Isolation and characterization of labile organic phosphorus pools in soilsfrom the Askov long-term field experiments

Labile soil organic phosphorus (P-o) plays a crucial role in plant P nutrit ion and in environmental eutrophication. This paper discusses recent studie s on the nature of labile soil P-o and its response to different fertilizat ion practices. Soil material was obtained from the Askov long-term experime nt on animal manure and mineral fertilizers. Our analytical approach combin ed a macroporous anion exchange resin to isolate labile P-o and P-31 NMR sp ectroscopy to assess the chemical composition and origin of NaOH-extractabl e P-o. The analyses were carried out on fine earth (< 2 mm) and particle-si ze fractions. The results suggest that the resin extraction isolates an act ive pool of soil P, that consists primarily of microbially-derived compound s. The size of the active P-o pool reflects seasonal variations and P ferti lization. However, the, source of added P (NPK vs. animal manure) appears t o have only little influence on the labile P-o. Our results further suggest that most of the active soil P-o is associated with clay sized separates ( < 2 mu m), indicating this fraction being important in the short-term turno ver of P-o.