Sources of spatial complexity in two coastal plain soil landscapes

Highlights

• Spatial heterogeneity of soils at two coastal plain sites

• Complexity assessed via spectral radius of soil adjacency matrix.

• Assess complexity due to soil-forming factors vs. contingent factors

• Greater complexity at partly forested site vs. cropland site

Abstract

The spatial pattern of soils and soil properties in soil landscapes is considered here as a function of (1) systematic variation along catenas or associated with spatial patterns of soil-forming factors; and (2) local pseudo-random variations associated with local disturbances or small, unobserved variations in soil-forming factors. The problem is approached at two study sites in the U.S. Atlantic Coastal Plain using algebraic graph theory and the spectral radius of the soil adjacency matrix as a measure of complexity. The matrix is constructed based on the observed spatial contiguity of soil taxa, and soil factor sequences (SFS) are defined for each site based on systematic soil variation associated with variations in parent material, topography, sandy surface thicknesses, and secondary podzolization. The spectral radii of the networks described by the adjacency graphs are compared to those associated with the maximum for a graph of the same size, and the maximum associated with control entirely by variations in soil forming factors. At the Clayroot study site, which is entirely cropland, complexity of the adjacency matrix is less than Λ, the maximum that could be accounted for by the four identified SFS, due to redundant information in the SFS. The Littlefield site, by contrast, has a spectral radius greater than Λ. Here, where about half the site is forested, the contingent variation is likely associated with effects of individual trees on soil morphology. The utility of the adjacency analysis is in identifying whether soil heterogeneity is likely associated with SFS or with contingent factors not captured in SFS.

Introduction

The spatial variability of soils and soil properties is well known to any field pedologist, and has been the subject of intensive research over the last three decades (e.g. Burrough, 1983, Campbell, 1979, Culling, 1986, Oliver and Webster, 1986, Trudgill, 1983). Soil variability is traditionally attributed to a systematic, predictable component, and an apparently random noise component (Burrough, 1983), with the apparent noise in many cases actually attributable to deterministic complexity associated with dynamical instability and chaos (e.g. Borujeni et al., 2010, Culling, 1988, Ibáñez et al., 1990, Ibáñez et al., 1994, Liebens and Schaetzl, 1997, Milan et al., 2009, Phillips, 1993, Phillips, 2000, Phillips, 2001a, Phillips, 2001b, Phillips and Marion, 2005, Phillips et al., 1996, Toomanian et al., 2006, Webster, 2000). The “noise” component is referred to here as contingent factors, as instability, local disturbances, and other deviations from systematic patterns are geographically and/or historically contingent. This recognizes that rather than randomness, the irregular variations are associated with local geographical variations in environmental controls and/or specific local (chains of) events.

The purpose of this study is to analyze the structure of soil spatial heterogeneity at the landscape level to determine the relative importance of pedological variability related to variation in soil forming factors (SFF) versus that associated with local disturbances and dynamically unstable magnification of minor initial differences, referred here collectively as contingent factors. The focus is on soil types rather than individual soil properties; the importance of and rationale for this type of analysis are discussed by, e.g., Campbell (1979), Phillips and Marion, 2005, Phillips and Marion, 2007, Toomanian et al. (2006); Bockheim and Haus (2013) and Ibáñez et al. (2013). The analysis is based on the pattern of spatial adjacency of soil types, rather than a spatially explicit analysis. The advantage of this approach is that it is unaffected by locally unmeasured or unobserved variation in SFF, as long as the state factors relevant to the area are identified.

A high degree of soil heterogeneity over short distances and small areas is common. This variation is sometimes, but not always, related to readily observed variations in SFF. Even when heterogeneity is related to (for instance) microtopography or localized bioturbation, which are included in the standard suite of SFF, variation may occur at a spatial scale too fine for relationships to be apparent at typical measurement and mapping resolutions. The multiple interrelated environmental factors that influence soils are not independent (of each other, or of soils themselves) and may include relic or inherited properties unrelated to contemporary environmental controls. Further, pedogenesis may sometimes be divergent, exaggerating the effects of minor initial variations or disturbances. Thus, notwithstanding technical and practical problems of measurement and observation of environmental heterogeneity, linking soil heterogeneity to variations in SFF is often no simple matter.

Soil landscape complexity is a function of the number of different soil types, the density of links or connections between them (here defined as spatial adjacency), and the (ir)regularity of the adjacency relationships. Soil forming factors are those environmental controls that are known to, or can potentially, result in variations in soil properties and soil types. These include the classic state factors of climate, biota, topography, parent material, and time or surface age, as well as any other locally or regionally significant factors such as land use, aeolian or other non-topographically driven soil redistribution, disturbance regimes, sea-level change, etc. Pedology and soil geography, as well as practical soil surveying and mapping, are based on the notion of sequences of soil variation based on SFF. Thus, for example, climo-, bio-, litho-, and toposequences represent systematic variation of soil properties along gradients of climate, biotic communities, lithology or parent material properties, and topography, respectively. Catenas, defined as sequences of soils developed from similar parent material under similar climatic conditions but whose characteristics differ because of variations in relief and drainage, are one example. Many are associated with spatial gradients—a climosequence, for instance, may occur along gradients of temperature or moisture. However, some sequences of SFF may be associated with categorical variations, which may involve multiple SFF. In a coastal landscape, for instance, fundamentally different suites of soils may be associated with tidal marsh, dune swale, and sand dune settings related to differences in parent material, topography, and drainage, which may or may not vary along a spatial gradient. All of these systematic sequences—catenas, factor sequences, soil-landform relationships, etc.—will be referred to here as soil factor sequences (SFS). SFS may include soil-forming factors such as parent material and topography, landscape elements such as landforms or geomorphic surfaces used to differentiate soil types, or catenary relationships reflected in soil properties themselves, such as horizon types and thicknesses, and redoximorphic features.