Quantifying microbial biomass phosphorus in acid soils

This study aimed to validate the fumigation-extraction method for measuring microbial biomass P in acid soils. Extractions with the Olsen (0.5 XI NaH-CO3, pH 8.5) and Bray-l (0.03 M NH4F-0.025 M HCl) extractants at two soil:solution ratios (1:20 and 1:4, w/v) were compared using eight acid soils (pH 3.6-5.9). The data indicated that the flushes (increases following CHCl3-fumigation) of total P (P-t) and inorganic P (P-i) determined by Olsen extraction provided little useful information for estimating the amount of microbial biomass P in the soils. Using the Bray-1 extractant at a soil:solution ratio of 1:4, and analysing P-i instead of P-t, improves the reproducibility (statistical significance and CV) of the P flush in these soils. In all the approaches studied, the P-i flush determined using the Bray-l extractant at 1:4 provided the best estimate of soil microbial biomass P. Furthermore, the recovery of cultured bacterial and fungal biomass P added to the soils and extracted using the Bray-1 extractant at 1:4 was relatively constant (24.1-36.7% and 15.7-25.7%, respectively) with only one exception, and showed no relationship with soil pH, indicating that it behaved differently from added P-i (recovery decreased from 86% at pH 4.6 to 13% at pH 3.6). Thus, correcting for the incomplete recovery of biomass P using added P-i is inappropriate for acid soils. Although microbial biomass P in soil is generally estimated using the P-i flush and a conversion factor (k(P)) Of 0.4, more reliable estimates require that k(P) values are best determined independently for each soil.