Ecological sustainability in Amazonian agroforests: An on-farm study of phosphorus and nitrogen dynamics following native forest conversion

Raising land productivity with perennial cash crops may allow Amazonian farmers to meet food demands and increase livelihoods with less forest clearing. Despite more efficient nutrient cycling in tree-based agroecosystems, maintaining phosphorus (P) availability to plants growing in weathered tropical soils challenges the sustainability of commercial plantation agroforests. The primary objective of this research project was to examine phosphorus and nitrogen (N) dynamics in a widely-adopted peach palm (Bactris gasipaes Kunth)-cupuassu (Theobroma grandiflorum)-Brazil nut ( Bertholetia excelsa) agroforestry system to evaluate the potential of commercial agroforests to offer a more sustainable alternative to other Amazonian land-uses. The research was conducted in Acre, Brazil, using a participatory approach so that farmers would benefit from both the investigative process and study results, perhaps enabling them to maximize the agroecosystem's potential for sustained production. A comparison of soils from eight agroforests and adjacent native forests demonstrated that despite greater cation exchange capacity and pH in agroforest soils, extractable P was significantly lower, suggesting a decline in P availability since conversion of forest to agroforest. Phosphorus limitations to productivity, assessed using a root ingrowth bioassay, were not apparent, although greater root growth by peach palm suggested a competitive advantage by this species. Monthly measurements of resin-exchangeable P demonstrated greater P availability in soil beneath peach palm litter than under cupuassu trees. Nitrogen and phosphorus were mineralized rapidly from decomposing palm litter but immobilized in P-poor cupuassu leaves. Soil P availability was greatest early in the rainy season, decreasing during the mid-rainy season when fruit production was highest. An annual budget for an eight-year-old agroforest revealed that P removal with harvest was half that expected for pasture and shifting cultivation, and that system-level P cycling was more rapid than in Amazonian forests growing in similarly P-poor soils. High N removal with harvest suggests that this nutrient may eventually limit agroforest productivity. Peach palm and cupuassu phosphorus-use-efficiencies were similarly low, while that of Brazil nut was comparable to Amazonian forest species. Leguminous cover crops and directed application of soil amendments, including plant residues, beneath cupuassu and Brazil nut canopies are recommended to increase soil nutrient availability and sustain productivity.