Soil plays a vital role in controlling the flow of water and chemicals between the atmosphere and the earth. As water passes through the soil it is filtered by plants, bacteria, fungi and soil particles, which remove various minerals and impurities before they are transported to surface waters. Rivers in catchments dominated by thin or impermeable soils are therefore more vulnerable to pollution inputs, such as acid rain in the uplands or fertilisers in the lowlands, as soils have a lower capacity to filter out harmful chemicals.
Soils also play an important role in flood control. In river catchments dominated by well-aerated soils rich in organic matter, large volumes of water can be trapped in the soil like a sponge, limiting water run-off into streams and reducing flood risk. Water retained in upper soil horizons is also key for agriculture: it has been estimated that humans withdraw around 8% of renewable river and lake freshwater per year, but more like 5% of surface soil water.
The biology, chemistry and physics of soils and our management of them is critical to how catchment systems function and deliver the multiple services and benefits that society requires of them.
The physical structure of soils contributes to how water moves through the landscape. Rural and urban management practices and land use change can alter the pathways and rate of water movement.
The presence of water in the landscape is key for chemical and biological processes that support ecological functions from the pore to field scale. The movement of water controls the transfer of diffuse substances that are either dissolved or carried as particulates. It is when we have too great or too little water or associated material in a particular location that the ecology and wider society notices impacts on the services that Scotland’s catchments provide.