A Biologically-Based Approach to Evaluating Soil Phosphorus Availability Across Complex Landscapes.

Plants employ a range of strategies to increase phosphorus (P) availability in soil. Current soil P extraction methods (e.g. Olsen P), however, often fail to capture the potential importance of rhizosphere processes in supplying P to the plant. This has led to criticism of these standard approaches, especially in non-agricultural soils of low P status and when comparing soil types across diverse landscapes. Similarly, more complex soil P extraction protocols (e.g. Hedley sequential fractionation) lack functional significance from a plant ecology perspective. In response to this, we present a novel procedure using a suite of established extraction protocols to explore the concept of a P protocol based on biologically significant P pools, fluxes and transformations. The biologically based P (BBP) extraction was conducted by using four extractions in parallel:  (1) 10 mM CaCl₂ (soluble P); (2) 10 mM citric acid (chelate extractable P); (3) phytase and phosphatase solution (enzyme extractable organic P); (4) 1 M HCl (mineral occluded P). To test the integrated protocol, we conducted the analyses on a total of 204 soil samples collected as part of a UK national ecosystem survey (Countryside Survey) in 1998 and repeated again in 2007. In the survey, Olsen P showed a net decline in national soil P levels during this 10 year period. In accordance with these results, soluble P, chelate extractable P and mineral occluded P were all found to decrease over the 10 year study period. In contrast, enzyme extractable organic P increased over the same period likely due to the accumulation of P as organic inputs to the mineral soil. This new method is simple and inexpensive and therefore has the potential to greatly improve our ability to characterise and understand changes in soil P status across complex landscapes