Soil Security (Sep 2021)
A framework to assess changes in soil condition and capability over large areas
Abstract
The assessment of changes in soil condition and capability requires the identification of a reference state specific to each soil class. This study develops a framework for mapping soil classes that can be used as a reference state. It identifies soil classes that should have undergone similar historic anthropedogenesis, and differentiate, within each class, zones that have been less affected by human activities. This approach could be used as a baseline for assessing contemporary soil change, as demonstrated in the state of New South Wales in Australia. First, we established soil classes with similar multimillennial natural pedogenesis and historic anthropedogenesis, called pedogenons. This was achieved by applying unsupervised classification (k-means) to a set of quantitative state variables, proxies of the soil-forming factors at the time of the European settlement in New South Wales (climate, relief, parent material, and estimated pre-1750s vegetation). Pedogenon classes were then stratified into subclasses (ranging from remnant pedogenons to different pedophenons) by combining information on native vegetation extent, status (remnant or cleared) and current land use (i.e., land use history). The stratification of 1000 pedogenon classes resulted in 5448 subclasses, ranging from remnant pedogenons (located in protected areas of intact native vegetation), quasi-remnant pedogenons (production with low intervention on remnant native vegetation), cleared, grazing, and cropping pedophenons. The median of the area proportion of the pedogenon that was still preserved as remnant vegetation was 5.3%. This quasi-remnant pedogenon or the less affected pedophenon could be used as reference state. Pedophenon grazing and cropping occupied larger areas, with mean values of 73 km2 and 153 km2, respectively. The application of this framework for assessing soil change is illustrated using legacy data of topsoil pH (5 – 15 cm) as one indicator of soil condition. The ability of the pedogenon and pedophenon subclasses for explaining the variation of three stable (total Si, total Al, clay) and three dynamic (bulk density, particulate organic carbon, pH) soil properties from agricultural soils. A generalised least squares model indicated that the effects of pedogenon, land use history and their interaction on topsoil pH were statistically significant (p < 0.001). Paired comparisons between pedogenon/pedophenon subclasses by pedogenon class were not statistically significant, although we observed the general trend: remnant pedogenon ≈ quasi-remnant pedogenon < pedophenon cleared ≈ pedophenon grazing < pedophenon cropping. Redundancy discriminant analysis indicated that pedogenons explained 40% of the variation of stable and dynamic soil properties, pedogenon/pedophenon subclasses explained 0.1% and the shared effect explained 18%, leaving 42% of unexplained variance. The effects of pedogenon/pedophenon subclasses on the location of group centroids were statistically significant only when dynamic soil properties were considered, but not for stable and dynamic soil properties. This framework can be integrated into a soil security assessment once the indicators of soil condition and capability are translated into soil functions and ecosystem services. Other potential applications include the design of soil monitoring sampling schemes and identifying thresholds of soil degradation.
