LandscapeDNDC: Simulation of the redox status for predicting soil-atmosphere GHG exchange

Background/Question/Methods

Production and consumption of the greenhouse gases CH₄ and N₂O in soils is strongly dependent on the redox or soil oxygen status. N₂O is primarily produced by nitrification and denitrification whose occurrence is regulated by the O₂ partial pressure (pO2). Under aerobic conditions nitrification converts NH₄ to NO₃ via NH₂OH and NO₂. Thereby N₂O can be produced either by the reduction of NO₂ within the same NH₄ oxidizing microorganism (nitrifier denitrification) or by the reduction of NO₃ by distinct microbes in situ (coupled nitrification denitrification). Both processes are favoured by short-term O₂ scarcity or existence of aerobic and anaerobic microsites in immediate vicinity. Under more developed anaerobic conditions N₂O is produced by incomplete denitrification of NO₃. CH₄ production is bound to strict anaerobic conditions and to the absence of alternative electron acceptors such as NO₃ or Fe₃. Passing aerob soil layers on its way to the atmosphere CH₄can be effectively oxidized by methanotrophic microbes.

The diversity and complexity of described processes in natural systems as well as the availability of required input data confronts model developers with the weighting of appropriate scales and accuracy of process description. Within the ecosystem model framework LandscapeDNDC the soil environment is represented by a vertical 1-D column with a discretization length of 0.02 [m] allowing specification of different production and consumption rates of N₂O and CH₄in different soil depths. The concurrently existence of aerobic and anaerobic microsites within the same soil depth is accounted for by the introduction of a layer-specific anaerobic volume, which is calculated in dependence of pO2. By these means main metabolic intermediate products as well as prevailing metabolic processes are distinguished between aerobic and anaerobic microsites. Transport of substances is calculated by soil layer diffusion and advection and plant-mediated diffusion (e.g. paddy rice) and ebullition.

Results/Conclusions

We will present results from a modelling study where LandscapeDNDC was used to predict and understand changes in GHG emissions due to management changes, i.e. in our case pollution swapping when double cropping lowland rice paddy systems are converted to rice-maize systems. The differences of CH₄ and N₂O emissions as well as regulating soil environmental conditions over two cropping seasons were modelled with high accuracy. However, applying LandscapeDNDC to rice paddy systems also shows that there is still a strong need for further research with regard to the availability of decomposable soil organic carbon and the oxidative potential of iron in the soil.