.5) from fresh soil fumigated with CHCl3 and the amount extracted from unfumigated soil. Some CHCl3-released Pi is sorbed by soil during fumigation and extraction: an approximate allowance for this is made by incorporating a known quantity of Pi during extraction and correcting for recovery. Most of the P released is in inorganic form and the proportion increases with duration of fumigation. Non-microbial P is little, if at all, affected by fumigation. Microbial biomass P is calculated from CHCl3-released Pi by dividing by 0.4, i.e. by assuming that 40% of the P in the biomass is rendered extractable as Pi by CHCl3. Measurements of biomass P must be done in fresh soil, CHCl3 releases much less P in air-dry soil.