P Speciation of German Forest Soils on Different Parent Materials As Assessed By Synchrotron-Based K-Edge XANES Spectroscopy.

There is increasing evidence of an insufficient P nutritional status of forest trees and forest ecosystems in Europe. Soils are the hub of P cycling in forest ecosystems, and the P nutritional status of forests is strongly dependent on the soil P status. Forest P nutrition depends not only on the total P content of soils, but also on soil P speciation. In principle, soil P can be speciated directly by modern methods like synchrotron-based K-edge X-ray absorption near-edge (XANES) spectroscopy. However, XANES spectra of samples with P concentrations <1 mg g-1 in most studies have been of poor quality, precluding reliable P speciation based on these spectra. Forest soils often have P concentrations < 1 mg g-1, and therefore P speciation in forest soils is mostly performed by sequential extraction and assignment of the different P fractions to distinct P species. In our study, for ten German forest soils with different geological parent materials, P contents (0.1 up to 2.0 mg g-1), and stage of pedogenesis, the P speciation was assessed by K-edge XANES spectroscopy. Using a beamline with a particularly powerful detector system and applying a recently-developed protocol for accurate and reproducible spectrum deconvolution, we were able to yield spectra with excellent signal-to-noise ratios and reliable P speciation results also for soil samples with total P contents between 0.2 and 1 mg g-1. This opens new perspectives on the application of synchrotron-based XANES spectroscopy on soil samples with low P contents, such as forest soils and agricultural soils without P fertilizer amendment. We present new information on the content of different P species in typical German forest soils and on relationships between their P speciation and other important physic-chemical properties (e.g. content of SOM, pedogenic Fe and Al oxyhydroxides, carbonate) of the respective soils.