Soil and litter

The storage and cycling of carbon and nutrients within the soil are key processes we aim to capture in the Virtual Ecosystem. This requires modelling soil specific processes, as well as litter decay processes that occur both within the topsoil (e.g. for dead roots) and within the above ground litter layer.

The soil theme is split into two separate models. On one hand, the litter model handles the breakdown of biomass that’s still in an identifiable form, both in the above-ground litter layer as well as below ground. On the other hand, the soil model handles the processes that are specific to the soil, i.e. the soil carbon and macronutrient cycles.

The litter model is significantly simpler than the soil model. The biggest uncertainty in this model are the rates at which different types of litter decay, and this can be parameterised relatively well from common empirical data. The model provides reasonably good estimates of the standing stocks of litter and the rates at which carbon, nitrogen and phosphorus enter the soil.

In contrast, the soil model is more detailed in order to address deeper uncertainties about nutrient transformations within the soil. The two biggest uncertainties in the soil model are the long-term fate of soil carbon and the impact of plant–microbe interactions on uptake rates of nutrients by plants. We believe that the soil-related uncertainties are of greater consequence, so we have made the conscious choice to focus more detailed modelling of soil processes than the litter processes. Therefore, the soil model explicitly represents the soil microbial community, which is the main driver of decomposition in soils. Explicitly represented microbes presents major data challenges, as soils are generally poorly characterised compared to the other constituent parts of terrestrial ecosystems.

Most processes in the soil are affected by both the environmental temperature and the soil moisture. Our model includes various response functions that capture these known links between the abiotic environment and soil processes

Future directions 🔭

There are ecologically explicit extensions to the litter model - such as modelling of litter microbial communities - that could increase the realism of litter model, but the current model is expected to be sufficient in the short term.