Organic Phosphorus Forms in Agricultural Soils under Mediterranean Climate

Abstract

Core Ideas Monoesters were the dominant OP form. Their degradation occurs under conditions of P limitation. Fe oxide type was crucial for explaining the stabilization of different organic P forms in soils. Inositol hexakisphosphate was mainly related to poorly crystalline Fe oxides. Monoesters different from inositol‐6‐phosphate were related to crystalline Fe oxides and carbonates. Future sustainable phosphorus (P) management in agriculture will require a full understanding of the effects of agricultural practices on soil organic P (OP) speciation in different regions. This work studied the effects of land use, available P status, and soil properties on OP forms in Spanish agricultural soils under Mediterranean climate. We used 31P nuclear magnetic resonance spectroscopy to characterize P in eight soils under two types of land use (field crops and olive or vine orchards), using two samples with different soil Olsen P status (“low” and “high” P samples) per soil type. Orthophosphate monoesters dominated OP in these soils, and inositol hexakisphosphates (IP6) were the most abundant identified group of monoesters. Land use affected total OP concentrations mainly through crop residue incorporation. The contribution of monoesters, including myo‐IP6, to total OP increased with higher Olsen P concentrations, but only in soils from orchards. Degradation of monoesters including IP6 occurred under conditions of P limitation, suggesting some contribution of these OP forms to the plant P supply. Total IP6 was mainly related to poorly crystalline iron (Fe) oxides, whereas other monoesters were related to crystalline Fe oxides and to active Ca carbonate equivalents. The dominant Fe oxide type, carbonates, and the low accumulation of crop residues explained the relative contents of different OP forms in these Spanish soils compared with soils from other geographical areas. The effect of oxides in the accumulation of different OP forms was attributed to the affinities of these forms for oxides and to differences in the density of binding sites depending on the oxide type.