Methods for assessing laterally-resolved distribution, speciation and bioavailability of phosphorus in soils

Supply of phosphorus (P) as an agricultural input depends on a limited mineral rock phosphate reserve, but inefficient use means this resource is not being used sustainably. Moreover, losses of P from historically accumulated soil reserves can cause substantial environmental harm. Development of sustainable P management will require optimization of P use efficiency, with this needing a better understanding of the distribution and chemical behaviour of P in soils, including both P applied as a fertilizer and native P. In this review, we discuss in-situ methods for microscopic mapping of P distribution, speciation and plant-available P fractions in soils, including scanning electron microscopy-based energy-dispersive X-ray spectroscopy, nanoscale secondary ion mass spectroscopy, X-ray fluorescence microscopy (and associated X-ray absorption spectroscopy), Raman and infrared spectroscopies, diffusive gradients in thin films coupled with laser ablation inductively coupled plasma mass spectrometry or colorimetry, and autoradiography. These methods can complement the traditional (bulk) methods for P analysis as well as deliver valuable information in their own right or in conjunction as part of an imaging cascade. We compare the merits and limitations of these techniques, which can serve to play a key role in answering questions about P behaviour in soil, from the microscopic to macroscopic scale, and provide some examples of how they may support researchers in their particular research.