Control of nitrogen export from watersheds by headwater streams

A comparative N-15-tracer study of nitrogen dynamics in headwater streams f rom biomes throughout North America demonstrates that streams exert control over nutrient exports to rivers, Lakes, and estuaries. The most rapid upta ke and transformation of inorganic nitrogen occurred in the smallest stream s. Ammonium entering these streams was removed from the water within a few tens to hundreds of meters. Nitrate was also removed from stream water but traveled a distance 5 to 10 times as long, on average, as ammonium. Despite low ammonium concentration in stream water, nitrification rates were high, indicating that small streams are potentially important sources of atmosph eric nitrous oxide. During seasons of high biological activity, the reaches of headwater streams typically export downstream Less than half of the inp ut of dissolved inorganic nitrogen from their watersheds.