Organic phosphorus mineralization studies using isotopic dilution techniques

Soil organic P (P-o) mineralization is an important process in P cycling. No accurate method for its quantification is available because any mineralized inorganic P (P-l) may be rapidly sorbed onto the soil solid phase where it cannot be separated from already present P-l. A method for measuring soil P-o mineralization is explored using isotopic dilution techniques under conditions of constant soil respiration rates. First, the specific activity (SA) as affected by physicochemical processes was extrapolated from an isotopic exchange kinetics batch experiment, Second, the SA was assessed during incubation after labeling soil with (PO4)-P-33 Lower SA measured during incubation than extrapolated from the batch experiment was attributed to the release of nonlabeled P-l due to mineralization of nonlabeled P-o. In order to separate biological from biochemical mineralization processes, one set of samples was gamma -irradiated to stop the microbial activity while maintaining phosphatase activity. The gamma -irradiated soil revealed higher mineralization rates than the corresponding nonirradiated soil. This was explained by an increase of the amount of easily mineralizable P-o derived from killed microbial cells by gamma -irradiation, Consequently, a gross, but overestimated, biochemical P mineralization can be assessed. In the nonirradiated soil, mineralization not only of nonlabeled, but also of recently synthesized labeled P-o resulting hum microbial turnover, may occur, Thus, in the nonirradiated soil, after several days a gross, biologically and biochemically mediated mineralization is increasingly underestimated. During the first 7 d, the mineralization rate in the nonirradiated soil was 1.7 mg P kg(-1) d(-1), which is an amount approximately equivalent to soil solution P in this soil, indicating that soil P mineralization is a significant process in delivering available P-l.