Effects of land use change on P bioavailability determined by chemical fractionation and 31P-NMR spectroscopy in a Nothofagus forest and adjacent grassland

The aim of this study was to compare P bioavailability in a Nothofagus rainforest Andisol (FS) and an adjacent clear-cut grassland soil (GS) in southern Chile to evaluate the effects of land use change on P chemical forms determined by chemical fractionation and P-31-NMR spectroscopy. Total phosphorus (P), Olsen P, microbial P, different soil P fractions (determined using a modified Hedley procedure), P-31-NMR spectroscopy results, acid phosphatase (P-ase) activity, pH and organic C were analyzed and compared. Forest samples were collected from the mineral soil at a depth of 2-20 cm and were compared with those collected from grassland soil at the same depth. Total P ranged from 2028 mg kg(-1) (FS) to 2157 mg kg(-1) (GL) and total organic P ranged from 829 mg kg(-1) (FS) to 1176 mg kg(-1) (GL). On the contrary, Olsen P, microbial P, labile P and P-ase activity were higher in the evergreen forest soil than in the grassland, with the predominance of the moderately labile (NaOH-P-o) fraction, which ranged from 668 to 720 mg kg(-1) in both soils. Phosphorus was mainly present in monoester-P form in the NMR extract in both soils (67 % on average). Other P-31-NMR signals were identified as C-2-myo-inositol phosphate and scyllo-inositol hexakisphosphate. The results suggest that land use change from forest to grassland will reduce P bioavailability and P-ase activity.