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

摘要

Commercial plantation agroforestry systems have emerged as a promising Amazonian land use with the potential to reduce soil degradation, improve living standards, and decrease pressures on remaining forested areas. While it is generally accepted that tree-based agroecosystems have more closed nutrient cycles that help conserve soil productivity, few studies of nutrient dynamics in Amazonian agroforestry systems exist, making it difficult to assess the extent to which this land use-is sustainable in the region's weathered soils. We quantified net primary productivity (NPP) and the stocks and fluxes of carbon (C), nitrogen (N), and phosphorus (P) over one year in a commercial plantation agroforestry system in Acre, Brazil, to determine how organic matter and N and P dynamics change when native Amazonian forest is converted to perennial crops, and how nutrient cycling in tree-based agroecosystems compares with other Amazonian land uses. We used a participatory approach that involved farmers throughout the research process, with the aim of stimulating a community dialogue on the importance of nutrient cycling dynamics to sustained agricultural production. As part of this process, management recommendations to protect and enhance nutrient cycling in a manner practical for resource-limited households were formulated by farmers during focus group discussions. Construction of a nutrient budget revealed that the 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 grown 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 resorption 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, despite 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 the 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 indicated that planting and pruning N-fixing legumes was a feasible way to add N, but the efficient use of scarce inorganic amendments would require further research. [References: 79]