Exploring the Seasonal Synchrony of Catchment Nitrogen Dynamics: The Search for a Unifying Theoretical Framework

Contrary to the predictions stemming from the nitrogen saturation hypothesis (Stoddard, 1994), some forested long-term research sites in Eastern North America exhibit peak streamwater nitrate exports during the growing season. Some initial hypotheses about climatic controls (Mullholland and Hill, 1997) have recently been questioned (Goodale et al., 2009). This is a proposal to fund a working group to advance a synthesis on the seasonal patterns of inputs, processing, and output of Nitrogen to watersheds across Eastern North America. Superimposed on the continental scale gradient are significant differences within research sites such as Turkey Lakes and Coweeta (Figure 1 in Attachment). Our conceptual model is that the controls on patterns of stream N concentrations and loads exported from watersheds emerge from a cascade of sources and sinks at multiple spatial and temporal scales that accumulate along converging flowpaths. This cascade integrates atmospheric, geologic, geomorphic, land use/land cover, water infrastructure and plant, soil and microbial responses. In order to synthesize controls from continental to patch (10-100 m2) scales, we must: (1) Understand how N is coupled to water and carbon cycling within reference forest ecosystems, broadly defined to include surface water drainage networks, across current climatic, atmospheric N deposition, geologic, geomorphic and vegetation gradients; and (2) Develop a mechanistic understanding of how human activity alters the timing, magnitude and pattern of these coupled processes. Time series of N export patterns from long-term experimental watersheds across a latitudinal gradient from Canada through the southeastern USA reveal marked differences in the seasonal timing and magnitude of export (sites listed in budget). De-convolving the controls requires an interdisciplinary approach that captures the progressive coupling of ecosystems and (a) atmosphere, (b) hydrology, and (c) human activity, from small watersheds to continental scales. Doing so will enable a mechanistic understanding and modeling framework connecting N cycling and export across a continuum of terrestrial through aquatic ecosystems. Our working group has recently collaborated on a NSF macrosystems biology proposal and aims to advance our analyses of long-term datasets.