The Effect of Management and Canopy Tree Functional Type on Nutrient Pools, Retention, and Loss from Coffee Agroforestry Systems in Costa Rica

This dissertation examines the effects of management techniques on nutrient dynamics in coffee farms in Costa Rica's Central Valley. I describe how fertilizer management and species alter the storage, movement, and release of nutrients. Coffee can be cultivated in monoculture or under the shade of trees (agroforest). While monocultures are always amended with mineral fertilizer, fertilizer is applied to agroforests in two forms: mineral and organic, with some farms receiving no fertilizer. A major finding of this study was the systematic difference in nutrient pools among management systems. Unfertilized agroforests had the largest aboveground nitrogen (N) and phosphorus (P) pools while fertilized agroforests had the largest soil nutrient pools. An approach based on nutrient balances indicated that mineral-fertilized agroforests are more likely to lose nutrients than organically-fertilized or unfertilized agroforests. Further, mineral-fertilized agroforests had higher concentrations of nitrate-N (but not phosphate-P) concentrations in leachate (at 15cm) than organically-fertilized agroforests, supporting the hypothesis that mineralfertilized agroforests are more likely to lose N. Despite greater nutrient inputs in mineralcompared to organically-fertilized agroforests, calculated N losses were similar. However, N losses were three times higher in the coffee monoculture. Deep soil N pools (50-80cm) were larger in mineralcompared to organically-fertilized agroforests, but correspondingly large soil carbon (C) pools appear to be capable of immobilizing N. High concentrations of iron and aluminum oxides provide a mechanism for P retention. Finally, over a year, more N is released under a mixed-species canopy than a monospecific canopy suggesting that increased canopy diversity may accelerate nutrient release to the crop. Further, N and P release from Erythrina pruning residues may be ii enhanced during periods high nutrient demand if Musa is simultaneously pruned. In sum, this dissertation suggests that nutrient optimization may be best achieved by a management strategy in which coffee is cultivated under the shade of several shade species. As the price organic coffee farmers receive for their beans continues to drop, this research shows that by farming under the shade of trees and applying moderate amounts of inorganic fertilizer, small-scale farmers may be able to enhance profitability while maintaining a low environmental impact.

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