A modelling framework for pedogenon mapping

Abstract

Soil entities are generally defined based on soil properties, using morphological, genetic, or utilitarian criteria. Alternatively, soil entities could be characterized by groupings of homogeneous soil-forming factors under the assumption that the dominant soil-forming processes occurring over a time period within each group are similar, and therefore develop unique soil entities with similar soil properties. We define the pedogenon as a conceptual soil taxon defined from a set of quantitative state variables that represent the soil-forming factors for a given reference time. The objective of this study was to develop a methodology for mapping pedogenon classes at the time of the European settlement in New South Wales (Australia). This period was chosen as reference because from 1788 onwards the intensification of land use has accelerated the rate of change of soil properties. We implemented a two-step modelling approach with a set of environmental covariates representing the soil-forming factors, including the estimated natural vegetation at 1750. The k-means algorithm was applied to generate pedogenon classes suitable for local management. Then, hierarchical clustering was applied to identify the organization of pedogenons into families or “branches” of higher level taxa. We tested the ability of the pedogenon classes for explaining the variance of stable soil properties (particle size fractions) in the subsoil (30–60 cm depth) with redundancy analysis (RDA). The results indicated that between 800 and 1000 pedogenon classes provide the desired level of detail for both local and regional management across New South Wales. The influence of the pre-1750 vegetation types (e.g. Acacia open woodlands and shrublands, Callitris forests and woodlands) was apparent in the distribution of some pedogenon branches. Pedogenon classes differed in their characteristics (median area ≈750 km2), but overall showed meaningful spatial patterns at local scale and formed regional assemblages. The RDA models indicated that pedogenon classes explained about 30% of the variance of silt and clay content. This flexible modelling framework allows the creation of pedogenon maps over large areas at high resolution (90 m) and is applicable at different scales. Potential applications of pedogenon maps include the quantitative assessment of soil change and designing soil monitoring surveys.