PnET-DNDC is a process based model that integrates three existing models (PnET, DNDC and a nitrification model) with several additional features. PnET is a forest physiology model that predicts photosynthesis, respiration, organic carbon production and allocation and litter production for forest ecosystems. DNDC is a process based model of carbon and nitrogen biogeochemistry in agro-ecosystems and the nitrification model was developed for prediction of nitrifier growth/death rates, nitrification rate and nitrifaction-induced nitric oxide (NO) and nitrous oxide (N2O) production (F. Stange, 2000b).
Importantly PnET-DNDC is able to model soils where aerobic and anerobic microsites exist simultaneously, as it can predict both nitrification and denitrification in the soil at the same time (Li et al. 2000a). The kinetic framework and interacting fractions of the model link ecological drivers to trace gas emissions.
The PnET-DNDC model is described by Li et al. (2000a) as having two components: the first was established to predict the effects of ecological drivers (climate, soil, vegetation and anthropogenic activity) on soil environmental factors (temperature, moisture, pH, redox potential and substrates concentrations). This component has a further three sub models which predict soil climate, forest growth and turnover of organic matter. The second component predicts the effects of the soil environmental factors on the biochemical or geochemical factors that control nitric oxide (NO) and nitrous oxide (N2O) production and consumption, which then contain two sub models for nitrification and denitrification.A kinetic scheme was developed to calculate the anaerobic status of the soil and divide the soil into aerobic and anaerobic fractions. Nitrification can only occur in the aerobic fraction and denitrification in the anaerobic fraction. Li et al. (2000a) describe this as a dynamic "anaerobic balloon" within a soil matrix. The balloon size is determined by the simulated oxygen partial pressure, which is calculated from oxygen diffusion and consumption rates in the soil. As the balloon swells and shrinks the model dynamically allocates substrates, including dissolved organic carbon (DOC), ammonium (NH4+), nitrate (NO3-), into the aerobic and anaerobic soil fractions. When the balloon swells more substrates (DOC, NH4+, NO3-, NO and N20) will be allocated to the anaerobic microsites for denitrification, less DOC and NH4+ will be left in the aerobic microsites for nitrification and the pathway for denitrification gas products to leave the balloon will become longer. The trends are reversed as the balloon shrinks. Rainfall duration, soil plus root respiration and soil texture affect the anaerobic volumetric fraction as a result of their effects on oxygen diffusion and oxygen consumption.
Partial pressue of oxygen (pO2) is estimated in the forest soil profile by a one-dimensional soil oxygen diffusion algorithm. The soil profile is divided into a series of layers and the oxygen flux between these is determined by the soil pO2 gradients, where oxygen diffusion rate is driven by soil gradient and texture (Li et al. 2000a).
Stange et al. (2000b) reported that PnET-N-DNDC can be successfully used to predict N2O and NO emissions from a broad range of temperature forest sites, based on the results obtained from sensitivity analysis and model validation with field data (from several forest sites in the United States and Europe).PnET-N-DNDC has since been integrated with WETLAND-DNDC to produce FOREST-DNDC (Giltrap et al., 2010).