Nitrogen and phosphorus cycling in an Amazonian agroforest eight years following forest conversion

Commercial plantation agroforestry systems have emerged as a promising Amaz onian land use with the potential to reduce soil degradation, improve livin g standards, and decrease pressures on remaining forested areas. While it i s generally accepted that tree-based agroecosystems have more closed nutrie nt cycles that help conserve soil productivity, few studies of nutrient dyn amics in Amazonian agroforestry systems exist, making it difficult to asses s the extent to which this land use-is sustainable in the region's weathere d soils. We quantified net primary productivity (NPP) and the stocks and fl uxes of carbon (C), nitrogen (N), and phosphorus (P) over one year in a com mercial plantation agroforestry system in Acre, Brazil, to determine how or ganic matter and N and P dynamics change when native Amazonian forest is co nverted to perennial crops, and how nutrient cycling in tree-based agroecos ystems compares with other Amazonian land uses. We used a participatory app roach that involved farmers throughout the research process, with the aim o f stimulating a community dialogue on the importance of nutrient cycling dy namics to sustained agricultural production. As part of this process, manag ement recommendations to protect and enhance nutrient cycling in a manner p ractical for resource-limited households were formulated by farmers during focus group discussions. Construction of a nutrient budget revealed that th e system's largest elemental stocks were located in the top 20 cm of soil. By the ninth year following establishment, agroforest productivity was high , with annual NPP approaching 3000 g.m(-2).yr(-1). Approximately 80% of the annual N and P requirements were taken up from soil stocks. Nearly 40% of the annual aboveground P requirement was allocated to reproductive tissues, half of which was removed with the harvest of agroforest products. Removal of P was half as much as would be expected for a first-year annual crop gr own in a shifting cultivation system, while N export was similar to that of annual crops. Inputs of N and P to the agroforest were negligible, and res orption of N and P from agroforest leaves was comparable to rates cited for mature forest growing on similarly nutrient-poor soils. Return of P to the soil in litterfall exceeded that reported for mature Amazonian forests, de spite similar rates of litter production and N flux, rendering agroforest P -use efficiency considerably lower than that reported for native vegetation growing on equally P-poor soils. Overall, NPP and rates of P cycling in th e 8-yr-old agroforest were high relative to Amazonian forests, but low soil contents of total N and extractable P, as well as moderate rates of N and P removal, suggest that this land use will require inputs of both nutrients to sustain system productivity in the future. Discussions with farmers ind icated that planting and pruning N-fixing legumes was a feasible way to add N, but the efficient use of scarce inorganic amendments would require furt her research.