The export of excess nitrogen input from rivers has been identified as one of the most significant problems facing coastal ecosystems, resulting in eutrophication and adverse environmental effects such as hypoxia and harmful algal blooms. Researchers at the GCE LTER constructed nitrogen budgets for the watersheds of all the major rivers in the southeast to determine the total input of nitrogen as well as the proportion that is exported to the coast in stream flow. They also determined the proportion of nutrients exported from watersheds on the US west coast, and evaluated changes in nutrient inputs in the watershed of the Altamaha River, which supplies most of the fresh water to the GCE domain.
The sources of nitrogen (N) considered in these studies included the application of fertilizer to farmland, input through biological N fixation (wherein legumes take up atmospheric N), the deposition of N compounds in precipitation (wet deposition; or without precipitation in a process known as dry” deposition), and, finally, the N in the food that is imported to feed the people and livestock that live in the watershed rather than produced locally. When all of these are added up, the sum of the inputs can be compared to the amount of N that leaves the system as riverine export to estuaries (measured as total N at the most downstream USGS water quality monitoring station).
It turns out that only a small proportion of the N that comes in actually leaves in streamflow. In an examination of N inputs and exports to 16 major watersheds in the northeast, previous investigators found that an average of 25% of the N that went in was exported, and this number had been used as the rule of thumb for several global models of N inputs to coastal waters. However, when researchers at the GCE LTER applied the same techniques to 12 watersheds in the southeastern US they found a vastly different story, with an average of only 9% export. The difference was not due to changes in methodology. Instead, they identified a threshold in N processing at 38°N latitude, where annual temperatures average 10-12°C. The warmer temperatures in the southeast, they proposed, stimulate the denitrifying bacteria that process the nitrogen, leading to nitrogen release to the atmosphere as N2 gas.
This research is important for three reasons. First, if export in lower latitudes is actually 9% rather than 25%, it means that globally-averaged estimates of export need to be shifted downward. Second, the link between temperature and watershed N export suggests that temperature exercises a fundamental control over N processing that has not been previously appreciated, with decreased capacity for N processing at higher latitudes. This finding is sure to stimulate additional research, as it suggests that warmer temperatures associated with climate change may reduce the amount of nitrogen that reaches estuaries. Third, these summaries of N inputs identify the most important sources of nutrients to each watershed, which can be useful to managers trying to reduce nitrogen pollution.