Can isotopic exchange kinetics give valuable information on the rate of mineralization of organic phosphorus in soils?

Although mineralization of soil organic phosphorus can play an important role in plant nutrition, its measurement has been impeded by methodological difficulties. Our objective was to show whether the determination of isotopic exchange kinetics could give information on the net mineralization of organic P compounds in soils. Using this technique, the mineralization of two organic phosphates (adenosine triphosphate, ATP, and nicotinamide adenine dinucleotide, NAD) was studied in an alfisol during an incubation. Parameters describing the isotopic exchange kinetics, r(1)R, n and cp, were used to calculate the quantity of phosphate ions (P) present in the pool of free ions and in the pool of P isotopically exchangeable between 1 min and 1 d. In addition resin-extractable P was measured in the same samples. Following the additions of ATP and NAD, P was rapidly released to the soil solution increasing the P content of the pool of free ions which subsequently decreased until the end of the incubation. Mineralized P in the soil amended with ATP or NAD and remaining either water-extractable or in the pool of free ions was positively correlated to the quantity of mineralized P which remained resin-extractable. The isotopic method measured the quantity of mineralized P which remained immediately plant-available. The changes in the P content of the pool of P exchangeable between 1 min and 24 h could not be related to organic-P mineralization. This lack of a relationship was related to variations in the n value which could have originated in organic-P mineralization during the isotopic exchange experiment itself or in sample heterogeneity. Therefore when studying soil organic-P mineralization with our proposed technique great caution should be taken before extrapolating the quantity of isotopically exchangeable phosphate to isotopic exchange times longer than 1 min.