Transformations and availability of phosphorus in three contrasting soil types from native and farming systems: A study using fractionation and isotopic labeling techniques

Background, aim, and scope: Despite the contribution of many sequential P fractionation schemes to the study of P transformations in agricultural soils, the nature of P in each fraction remains qualitative rather than mechanistic. This study used the sequential extraction and isotopic dilution techniques to assess the recovery of a tracer (32P) in soil P fractions and to elucidate the transformation of soil P in different P pools and its lability. Materials and methods: Three contrasting soils (Vertosol, Calcarosol, and Chromosol) were collected from paddocks with a long history of P fertilization and from an adjacent virgin area under native vegetation. The soils were labeled with 32P and then incubated for differing periods before being sequentially extracted for P fractions. Recovery of 32P in each P fraction was measured. Results: The P history increased total and available P in all soils but decreased phosphorus buffering capacity only in the Calcarosol. The previously applied P was distributed into all Pi fractions, and the proportion of the P transformed into individual fractions depends on soil characteristics. Adding P significantly increased the 32P recovery in the water-Pi fraction of the Calcarosol. In contrast, the higher proportion of the label was recovered in the bicarbonate-Pi of the Vertosol and in the NaOH-Pi of the Chromosol. Discussion: The recovery of 32P in all soil P fractions showed that 32P had undergone exchange with the native P. The exchange reaction was most dominant in the Pi fractions. The greater level of the 32P recovered in the water-Pi fraction of the P-amended Calcarosol indicates that the added P transformed into this fraction remains highly exchangeable. In contrast, the significantly greater amount of 32P recovered in the NaOH-Pi fraction of the Chromosol suggests that this fraction is of great importance in P fertility of this soil type. Conclusions: The transformation of soil P fraction was dependent on soil type and P fertilization history. However, during the short-term (42 days), the applied P preferably remained in the form that can be exchangeable with solution P and, therefore, can be plant-available. Recommendations and perspectives: Long-term history of P fertilization has resulted in P accumulation which is associated with an increased P availability and decreased sorption. The fertilizer P is shown to distribute into all the P fractions. Further studies are warranted to examine the accessibility of these P fractions by plants. The isotopic dilution technique using 32P has been verified to be useful for quantifying P transformation and contributes to a further understanding of P dynamics in native and farming systems. � Springer-Verlag 2009.