The Natural Capital Value of Soil: Carbon, Macropores and Hydrophobicity.

The natural-capital concept integrates economic thinking with ecological principles by considering nature’s stocks of materials and energy as capital.  Nature comprises an assemblage of natural capital stocks, and they, in connected sum, form our ecological infrastructure. Soil is a key component of ecological infrastructure. Soil is a prime natural-capital stock that provides valuable ecosystem services. In 2005 the Millennium Ecosystem Assessment classified ecosystem services into four broad, and often overlapping, types: supporting, provisioning, regulating and cultural services.  We focus here on regulating services, and show how investment of carbon into the soil’s natural capital can provide valuable returns on investment. We show how changes in the soil’s carbon content alter the manageable properties of the soil’s natural capital, such as its connected macroporosity, labile carbon content, and hydrophobicity. These affect the nutrient, gaseous and water regulating services that flow from the soil’s natural capital. We highlight this through case studies. We show how trees and vines can increase the soil’s carbon content down to great depths – up to 9 m.  This carbon investment has multiple benefits on the regulating services provided by soil. Changing the soil’s carbon content affects connected macroporosity and soil microbial functioning, which changes the soil’s regulating services in relation to nutrients, water and gases.  We contrast the impact of macropores on the buffering and filtering services provided by the soil to endogenously generated nitrogen and to exogenously applied nitrogen.  The effect of soil carbon on soil water repellency is outlined. Both the water regulating services, and dis-services, provided by water repellency are discussed.  The measurement and proximal sensing of soil water repellency is detailed and a new device for measuring the impact of water repellency on water regulation services is described.  We highlight the manifold ecosystem-service values from investing carbon into the soil’s natural capital.