Phosphorus sustainability in tropical ecosystems

Highly weathered soils in the tropics display nutrient deficiencies, especially of phosphorus (P) in low-activity clay soils. The objective of this work is to evaluate P distribution in soil fractions (> 8 mm, 2–8 mm, 0.25–2 mm, 0.10–0.25 mm, < 0.10 mm) and to study the effect of management practices on P sustainability and availability. Surface and subsurface soil samples were taken from three different situations: virgin soil, degraded plantation of Ilex paraguariensis Saint Hil. (yerba mate), and a similar plantation consociated with Pennisetum purpureum Schumach (elephant grass), used as a mulch.For virgin soils, the highest P values were present in the < 0.1 mm fraction, where the organic-clay complex is more stabilized. Degraded agroecosystem presented low phosphorus values in the lines of the crops (rows) due to extraction of foliar nutrients with the harvests. Interrows (spaces between the lines of the crop) also had low P due to erosion phenomena. The influence of elephant grass was to increase P values in all fractions. This effect was remarkable in the >8 mm fractions; macroaggregates P is associated with decomposable material which is related to short-term fertility. Its beneficial action was also observed below the soil surface. Protecting the soil with mulch improved P sustainability in ‘yerba mate’ agroecosystems.