Remote sensing of soil processes

Abstract

Estimates of ecosystem structure and function using remotely sensed data are based on the information contained in reflected and emitted radiation. Soil plays an integral role in almost all remote observations of the land surface either because the vegetation cover is discontinuous or because multiple scattering of the radiation occurs between vegetation and the underlying soil surface. However, since vegetation covers most of the land surface, we should consider the plant canopy as a primary interface to soil processes. In many ways, the canopy is an integrated expression of (and is mutually linked to) below-ground processes and can, as such, be used to infer ecosystem properties. Classifications of satellite imagery into vegetation communities in which soil processes have been characterized can be used to extrapolate those processes across the landscape based on the areal extent of the associated cover type. Ratios of reflectance values measured in the region of chlorophyll absorption to those in the highly reflected near-infrared region are sensitive to biomass quantity and, when integrated over time, strongly correlate with primary productivity. Imaging spectrometers which measure the reflected spectrum in many narrow contiguous bands enable the development of spectral mixture models to deconvolve the reflectance signal from each pixel in an image into each of the components contributed by spectrally distinct materials on the ground, e.g. vegetation and soil. High spectral resolution measurements of the absorption characteristics of major foliar chemical constituents may permit assessment of canopy-level concentrations and related processes such as decomposition and nitrogen mineralization. Predicting the types of soil processes and their rates from remote sensing data will depend on strong correlations between canopy variables and the process of interest. This paper discusses the potential for measuring canopy indicators of below-ground processes using current and emerging remote sensing technology, and how we might begin to construct a set of measurements relevant to studies of soil processes and their dynamics over time and space. Basic remote sensing principles are presented with an emphasis on the visible and short-wave infrared spectral region (0.4–2.5 μm), followed by a discussion of the spectral properties of soil and vegetation.