Land‐use and fertilization effects on P forms in two European soils: resin extraction and 31P‐NMR analysis

Macroporous anion‐exchange resin extraction and 31P‐NMR spectroscopy of dialysed NaOH extracts were used to investigate the effects of land use (Taubenberg, Bavaria: spruce forest, deciduous forest, permanent grass, arable) and fertilization (Askov, Denmark: unmanured, mineral fertilizer, animal manure) on forms of phosphorus in soil with emphasis on the potentially labile organic (Po) and inorganic (Pi pools. Carbon content ranged from 12.5–118.1 g kg−1 and total P (Pi) content from 511 to 2063 mg kg−1. For all soils, the C:Po ratios of SOM decreased in the order: whole soil, 150: 1–44:1; alkali extract, 57:1–41: 1; resin extract, 20:1–9:1; suggesting an increasing P functionality of the OM with increasing Po lability. Analysis of functional relation showed a close relation between resin Po and 31P‐NMR estimates for diester‐P including teichoic acid‐P, indicating that these species contributed significantly to the labile Po pool as determined by the resin method. The most marked effects of land‐use were an increase in Pi under grass and arable, a concurrent sequestration of Po and SOM under grass, and a depletion of Po under arable. The amount of resin Pj appeared to be a function primarily of fertilization, and amounted to around 100 mg kg−1 in the fertilized soils irrespective of the SOM content and P source. The forest soil and the unfertilized agricultural soil had much smaller resin Pj values. The soil under grass had the largest amounts in resin Po and diester‐P including teichoic acid‐P, indicating a rapid turnover of Po with build‐up of a large potentially labile, microbially derived Po pool. 31P NMR also showed large proportions of labile Po species in soils where microbial activity is restrained by acidity (Taubenberg spruce forest, phosphonates) or where highly microbially altered OM is relatively enriched (Taubenberg arable, diester‐P including teichoic acid‐P). We conclude that the resin used in this study isolates a structurally and functionally reasonably uniform pool of potentially labile soil Po.