Air-drying changes the distribution of Hedley phosphorus pools in forest soils

Hedley labile phosphorus(P) pools in soil tend to be several times larger than annual forest requirements, even in highly weathered soils characterized by P limitation. The discrepancy between plant and soil P status could be partly attributable to the frequently adopted air-drying pretreatment that tends to increase soil P solubility. In this study, the effects of air-drying on the distribution of Hedley P fractions were examined using soils collected under 4 forest types at Gongga Mountain, southwestern China. The results showed that the microbial biomass P(Pmic) in the organic horizon decreased markedly after air-drying. The concentrations of Hedley labile P in the air-dried samples were 31%–73% more than those in the field-moist samples. Consequently, the air-drying-induced increments of Hedley labile P pools in the surface soil horizons were 0.8–3.8 times the annual plant P requirements. The organic horizon was more susceptible to the air-drying-induced increases in Hedley labile P than the mineral horizon, probably because of the stronger release of Pmicand disruption of soil organic matter. The quality of P, indexed by the ratio of Hedley labile P to slowly cycling P, shifted in favor of the Hedley labile fractions after air-drying, further revealing that air-drying changed the distribution of Hedley P pools in forest soils. These indicated that the effects of air-drying could not be ignored when interpreting the discrepancy between the P status of plants and the Hedley labile P pools in forest soils. To more efficiently evaluate the P status in forest soils via the Hedley fractionation procedure, the use of field-moist soils is recommended.