## 2 Materials and Methods
### The HJA-N Model
#### 2.3.4 Soils Model  
The soil organic matter (SOM) sub model is defined similarly to that utilized in the PNET-CN model of [17] in that the number of SOM pools is very small, particularly when compared with standard SOM models. The current version of HJA-N includes two SOM pools, the first representing root zone SOM and the second representing below root zone SOM, with carbon and nitrogen explicitly represented in both (Table 2). Although the inclusion of additional pools could be used to more precisely describe the wide distribution of temporal SOM stability, an evaluation of the simpler definition against the long term measured data represents a useful first step, and is consistent with the soil nitrogen budget developed in WS10 in the early 1980s [29].  
Four additional below-ground nitrogen pools are included to represent DIN and DON in both the rooting and below rooting zones. A kinetic sorption isotherm is used to separate soil bound nitrogen from dissolved forms, assuming that the proportion of each stays constant through time. Hydrologic losses are defined based upon the flux rates calculated by the hydrologic components of the model and the concentrations of freely available DON and DIN. Landscape scale denitrification rates are not well understood, but the model does maintain a first order denitrification loss pathway from the DIN storage.  
The soil respiration model is defined similarly to that for respiration from plant residues as a first order rate, which includes temperature dependence based upon the q10-based temperature modifier. The respiration rate is assumed to represent the production of both CO\({}_{2}\) and DOC. The mobilization of both DIN and DON is calculated as a proportion of the soil respiration rate.  
The incorporation of DON production and loss is a key feature of the model. Very few ecosystem models include DON as a component of the nitrogen cycle, however [19] proposed and evaluated four potential definitions of DOC mobilization, and then used soil CN ratios to proportionally estimate the production of DON. These definitions included a constant loss model, a first order model, a model based upon soil CN ratios, and lastly a model where the rate of mobilization was proportional to the microbial respiration rate. The last of these rate definitions is consistent with our definition of SOM production, and as such was incorporated into the model.  
At WS10 we have a long term record of streamwater DON and DIN export, but production rates in soils have not been studies. Our model reflects this in its simplicity. We assume that the production of dissolved N, in total, is proportional to the soil respiration rate, depending upon the soil CN ratios. This overall production rate of dissolved nitrogen is then split into DIN and DON assuming a fixed portioning constant. The DON pool includes an additional respiration term that is used to simulate the continuous decomposition processes of DON, which we assume result in the further production of DIN.  
A more compelling definition of these dissolved N pools would separate plant available N from unavailable N, rather than organic from inorganic [36,37]. Such a distinction would recognize the fact that organic nitrogen is a term that represents a wide variety of compounds, with a significant range in molecular weights [38]. This would then allow for the lower molecular weight fraction of that distribution to interact more directly with the vegetation and microorganisms. However, in this case we are limited by available long term records of aquatic DON and DIN, which do not support such distinction. To be consistent with these data, we make the simplifying assumption that the DON pools, throughout the model domain, are unavailable forms. A complete description of the rate terms defining the SOM sub-model is included in Table 5.