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 agriculturaI 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.sub.o]), 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 [P.sub.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 I60, 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 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 [P.sub.o]. The 360[degrees]C treatment was best able to distinguish between recalcitrant and labile Po' doi: 10.2136/sssaj2009.0268