Determination of microbial phosphorus Kp factors in a spodosol:: influence of extractant, water potential, and soil horizon

The acidic, sandy soils in the southeastern US are phosphorus (P) limited for forest production and are commonly fertilized with P. There is no P retention capacity in the A horizon. However, microbial biomass may immobilize and retain P fertilizer before it is leached below seedling rooting depth making P fertilization more efficient. An accurate estimation of microbial P is dependent on measuring the K-p factor in the fumigation-extraction method. The overall purpose of this study was to examine the fumigation-extraction method for microbial P in acidic, forested, sandy soils. The three objective were: to determine which extractant was the most useful extracting microbial P by comparing the standard basic extractant, 0.5 M NaHCO3 at pH 8.5, against several acidic and oxalate extractants; to evaluate whether soil water potential influenced the K-p factor; and to test whether the K-p factor differed by soil horizon within the profile of a representative Flatwoods Spodosol. Three millimolar oxalate was determined to be the preferred extractant due to its efficient removal of microbial P and ease of analysis. The K-P factor was dependent on soil water potential and horizon. The range in K-p at different water potentials using 3 mM oxalate was from 0.31 to 0.67 in the A horizon, 0.48 to 0.91 in the E horizon, and 0.22 to 0.45 in Bh horizon. The highest K-P factors tended to be at water potentials near saturation and under the driest condition. Differences in K-p were attributed to the influence that water potential and soil horizon had on microbial assemblages and diversity. Using a literature value of K-p, instead of measuring K-p directly, caused an overestimate of -7 to 63% in the A horizon, 63-160% in the E horizon and 7-32% in the Bh horizon. The best estimate of microbial P required that K-P be evaluated for specific soil conditions. (C) 2004 Elsevier Ltd. All rights reserved.