Tropical forests store large amounts of Earth’s terrestrial C, but many tropical montane streams have low dissolved organic matter (DOM). This low availability of energy likely limits certain pathways of inorganic N uptake, as evidenced by the high rates of nitrification and predominance of nitrate (NO3−) in the total pool of dissolved N seen in many tropical montane forests. To explore the influence of DOM availability on tropical stream N cycling, we performed nutrient pulse additions of NO3− with or without an added C source (acetate or urea) in streams of the Luquillo Experimental Forest, Puerto Rico. In the absence of added DOM, NO3− uptake was either undetectable or had very long (>3000 m) uptake lengths (Sw). When DOM was added with NO3−, Sw values for NO3− were much shorter (97–1500 m), with the shortest lengths resulting from additions of acetate. Comparing uptake metrics of the added C sources, there was greater demand for acetate compared to urea, and measurable urea uptake was detected much less frequently. During NO3−-only additions, ambient concentrations of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) decreased in some cases, suggesting increased metabolic demand for energy from the ambient organic matter pool under elevated levels of inorganic nutrients. Collectively, these results demonstrate that pathways of inorganic N cycling are tightly tied to energy availability at this tropical site. The response of ambient DOC and DON to increases in NO3− concentrations points to important feedbacks between inorganic N and DOM including organic N. Understanding the controls on NO3− processing in these streams is important to predicting network-scale exports of N from tropical ecosystems.

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University of Chicago Press

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Freshwater Science

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© 2021 by The Society for Freshwater Science. This is an article published by University of Chicago Press in Freshwater Science in 2021, available online: