Soils comprise various heterogeneously distributed pools of lithogenic, free organic, occluded, adsorbed, and precipitated phosphorus (P) forms, which differ depending on parent material, soil depth, and stage of pedogenesis. Small-scale heterogeneity of element distributions recently has received increased attention in soil science due to its influence on soil functions and soil fertility. We investigated micro-scale P distribution and binding forms in siliceous soil aggregates by combining advanced spectrometric and spectroscopic techniques to introduce new insights on P accessibility and availability in soils. Here we show that soil substrate and soil depth determine micro-scale P heterogeneity in soil aggregates. At the P-rich areas in all investigated soil aggregates, P was predominantly co-located with aluminium and iron oxides and hydroxides, which are known to strongly adsorb P. Clay minerals were co-located with P only to a lesser extent. In the low-P topsoil, more than half of the P was bound organically, probably as a result of retarded organic P mineralization. Phosphorus minerals predominated in the quartz-rich low-P subsoil aggregate. Sorbed and mineral P phases determined P speciation in the high-P top- and subsoil and apatite was only detected in the high-P subsoil aggregate. Our results indicate that micro-scale spatial and chemical heterogeneity of P influences P bioavailability.

Supplement to: Werner, Florian; Mueller, Carsten W; Thieme, Jürgen; Gianoncelli, Alessandra; Rivard, Camille; Höschen, Carmen; Prietzel, Jörg (2017): Micro-scale heterogeneity of soil phosphorus depends on soil substrate and depth. Scientific Reports, 7(1)