Phosphorus fractions in montane forest soils of the Cordillera de Piuchue, Chile: biogeochemical implications

The Hedley fractionation procedure as modified by Tiessen and Moir (1993) was used to evaluate the amounts of P in several soil chemical pools in an old, unglaciated landscape at 600 m elevation in the Cordillera de Piuchue, Chile (42 degrees 30' S. 74 degrees W). This is an area of primary forests which have escaped disturbance from forest harvesting, land clearing and the deposition of anthropogenic chemicals. Two study watersheds are conifer-dominated with moorland on wind-exposed ridgetops. In a third study watershed, vegetation is dominated by evergreen broadleaf trees. Soils are thin (ca. 40 cm) and have a high organic matter content. Across all communities, most of the soil P is in non-labile forms in organic combinations or in combination with secondary soil minerals. Little P was present in primary minerals. The remainder (ca. 20%) was in labile forms extractable with anion exchange resin or bicarbonate solution. From litterfall and allometric relationships, we estimated the annual P requirement of growing vegetation to be <1 kg ha(-1) in the moorland and < 3 kg ha(-1) in the conifer and mixed forests. This is substantially less than the standing pool of resin-extractable P (ca. 20 kg ha(-1)), which is considered to be P fraction most readily available to plants. Resin-extractable P was strongly correlated with soil carbon content (R-2 =0.72 0.87, p < 0.001) suggesting that soil organic matter is the likely proximate source of plant-available P. On a kg ha(-1) basis, the most labile forms of P did not differ significantly across 3 of the 4 community types despite dramatic differences in species, live biomass and annual P requirement, suggesting little control of available P pools by forest vegetation type. On a more detailed level, resin-extractable P was strongly correlated with HCO3-extractable organic (and inorganic) P. This is consistent with other findings of P behavior in acid soils high in organic matter in which microbial transformations are key in regulating pools of plant-available P.