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

Commercial plantation agroforests offer a promising land-use alternative fo r small-scale farmers in tropical America because of their potential to pro duce high-value cash crops with less soil degradation than traditional annu al cropping systems. In low- to no-input tree-based agroecosystems growing in tropical Ultisols and Oxisols, soil P availability depends heavily on fa ctors that influence mineralization from decomposing litter. Using anion ex change resin membranes (AERMs), we monitored monthly fluctuations in soil s olution P in an eight-year-old peach palm (Bactris gasipaes)-cupuassu (Theo broma grandiflorum) commercial plantation agroforest to determine if change s in P availability were related to environmental factors controlling organ ic 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 (ini tial leaf C, N and P contents) and thus differing rates of P release or imm obilization. Although AERMs acted as dynamic exchangers, they appeared adeq uately sensitive to detect fluctuations in monthly soil P availability, des pite 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 se ason when fruit production peaked. AERM P was greatest in superficial palm root mate, where decomposing litter accumulates, and in mineral soil beneat h palm litter. This corresponded with greater N and P release from more P-r ich palm leaf litter. Phosphorus immobilization in initially P-poor cupuass u leaves appears to have contributed to lowered P availability in soil unde rlying this species' litter. Phosphorus availability was lowest in bare min eral soil located agroforest alleys where litter accumulation was minimal. Greater overall P availability in peach palm litter-covered soil and root m ats may contribute to observed higher productivity in this species. (C) 200 0 Elsevier Science B.V. All rights reserved.