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 s eparates, 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 P-31-nuclear magnetic resonance (NMR) spec troscopy, (ii) to determine the labile P-o pool in:the size separates by an ion-exchange resin extraction, and (iii) to characterize the labile P-o poo l, We used soils from two long-term experimental: sites, one in Bavaria (un der spruce and deciduous forests, permanent grassland, and arable farming) and one in Denmark (with arable rotation and different fertilization strate gies - unfertilized, mineral fertilizer, and animal manure), Total P-o cont ent 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 ha d larger proportions of teichoic acid-P and other diester-P forms and was r icher in resin extractable P-o than clay from arable soil. There was a line ar relationship between the proportion of the P-31-NMR spectra allocated to diester-P (including teichoic acid-P) and resin-P-o Our results suggest th at the highly active and easily mineralized soil P-o was mainly associated with clay. The larger part of the day-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 o f NaOH-extractable P-o in the clay fraction was influenced to a greater ext ent by land use than bg fertilizer inputs.