Carbon and nitrogen cycling in the Peruvian Andean Amazon



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This dissertation consists of several studies conducted at various spatial and temporal scales designed to identify the important processes that affect organic matter (OM) inputs from the Andes mountains to the Amazon headwaters, as well as carbon (C) and nitrogen (N) cycles in the rivers themselves. Andean rivers supplied approximately equal amounts of fine and coarse sediments to the Amazon, but most coarse sediments were retained in the Andean foreland while fine sediments continued downstream. Terrestrial plant δ¹³C increased with elevation, but terrestrial soil δ¹³C did not and was enriched by 1-3% over plants. Particulate organic matter (POM) concentrations were generally low, with periodic high concentrations during storms. There were significant differences in the isotopic composition of POM between seasons (wet vs. dry), reflecting changes in sediment source. During high flow, POM resembled terrestrial materials, but during drier periods there was evidence for a resuspended bottom sediment or algal source. During wet periods, OM content of soils and river POM decreased downstream, but the POM trend was complicated during drier months. Δ¹⁴C of POM decreased downstream, suggesting that young, fresh OM introduced in small headwater streams was respired preferentially in rivers or diluted with older material downstream. Fine suspended POM was higher in δ¹⁵N and δ¹³C than coarse POM, indicating either greater degradation in the fine fraction or sorption of isotopically enriched dissolved OM. Deforestation is reflected in suspended POM in small headwaters, with enrichment in both ¹³C (due to introduced C₄ grasses) and ¹⁵N. Epiphytic plants living in forest canopies were δ¹⁵N-deplete compared to rooted plants, and the biomass of these epiphytes was so high that their δ¹⁵N was reflected in stream POM. Precipitation was a major source of isotopically enriched dissolved organic N (DON) to cloudforests in the central Andes. Inorganic N export from these systems in streams was very low, but this seems to be the result of high demand for DIN by microbes and fine roots in riparian soils, not a high reactivity of dissolved inorganic N (DIN) in the stream. Low deposition of and demand for inorganic N in these forests appear to explain low ambient stream DIN concentrations.