Phosphorus association with phyllosilicates and organic matter in temperate soils

Aluminum (Al) and iron (Fe) (hydr)oxides have been considered the primary phosphorus (P) sinks in many acidic soils. However, low contents of Al/Fe (hydr)oxides and their limited pH-dependent sorption capacity in temperate soils with pH 5.5-7 question their dominant role in P retention and suggest the importance of other P retention pathways. This study presents a newly developed nanoscale secondary ion mass spectrometry (NanoSIMS) method that spatially and operationally identifies inorganic phosphorus (IP) and organic phosphorus (OP) in natural soils by integrating 31 P 16 O 2 - /( 31 P - + 31 P 16 O 2 - ) and 12 C 14 N - /( 12 C - + 12 C 14 N - ) ratios. This method enables mapping IP and OP distributions on natural soil microaggregate surfaces. Our results exhibit that phyllosilicates and their associated OM spatially associated with over 85% of total P observed on microaggregate surfaces in four loamy, slightly acidic temperate soils in Southeast Germany. The association of phyllosilicates with OM doubles the P retention density (the proportion of surface area occupied by P within each mineral/OM constituent), highlighting the important role of phyllosilicate-OM associations in P retention beyond the individual P retention capacity of OM-free phyllosilicates. Spatial mapping revealed two contrasting P associations: (i) IP and myo -inositol hexaphosphate predominant on phyllosilicate surfaces with minimal N signals, and (ii) OP predominates in phyllosilicate-associated OM in the form of N-rich microbial biomass/necromass and plant residues. These distinct spatial patterns may imply potentially different microscale P associations, although their functional significance for soil P cycling requires further investigation with complementary spectroscopic methods. Our study demonstrates the spatial importance of phyllosilicates and phyllosilicate-associated OM for P retention through two distinct P association patterns in slightly acidic temperate soils with low Al/Fe (hydr)oxide contents. Establishing the functionality of these spatial patterns in controlling soil P availability requires further research integrating molecular speciation, desorption kinetics, and mineralization monitoring. • NanoSIMS spatially identifies organic (OP) and inorganic phosphorus (IP) patterns • Phyllosilicates associate >85% of total phosphorus (P) on microaggregate surfaces • Phyllosilicates associated with organic matter (OM) double P retention density • OP is primarily concentrated in nitrogen-rich OM • P retention dominates by phyllosilicate–OM associations in pH 5.5-7 temperate soils