Soil phosphorus availability and fine root proliferation in Amazonian agroforests 6 years following forest conversion

In the Amazon Basin, where deforestation rates are among the highest in the world, raising land productivity with perennial crop-based agroforestry sy stems may allow small farmers to meet economic demands with less forest cle aring. Although tree-based agroecosystems may cycle nutrients more efficien tly than other agricultural systems, difficulties maintaining phosphorus (P ) availability to plants growing in tropical Ultisols and Oxisols may threa ten the long-term sustainability of Amazonian agroforests. To determine how soil nutrient dynamics are altered when primary forest is converted to per ennial crops, soils from eight 6-year-old peach palm (Bactris gusipaes Kunt h)-cupuassu (Theobroma grandiflora) plantation agroforests were compared to those of adjacent native forests in Acre, Brazil. The response of fine roo ts to P microsite enrichment by agroforest and native forest plants on eigh t farms was measured using a root ingrowth bioassay to determine if a P-lim itation already threatened agroforest sustainability. Localized root prolif eration by individual agroforest species was also examined as a component o f interspecific competition. The impact of agroforest adoption by small far mers on forest clearing was qualitatively examined through interviews and f ocus groups held with farmers participating in the field studies. Six years after establishment on sites initially occupied by native terra firme fore st, exchangeable base cations, ECEC and pH were greater in agroforest soils than in those of adjacent forests. Extractable inorganic P was 30-50% lowe r in agroforest soil, suggesting that P uptake by the aggrading agroecosyst em outpaced its restoration in the soil solution by other P pools. After 10 0 days, cupuassu root length and mass were greater in P-treated cores burie d in agroforest alleys, but not in cores buried in rows. 'While a P deficie ncy in any of the agroforest components could not be inferred using the roo t ingrowth technique, the study of root proliferation demonstrated both the potential for interspecific competition among peach palm and other agrofor est components. as well as opportunities for alleviating the impact of comp etition through management practices. Discussions with farmers revealed tha t this type of commercial plantation agroforest, unless managed to maintain productivity, may actually contribute to more forest clearing on small far ms because of a lack of farmer confidence in the future of the system. Thus , despite more efficient nutrient cycling in tree-based agroecosystems, the decline in agroforest soil P following establishment must be addressed thr ough management practices if plantation agroforests are to remain a sustain able land use in Amazonia. (C) 2001 Elsevier Science B.V. All rights reserv ed.