Site: Coweeta LTER

Understanding how global change will affect patterns of nitrogen (N) loss from forests is an area of special importance for researchers. Atmospheric deposition of N associated with anthropogenic activities and the sensitivity of microbial processes that convert organic N to plant available forms to temperature both point to the likelihood of increased N export from forests as increased rates of deposition and warming continue. Increased N export reduces water quality, long term forest productivity, and the ability of the forest to sequester carbon.

Microbes convert organic nitrogen (N) from decaying matter into plant available forms in a process of mineralization that is highly sensitive to temperature, increasing in rate as the temperature warms. This converted N is a critical determinant of plant growth and carbon sequestration, especially in N limited forests such as those that characterize the southern Appalachian region of the Coweeta LTER. However, though theoretical research and experiments at small temporal and spatial scales indicate high climate sensitivity of N mineralization, results have been difficult to scale to broader spatial scales, multiannual time periods, and more complex ecosystems. To address these gaps, Coweeta researchers modeled data representing 21-32 years of measurements of climate and water chemistry in two separate watersheds in order to disentangle the effects of atmospheric deposition and increased rates of N mineralization from temperature changes and to estimate the effects of climate change on N export. Separating the effects of global environmental change is a daunting task in terrestrial systems with extremely complex biogeochemical cycles, but one that can be facilitated by the combination of long term records and process-based evaluations.

The researchers were able to observe strong seasonal correlations between temperature and N export in streams, a pattern contrary to that observed in colder, more northerly forests that allowed them to separate the effects of N deposition from N mineralization. Results also indicated that a strong internal sink persists regardless of temperature, suggesting that seasonal increases in export during the summer months are driven by the supply of dissolved inorganic nitrogen made available through increased rates of microbial mineralization. Extrapolation of trends using the model indicate that future climate warming may increase N export by greater than threefold more than from increased deposition. The ability to anticipate the impact of future changes on water quality is of extreme importance in the southern Appalachian region, where the mountains are the effective “water towers” for the rapidly growing surrounding lowland regions.

Graph for
Stream nitrate concentrations as observed (circles) and modeled (gray line) over one year at two watersheds monitored by the Coweeta LTER. Annual temperature (black line) shows a clear positive correlation with nitrate concentrations
Brookshire et al 2010