Nitrate uptake enhanced by availability of dissolved organic matter in tropical montane streams

Tropical forests store large amounts of Earth’s terrestrial carbon (C), but many tropical montane streams have low dissolved organic matter (DOM). This low availability of energy likely limits certain pathways of inorganic nitrogen (N) uptake, as evidenced by high rates of nitrification and predominance of nitrate (NO3-) in the total pool of dissolved nitrogen, seen in many tropical montane forests. To test this hypothesis of energy limitation to N cycling, we conducted a series of experiments to explore the influence of DOM availability on tropical stream N cycling. Nutrient pulse additions of NO3- with or without an added carbon (C) source (acetate or urea) were conducted in streams of the Luquillo Experimental Forest, Puerto Rico. In the absence of added DOM, NO3- uptake was either undetectable or had very long (>3,000 m) uptake lengths (Sw). When DOM was added with NO3-, NO3- Sw were much shorter (80 to 1,200 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 additions of NO3--only, 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 nitrogen 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 nitrogen and DOM including organic nitrogen. Understanding the controls on NO3- processing in these streams is important to predict network-scale exports of nitrogen from tropical ecosystems.