Correction for P-sorption in the measurement of soil microbial biomass P by CHCl3 fumigation

Microbial phosphorus in soils is commonly assessed by comparing extractable P in chloroform-treated samples with that of parallel untreated samples. The values obtained are corrected for P sorption occurring during the extraction using a P spike, but sorption during the fumigation period is ignored. We assessed the extent to which P sorption occurring during the chloroform fumigation affects the measure of microbial P and propose a method that can correct for all sorption reactions occurring during the procedure. A short fumigation period (2 h 15 min) was used to release the microbial P as a spike of P into the soil solution. This was followed by a longer equilibration period (40 h). In parallel 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 fumigated soils to characterize P sorption reactions by measuring increases in the solution P concentration with P additions. After the equilibration period, the amount of microbial P is calculated by determining the increase of solution P caused by chloroform fumigation and correcting this value using the P sorption curve. For different soil types cultivated with various cropping systems, the microbial P thus determined (Pμsrp) was compared to the conventional measurement of the microbial P released by a 24 h CHCl3 fumigation followed by a 0.5 m NaHCO3 extraction (Pμext). The Pμext values ranged from 22 to 50% of the Pμsrp values. Differences were explained by the soilsʹ different P sorption capacities. Consequently, P sorption occurring during the 24 h fumigation before the NaHCO3 extraction significantly affects results, and the Pμsrp value is a more accurate measure of microbial P. Since the estimation of microbial P by Pμext is affected by P sorption which in turn is affected by soil management, Pμext gave 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 Pμsrp method.