Micro-scale assessment of soil phosphorus in aggregates

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.