Determination of soil microbial biomass phosphorus in acid red soils from southern China

A CHCl3 fumigation and 0.03 M NH4F-0.025 M HCl extraction procedure was used to measure microbial biomass P (P-mic) in 11 acid red soils (pH <6.0) from southern China and the results compared to those obtained by the commonly-used CHCl3 fumigation and 0.5 M NaHCO3 extraction method. Extraction with NH4F-HCl was found to be more effective and accurate than NaHCO3 extraction for detecting the increase of P from microbial biomass P following chloroform fumigation due to its higher efficiency in extracting both native labile phosphate and added phosphate (P-32) in the soils. This was confirmed by the recovery of P-32 from in situ P-32-labeled soil microbial biomass following fumigation and extraction by the NH4F-HCl solution. Soil microbial biomass P, measured by the NH4F-HCl extraction method, was more comparable with soil microbial biomass C (with a more narrow C:P ratio range of 4.3 to 22.3 and a mean of 15.6 in the microbial biomass), than that obtained by NaHCO3 solution (with a mean C:P ratio of 30.7 and a wide range of 14.9 to 48.9). K-p, the fraction of soil microbial biomass P extracted after CHCl3 fumigation, by the NH4F-HCl solution was 0.34. The amount of microbial biomass P determined (using K-p =0.34) was 3-400% (mean 131%) higher than that obtained by the NaHCO3 extraction (using K-p =0.40) for the 11 red soils studied. The results suggest that the CHCl3 fumigation and NH4F-HCl extraction method is more reliable for measuring microbial biomass P than the NaHCO3 extraction method in acid red soils.