Carbon and Nitrogen Export from Semiarid Uplands to Perennial Rivers: Connections and Missing Links, San Pedro River, Arizona, USA

Investigations of biogeochemical processes in semiarid environments have largely focused on either plot studies in the uplands or on in-stream and near stream reaction or transport studies. Recent research permits us to synthesize a conceptual model of how uplands and riparian systems are linked in semiarid climates specific to the San Pedro River basin in Arizona. These studies have demonstrated significant export of both dissolved and sediment associated carbon and nitrogen from the uplands into the channel network of semiarid river systems. Likewise research has demonstrated that riparian areas are biogeochemically active, with the potential to rapidly respire inputs of organic matter, releasing carbon back to the atmosphere and inorganic nitrogen into the water column or back to the atmosphere through denitrification. For the San Pedro, a total of more than 80% of both carbon and nitrogen export from the uplands that is observed in small catchments is not observed at the larger river system scale, indicating that this carbon and nitrogen must be either stored, taken up by vegetation or returned to the atmosphere at scales between the small catchment (1-1000ha) and large river systems scale (∼320,000ha). In summary, existing research shows that the uplands contribute significant amounts of material into the stream and near stream zone, and that this imported material influences nutrient conditions in aquatic systems. These conclusions point out significant gaps in developing a complete understanding of the reactions of carbon and nitrogen as material is transported from the uplands through ephemeral and perennial channel networks.