Estimating the Stability of Organic Phosphorus in Wetland Soils

Abstract

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 (Po), particularly with respect to stability. We hypothesized that the extent to which P could be extracted from organic soils, after exposure to heat, may be related to environmental recalcitrance. We investigated two thermal methods for characterizing Po 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°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°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 P in 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 Po The 360°C treatment was best able to distinguish between recalcitrant and labile Po