Modelling changes in soil structure caused by livestock treading

Increased soil compaction resulting from livestock treading and use of heavy machinery is a major environmental hazard often linked to degradation of the soil ecosystem and economic services. However, there is a weak quantitative understanding of the spatial and temporal extent of soil compaction and how it modifies soil properties and associated functions. To address this challenge, we developed a framework for systematic modelling soil compaction caused by grazing animals. We considered random movement of livestock in a confined field to describe the spatial variation in the soil that is discretized in square cells with given properties. We then used a rheology model based on Bingham’s law to infer compaction-induced changes in soil bulk density and porosity. An associated reduction of saturated hydraulic conductivity is obtained from soil porosity predictions by empirically accounting for macroporosity reduction using a dual-porosity permeability model. This model is coupled with an empirical model of soil structure recovery to account for biological activity (i.e., earthworms and roots). The modelling framework effectively captures primary effects of soil compaction on key soil properties despite lack of explicit consideration of complex effects of compaction such as redistribution of pore sizes and changes in pore connectivity. We tested the model using bulk density, macroporosity and saturated hydraulic conductivity data from a grazing study at the Tussock Creek experimental platform in New Zealand. Data were successfully reproduced by the model. Compaction and recovery trends can be interpreted in terms of model properties associated with management, soil texture and environmental conditions. If data are available for calibration of such properties, the model could be used in agro-ecosystem modelling applications to assess the environmental impacts (such as surface runoff and green-house gas emissions) of livestock-grazing systems and inform management strategies for ameliorating these.