Estimating the Stability of Organic Phosphorus in Wetland Soils

The distribution of soil P among labile and nonlabile forms can be a major determinant of agricultural and natural ecosystem productivity. Determination of soil P pools is typically performed using operationally defined chemical fractionation methods. Most of the current fractionation techniques were developed for predominately mineral soils, thus they provide only limited information on organic P (P-o), particularly with respect to stability,. We hypothesized that the extent to which P could be extracted from organic soils, after exposure to hear, may be related to environmental recalcitrance. We investigated two thermal methods for characterizing P-o stability in organic wetland soils, an autoclave-based and a dry heat technique. Soils from two subtropical wetlands were collected to a depth of approximately 1 m. Autoclave-extractable P was determined by subjecting soils to 128 degrees C and 170 kPa for 90 min in an autoclave. A second set of samples was exposed to dry heat at temperatures of 160, 200, 260, 300, 360, and 550 degrees C. The results were compared with data from a conventional chemical P fractionation scheme. Phosphorus that could be extracted using the hot water technique declined with soil depth, representing 10 to 50% of total Pin surficial soils, to 5 to 10% at a depth of 60 cm. Microbial biomass P was correlated with hot water extractable P. and represented approximately 50% of the hot water extract. In the dry heat technique, increasing the extraction temperature resulted in significantly greater extraction of P-o. The 360 degrees C treatment was best able to distinguish between recalcitrant and labile P-o.