Organic phosphorus speciation in Australian Red Chromosols: stoichiometric control

Organic phosphorus (P) plays an important role in the soil P cycle. It is present in various chemical forms, the relative amounts of which vary among soils, due to factors including climate, land use, and soil type. Few studies have investigated co-variation between P types or stoichiometric correlation with the key elemental components of organic matter- carbon (C) and nitrogen (N), both of which may influence P pool structure and dynamics in agricultural soils. In this study we determined the organic P speciation of twenty Australian Red Chromosols soils, a soil type widely used for cropping in Australia. Eight different chemical forms of P were quantified by P-31 NMR spectroscopy, with a large majority (>90%) in all soils identified as orthophosphate and humic P. The strongest correlations (r(2)=0.77-0.85, P<0.001) between P types were found among minor components: (i) between two inositol hexakisphosphate isomers (myo and scyllo) and (ii) between phospholipids and RNA (both detected as their alkaline hydrolysis products). Total soil C and N were correlated with phospholipid and RNA P, but not the most abundant P forms of orthophosphate and humic P. This suggests an influence of organic matter content on the organic P pool consisting of phospholipid and RNA, but not on inositol P or the largest organic P pool in these soils - humic P.