Effects of stream chemistry and leaf litter chemistry on leaf decomposition in neotropical streams

L.A. Stallcup¹, M. Ardón¹, K.R. Maynard², and C.M. Pringle¹. ¹Institute of Ecology, University of Georgia, Athens GA 30602, ²Department of Ecology and Evolutionary Biology, Yale University, New Haven CT 06520

Stream ecosystems are influenced by the availability of inorganic nutrients, as well as by the quality of organic matter from the riparian zone. We tested the hypothesis that at high phosphorus (P) concentrations, nitrogen (N) becomes secondarily limiting to leaf decomposition processes in streams draining La Selva Biological Station, Costa Rica. We also predicted that decomposition would occur more rapidly in leaves with a lower carbon to nitrogen ratio (C:N) than in leaves with high C:N. We selected leaves from two common riparian species with high (Ficus insipida) and low (Trema integerrima) C:N. Both species were similar with respect to secondary and structural compounds. Leaves were incubated in three low-order streams with different P concentrations: naturally low (10 ug SRP L^-1); naturally high (>100 ug SRP L^-1) due to solute-rich groundwater; and within a whole-stream P enrichment experiment (200 ug SRP L^-1) designed to isolate P effects. To achieve localized N enrichment, fertilizer was added upstream of half of the litter bags. Results suggest P effects; however, our short-term N addition had no apparent effect on leaf decay. In all nutrient treatments, the leaf type with lower C:N ratio decayed most rapidly. Future experiments will examine a larger range in leaf quality and the effects of a whole stream N enrichment.