Comparison of Four Methods of Measuring Gross Phosphorus Mineralization

This study compared three previously published methods of estimating gross P mineralization: (i) an isotope dilution method that relies on specific activity, (ii) a later isotope dilution method that uses isotopically exchangeable P as the response variable, and (iii) a differential extraction method. We adapted the isotope dilution method (KB) commonly used for gross N mineralization for gross P mineralization. We evaluated two methods used to correct for adsorption of ³²P: sterilized soil incubations and a simulation model. Finally, we examined the necessity of including microbial biomass P as a component of labile P for the isotope dilution methods. The three previously published methods gave highly variable estimates of gross P mineralization, and our data suggest that critical assumptions of each method were violated. We recommend the KB method because its assumptions were generally met and it requires no sterilized samples. The KB method represents net mineralization because there is no correction for adsorption/desorption, which we have shown to be complex and difficult to interpret in nonsterilized samples. Modeled and estimated adsorption were often different, and relative differences varied among soil types. We also recommend combining the extractable inorganic P and microbial biomass P fractions into a single "labile" pool for isotopic-dilution studies and that the incubations are run over no more than 3 to 5 d. Although the KB method represents a conservative estimate of P mineralization as net P mineralization, it corresponds to a useful indicator in comparing potentially any soil type.