| The microbial biomass is significant in nutrient cycling and nutrient bioavailability. The acidic, sandy soils in the southeastern U.S. are phosphorus (P) limited for forest production and commonly fertilized with P. The surface horizon has no P retention capacity, resulting in possible leaching of P fertilizer, so the microbial biomass may be a significant sink and source of labile P for plant uptake in these soils. The ability of the microbial biomass to immobilize and retain P fertilizer before it is leached below seedling rooting depth would determine how important it might be as a source of bioavailable P. Spodosols in the southeastern U.S. have a fluctuating water table that influences the water potential of the surface horizon. Microbial communities are linked to water potential and so may influence microbial immobilization of P fertilizer. An accurate estimate of microbial P is dependent upon a correct K p factor that may also be affected by water potential.; The extractant, 0.5 MNaHCO3 (pH 8.5), used to measure microbial P was questioned for use on these acidic soils. Kp factors were measured using six extractants, 0.5 MNaHCO3, Mehlich 1, Bray and Kurtz, and 1 mM, 2 mM, and 3 mM oxalate, at five water potentials ranging from -0.1 to -1000 kPa in the A, E, and Bh horizons. The superior extractant for estimating microbial P in this soil was determined to be 3 mM oxalate. Kp factors were determined to change with soil horizon and soil water potential.; Five treatments, control, 30, 60, and 100 kg P ha-1, and a DAP treatment, were added to the A horizon at five water potentials. Microbial P and microbial biomass were measured over time until 77 days after fertilizer addition. The microbial biomass in this soil is not P limited but is N limited. After fertilization, up to three or more times the P than the annual demand by pine plantations was sequestered and held for several weeks. Water potential did not affect P immobilization. Microbial P would become slowly available to plants over time, possibly after fertilizer is leached from the rooting zone. |
