A process-oriented model of N2O and NO emissions from forest soils: 1. Model development

To predict emissions of nitrous oxide (N2O) and nitric oxide (NO) from fore st soils, we have developed a process-oriented model by integrating several new features with three existing models, PnET, Denitriftcation-Decompositi on (DNDC), and a nitrification model. In the new model, two components were established to predict (1) the effects of ecological drivers (e.g., climat e, soil, vegetation, and anthropogenic activity) on soil environmental fact ors (e.g., temperature, moisture, pH, redox potential, and substrates conce ntrations), and (2) effects of the soil environmental factors on the bioche mical or geochemical reactions which govern NO and N2O production and consu mption. The first component consists of three submodels for predicting soil climate, forest growth, and turnover of soil organic matter. The second co mponent contains two submodels for nitrification and denitrification. A kin etic scheme, a so-called "anaerobic balloon," was developed to calculate th e anaerobic status of the soil and divide the soil into aerobic and anaerob ic fractions. Nitrification is only allowed to occur in the aerobic fractio n, while denitrification occurs only in the anaerobic fraction. The size of the anaerobic balloon is defined by the simulated oxygen partial pressure which is calculated based on oxygen diffusion and consumption rates in the soil. As the balloon swells or shrinks, the model dynamically allocates sub strates (e.g., dissolved organic carbon, ammonium, nitrate, etc.) into the aerobic and anaerobic fractions. With this approach, the model is able to p redict both nitrification and denitrification in the same soil at the same time. This feature is important for soils where aerobic and anaerobic micro sites often exist simultaneously. With the kinetic framework as well as its interacting functions, the PnET-N-DNDC model links ecological drivers to t race gas emissions. Tests for validating the new model are published in a c ompanion paper [Stange er al., this issue].