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) w as used to evaluate the amounts of P in several soil chemical pools in an o ld, 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 th e deposition of anthropogenic chemicals. Two study watersheds are conifer-d ominated with moorland on wind-exposed ridgetops. In a third study watershe d, vegetation is dominated by evergreen broadleaf trees. Soils are thin (ca . 40 cm) and have a high organic matter content. Across all communities, mo st of the soil P is in non-labile forms in organic combinations or in combi nation with secondary soil minerals. Little P was present in primary minera ls. The remainder (ca. 20%) was in labile forms extractable with anion exch ange resin or bicarbonate solution. From litterfall and allometric relation ships, 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 fores ts. 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 ava ilable to plants. Resin-extractable P was strongly correlated with soil car bon 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 a nnual P requirement, suggesting little control of available P pools by fore st vegetation type. On a more detailed level, resin-extractable P was stron gly correlated with HCO3-extractable organic (and inorganic) P. This is con sistent with other findings of P behavior in acid soils high in organic mat ter in which microbial transformations are key in regulating pools of plant -available P.