Organic Phosphorus in Soil Size Separates Characterized by Phosphorus‐31 Nuclear Magnetic Resonance and Resin Extraction

Land use and soil management affect soil organic C in whole soil and size separates, but knowledge of the accompanying soil organic P (P o ) is limited. The objectives of this study were (i) to identify the structure of P o in soil size separates by solution 31 P‐nuclear magnetic resonance (NMR) spectroscopy, (ii) to determine the labile P o pool in the size separates by anion‐exchange resin extraction, and (iii) to characterize the labile P o pool. We used soils from two long‐term experimental sites, one in Bavaria (under spruce and deciduous forests, permanent grassland, and arable farming) and one in Denmark (with arable rotation and different fertilization strategies — unfertilized, mineral fertilizer, and animal manure). Total P o content increased with decreasing particle size. The dialyzed NaOH extracts of clay were enriched in microbial‐derived teichoic acid‐P and other diester‐P forms compared with silt and sand. Clay from permanently vegetated soil had larger proportions of teichoic acid‐P and other diester‐P forms and was richer in resin extractable P o than clay from arable soil. There was a linear relationship between the proportion of the 31 P‐NMR spectra allocated to diester‐P (including teichoic acid‐P) and resin‐P o Our results suggest that the highly active and easily mineralized soil P o was mainly associated with clay. The larger part of the clay‐associated P o was tightly bound and not extractable. Although the composition of this P o remained unknown, it was probably inaccessible to rapid microbial utilization. The composition of NaOH‐extractable P o in the clay fraction was influenced to a greater extent by land use than by fertilizer inputs.