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.