## 1 Introduction  
Human-induced changes to global biogeochemical cycles are large, having for example effectively doubled the global rate of annual N fixation [1] yet the ultimate response of ecosystems to these changes remains unclear. This lack of clarity is due in part to the inter-dependencies and feedbacks between system components as well as to an incomplete understanding of the complex processes governing biogeochemical cycles. These relationships result in the inherent complexity and non-linearity of environmental systems. Components comprising environmental systems include functionally discreet entities like the atmosphere, vegetation, and soils; spatial assemblages of these entities, including hillslopes, riparian transitions, and instream environments; and also the different elements that cycle through them, including nitrogen, carbon, water, and a host of others. The detailed study of each of these individual units tends to be well-covered by traditional disciplines, such as hillslope hydrology, aquatic ecology, soil biogeochemistry, forest ecology, _etc._, but with less attention given to the relationships between components. That these dependencies, in some sense by definition, fall at interfaces between traditional disciplinary boundaries is an ongoing challenge for ecosystem science, though the emergence of cross-cutting disciplines, such as ecohydrology and hydrobiogeochemistry, is an indication that this challenge is being addressed. The use of numerical models, which are designed to capture relationships between system components, represents one method that can be used to contribute to our understanding of how ecosystems function, and how they may respond to future change in both climate and landcover. The retention and release of N from temperate forested ecosystems provides a clear example of this need for understanding, at an integrated catchment scale. Its relevance is, of course, well established, with tremendous progress having been made throughout the last three decades [2; 3; 4]. Much of this progress has been driven by the need to better understand the response of forested catchments, which are commonly limited by N, to environmental changes, including climate change, disturbance, and increased N deposition [5]. Increasing N deposition provides a useful example, where the potential effects have been succinctly synthesized as the \"nitrogen saturation\" hypothesis [5; 6]. Under this hypothesis, the chronically increased rates of N deposition invoke a variety of ecosystem responses, culminating in increased nitrate mobility in soils, nutrient imbalances in vegetation, forest decline, the acidification of soils, as well as increased surface water N loading leading to the potential for both acidification and eutrophication [5]. Numerous catchment scale experiments have been developed to test the saturation hypothesis [7; 8], and along with them, models have been developed to predict the response of catchments to different deposition patterns [9]. Because nitrate in solution is the dominant form of N lost from these systems, the models that have been developed tend to emphasize inorganic N (e.g., [10]). Despite the strides made in understanding catchment nitrogen cycling under the saturation hypothesis, in many temperate forested catchments, notably those on the Western side of both North and South America, current N deposition rates remain similar to those found historically [11]. In these areas that lie outside of urban or other point sources of atmospheric nitrogen, N continues to be a major limiting nutrient, losses of plant-available N remain very low, and nitrogen saturation is not currently part of the developmental trajectory [12]. The models that have been developed in regions of excess N deposition are not necessarily applicable in these places, a fact that underscores the suitability of low deposition regions as a useful counterpoint in the evaluation of ecosystem N cycling. The HJ Andrews Experimental forest (HJA) in the Pacific Northwest of the USA is one such region.