Scaling aquatic ecosystem processes like nutrient removal is critical for assessing the importance of streams and rivers to watershed nutrient export. We used pulse NH4+ enrichment experiments and measured net NH4+ uptake in 7 streams throughout a mountainous tropical river network in Puerto Rico to assess spatial variability in NH4+ uptake and to infer the physical, chemical, and biological characteristics that most influence its variation. Across 14 experiments, NH4+ uptake velocity (v(f)) ranged from 0.3 to 8.5 (mean = 2.7) mm/min and was positively related to algal biomass standing stock, measured as chlorophyll a. On average, 49% of experimentally added NH4+ was immediately transformed to NO3-, suggesting that nitrification can rival microbial and algal assimilation as a fate of streamwater NH4+. We considered the implications of our empirical results at the river-network scale based on a simple mass-balance model parameterized for the Rio Mameyes watershed. Most catchment NH4+ inputs are delivered to 1(st)-order streams. Therefore, model results indicated that high NH4+ uptake rates in headwater streams limit NH4+ inputs to downstream reaches, thereby decreasing the role of larger streams in NH4+ removal at the river-network scale. In-stream nitrification resulted in additional NO3- inputs, which were more likely than NH4+ to be transported downstream because of lower biological demand for NO3- relative to NH4+. Given our estimates of catchment N loading to streams and rivers, we estimated that 39% of modeled watershed NO3- export was produced within the river network by nitrification. Together, these results suggest that streams and rivers can significantly transform the N load from their catchments.