Fully Recovered: Can past Site Disturbance Be Determined from Soil Features?.
Geoff S. Humphreys, Dept of Physical Geography, Macquarie Univ, Sydney, NSW 2109, Australia
Changes to the soil occur as land-use practices are modified. These changes may include improvements but more commonly result in degradation, which may be obvious or subtle. The latter type is addressed here. In this case the degradation represents limited changes to the soil such that once recovered there is little apparent evidence that recalls this disturbance history. How this history is deciphered represents a compelling challenge to soil science that necessarily incorporates obvious signs of soil degradation, stratigraphic principles, archaeological principles and pedo-forensics at scales ranging from micro to macro. The challenge is to determine which features can be used in a diagnostic sense. Undoubtedly, these will differ between different environments, land-use practices and recovery time.Morphological indicators of different land use regimes may be revealed in soil colour, structure and texture and this leads naturally to confirmation using microscopic techniques that includes microstructure as well as the presence of exotic components. Often a stratigraphic approach is required to provide a geomorphic context such as when former upslope soil material now constitutes the downslope topsoil. Chemical changes often occur too. Several examples are used to illustrate this. Forest clearance offers the most compelling case especially where the litter layer and the bioturbated surface layer are replaced by very different features such as surface crusting and the presence of mineral soil at the surface. This is often associated with an increase in bulk density, reduced organic matter, and a reduction in larger biovoids. Changes within grassland systems may be less obvious if little disturbed. Further changes are apparent when compaction occurs, which must lead to a decrease in pore size, even of simple packing voids. More telling is the appearance of horizontal planar voids, which imparts a platy structure in the near surface that reflects the applied stresses imposed from above. Biovoids may display a radial fracture pattern in this situation. Changes in drainage that affects the redoximorphic status of the soil may lead to the superimposition of a set of features commensurate with this. However, many of these features are gradually lost as recovery occurs and this results in a greater reliance on general stratigraphy and the presence of exotics and perhaps local historical knowledge of land use change. Such information was used by soil surveyors in Eastern USA in the early 1900's in delineating phases. In South-Eastern Australia, post-settlement alluvium (i.e. deposited since European settlement and mostly <200 years old) blankets many valley floors. For many years it remained unknown in soil science circles since the material is often cultivated and even built upon (often an effective disguise) and treated as a natural part of the soil-landscape. Nevertheless, it often displays a paler colour, lower organic matter, preservation of bedding and a platy structure which readily distinguishes it from pre-settlement material. European artefacts, including partly to completely buried fence posts, fragments of porcelain etc., and/or OSL dating confirm the interpretation. Its identity was revealed during geomorphic investigations of valley fills. The source of these deposits has often been attributed to reworking of older valley fills but in at least one study the main source area is the adjacent valley sides with the evidence seemingly hidden within the topsoils of varying thickness.