Anthropogenic effect on soil organic phosphorus fractions in tropical ecosystems

Total organic phosphorus (P), labile organic P, and total carbon were deter mined in composite surface soil samples, with three replicates, in a Kandih umult from Misiones province, Argentina. The analysis was performed on <0.1 mm, 2-0.5 mm, and >8 mm aggregate fractions obtained by dry sieving. Repre sentative situations were selected to evaluate changes in P dynamics by soi l use: a native subtropical forest, a degraded agroecosystem with a 50-year yerba mate plantation (Ilex paraguariensis Saint Hill.), and a similar pla ntation with a 6-year associated elephant grass (Pennisetum purpareum Schum ach). Native pristine soil from the forest presented the highest content of phosphorus in more humified fractions (<0.1 mm). Fifty-year cultivation of "yerba mate" produced a degraded agroecosystem with 10% decrease of total organic phosphorus forms in humified fractions. Six-year-old elephant grass mulch incorporation increased total organic phosphorus both in more humifi ed fractions and in macroaggregates (>8 mm), due to addition of decomposabl e organic materials. Labile organic phosphorus was higher under stress cond itions.