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

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 (P-32) in soil P fractions and to elucidate the transformation of soil P in different P pools and its lability. 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 P-32 and then incubated for differing periods before being sequentially extracted for P fractions. Recovery of P-32 in each P fraction was measured. 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 P-32 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. The recovery of P-32 in all soil P fractions showed that P-32 had undergone exchange with the native P. The exchange reaction was most dominant in the Pi fractions. The greater level of the P-32 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 P-32 recovered in the NaOH-Pi fraction of the Chromosol suggests that this fraction is of great importance in P fertility of this soil type. 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 P-32 has been verified to be useful for quantifying P transformation and contributes to a further understanding of P dynamics in native and farming systems.