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 Kp 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 Kp factor; and to test whether the Kp 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 Kp factor was dependent on soil water potential and horizon. The range in Kp 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 Kp factors tended to be at water potentials near saturation and under the driest condition. Differences in Kp were attributed to the influence that water potential and soil horizon had on microbial assemblages and diversity. Using a literature value of Kp, instead of measuring Kp 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 Kp be evaluated for specific soil conditions.