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 (P_o) and inorganic (P_i pools. Carbon content ranged from 12.5–118.1 g kg^?1 and total P (P_i) content from 511 to 2063 mg kg^?1. For all soils, the C:P_o 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 P_o lability. Analysis of functional relation showed a close relation between resin P_o and ³¹P-NMR estimates for diester-P including teichoic acid-P, indicating that these species contributed significantly to the labile P_o pool as determined by the resin method. The most marked effects of land-use were an increase in P_i under grass and arable, a concurrent sequestration of P_o and SOM under grass, and a depletion of P_o under arable. The amount of resin P_j 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 P_j values. The soil under grass had the largest amounts in resin P_o and diester-P including teichoic acid-P, indicating a rapid turnover of P_o with build-up of a large potentially labile, microbially derived P_o pool. ³¹P NMR also showed large proportions of labile P_o 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 P_o.