Examining the relationship between soil structure and soil reflectance using soil pore structure characteristics obtained from image analysis

With new environmental perspectives and policies, such as the Water Framework Directive, the need to understand the processes governing the transfer of water from soils to freshwater systems at a catchment and regional scale has never been more important. It is widely understood that soil texture and structure strongly influence water flow in soils. However, less is known how specific size, shape and aggregation of particles and the associated distribution of pores affect the path of light transmission. In this study, we hypothesize that reflectance spectra exhibit a primary response to soil structure. Using a field study site near Nottingham, United Kingdom that comprised of randomized plots with different crop histories, soil reflectance factor measurements were taken between 350 and 2500 nm on undisturbed soil samples from the top horizon using an Analytical Spectral Devices (ASDs) Field Spec (R) Pro spectrometer (ASD Inc., Boulder, CO, USA). The undisturbed soils were subsequently impregnated to preserve their structure, and the soil structure properties (in terms of macropore characteristics) were measured in horizontally orientated two-dimensional sections through the soil at different depths. The soil reflectance and transformed spectra (first- and second-order derivatives) were correlated with the soil structural properties and empirical relationships were found between the surface macroporosity, pore size and shape attributes and reflectance values and derivatives of reflectance. Surface macroporosity was highly negatively correlated with reflectance in the visible domain at water absorption bands at similar to 1400 and similar to 1900 nm and throughout the shortwave-infrared region from 1915 nm onwards.