CORRECTION FOR P-SORPTION IN THE MEASUREMENT OF SOIL MICROBIAL BIOMASS P BY CHCL(3) FUMIGATION

Microbial phosphorus in soils is commonly assessed by comparing extrac table P in chloroform-treated samples with that of parallel untreated samples. The values obtained are corrected for P sorption occurring du ring the extraction using a P spike, but sorption during the fumigatio n period is ignored. We assessed the extent to which P sorption occurr ing during the chloroform fumigation affects the measure of microbial P and propose a method that can correct for all sorption reactions occ urring during the procedure. A short fumigation period (2 h 15 min) wa s used to release the microbial P as a spike of P into the soil soluti on. This was followed by a longer equilibration period (40 h). In para llel non-fumigated samples, the soil solution was spiked with 0, 2, 5, 10, 20 or 50 mg P kg(-1), and equilibrated for the same time as the f umigated soils to characterize P sorption reactions by measuring incre ases in the solution P concentration with P additions. After the equil ibration period, the amount of microbial P is calculated by determinin g the increase of solution P caused by chloroform fumigation and corre cting this value using the P sorption curve. For different soil types cultivated with various cropping systems, the microbial P thus determi ned (P mu(srp)) was compared to the conventional measurement of the mi crobial P released by a 24 h CHCl3 fumigation followed by a 0.5 M NaHC O3 extraction (P mu(ext)). The P mu(ext) values ranged from 22 to 50% of the P mu(srp) values. Differences were explained by the soils' diff erent P sorption capacities. Consequently, P sorption occurring during the 24 h fumigation before the NaHCO3 extraction significantly affect s results, and the P mu(srp) value is a more accurate measure of micro bial P. Since the estimation of microbial P by P mu(ext) is affected b y P sorption which in turn is affected by soil management, P mu(ext) g ave an erroneous result for the effect of soil management on microbial P for one of the soils studied. This error is avoided by the sorption correction contained in the proposed P mu(srp) method. (C) 1997 Elsev ier Science Ltd.