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Soil Biology & Biochemistry

A Comparison of Four Methods of Measuring Gross Phosphorus Mineralization

^a Dep. of Ecology, Evolution, and Behavior, Univ. of Minnesota, 100 Ecology, Saint Paul, MN 55108
^b Center for Ecology and Evolutionary Biology, 5289 Univ. of Oregon, Eugene, OR 97403-5289
^c Laboratorio de Estudios Ambientales, Instituto de Zoología Tropical, Facultad de Ciencias, Universidad Central de Venezuela, Apartado 47058, Caracas 1041-A,Venezuela

^* Corresponding author (kello036{at}gmail.com)

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.

Abbreviations: AC, sodium azide plus autoclaving treatment incubated for 1 h in the Z method, correcting for lysing of microbial cells • E, isotopically exchangeable P • I, immobilization rate • KB, isotope-dilution method of Kirkham and Bartholomew adapted for gross P mineralization • LH, isotope-dilution method for gross P mineralization of López-Hernández et al.) • M, mineralization rate • MA, sodium azide plus autoclaving treatment incubated for 24 h in the Z method, representing the solubilization of inorganic P • N, slope of ln (³²P_i,t/³²P_a) versus ln t in the model of ³²P adsorption in the LH method • PM, sodium azide treatment in the Z method, representing solubilization of inorganic P and the mineralization of organic P by exoenzymes but no immobilization by the microbial community • SA, specific activity • slope, slope of ln of SA of P_i+m pool versus t in WV method • t, time • WV, isotope dilution method for gross P mineralization method of Walbridge and Vitousek (1987) • X, correction term for adsorption of ³²P_i in WV method • Z, differential extraction method for gross P mineralization of Zou et al.