Revisiting the fundamentals of phosphorus fractionation of sediments and soils

Sequential fractionation procedures have been an important tool in the study of phosphorus (P) dynamics in soils and sediments for over 50 years. Throughout this period, the various methodologies have been thoroughly reviewed, and several limitations are widely acknowledged, but there are also aspects of P fractionation that have received little or no discussion. As there are few alternatives to fractionation procedures, in order to advance the usefulness of these techniques, we need to explore the undisclosed variables that may falsely bias our interpretation. This article highlights specific fundamental components of P fractionation, including sample preparation and handling, determination of P in extracts, fraction validation and residual P. While researchers have project-specific objectives, we provide guidelines on how to minimise the influence of these confounding variables. For example, we recommend that samples be maintained and extracted under in situ moisture and oxic/anoxic conditions, especially with sediments. In addition, care needs to be taken to ensure the extraction process supports the operational definition, avoiding carryover and neutralisation between successive extractions. Despite the extensive use of these procedures, we will only be able to move away from operational definitions by validating links between extraction and reactivity, and our understanding of this key component is still limited. To provide some focus for this, we make specific recommendations on fractionation procedures to be used in the assessment of P transformations in various types of soil and sediments, dependent on the study objective.