Litter dynamics and monthly fluctuations in soil phosphorus availability in an Amazonian agroforest

Commercial plantation agroforests offer a promising land-use alternative for small-scale farmers in tropical America because of their potential to produce high-value cash crops with less soil degradation than traditional annual cropping systems. In low- to no-input tree-based agroecosystems growing in tropical Ultisols and Oxisols, soil P availability depends heavily on factors that influence mineralization from decomposing litter. Using anion exchange resin membranes (AERMs), we monitored monthly fluctuations in soil solution P in an eight-year-old peach palm (Bactris gasipaes)-cupuassu (Theobroma grandiflorum) commercial plantation agroforest to determine if changes in P availability were related to environmental factors controlling organic matter decomposition (precipitation, soil moisture and temperature) and seasonal fluctuations in litterfall and fruit harvest. Decomposition and C, N, and P dynamics in leaf litter were also studied to determine if soil P availability might be related to species differences in litter quality (initial leaf C, N and P contents) and thus differing rates of P release or immobilization. Although AERMs acted as dynamic exchangers, they appeared adequately sensitive to detect fluctuations in monthly soil P availability, despite inherently low soil extractable P concentrations. Soil P availability was greatest early in the rainy season, when both litterfall and a cycle of soil-wetting and drying were initiated, decreasing during the mid-rainy season when fruit production peaked. AERM P was greatest in superficial palm root mate, where decomposing litter accumulates, and in mineral soil beneath palm litter. This corresponded with greater N and P release from more P-rich palm leaf litter. Phosphorus immobilization in initially P-poor cupuassu leaves appears to have contributed to lowered P availability in soil underlying this species' litter. Phosphorus availability was lowest in bare mineral soil located agroforest alleys where litter accumulation was minimal. Greater overall P availability in peach palm litter-covered soil and root mats may contribute to observed higher productivity in this species. (C) 2000 Elsevier Science B.V. All rights reserved.