Changes to the dispersive characteristics of soils along an evolutionary slope sequence in the Vera badlands, southeast Spain: Implications for site stabilisation

Abstract

<p>In dispersive marls near Vera, badland gullies are eroding into a previously stable matorral-covered landscape. This relatively recent activity was probably triggered by minor climatic or land-use change. In this landscape, Calvo-Cases and Harvey [Earth Surface Processes and Landforms 21(1996) 725] argued that new gully-head slopes with differing aspect represent a sequence of progressive erosion. Using this interpretation as a basis, slope surface materials were explored for evidence of material (chemical) stabilisation by sampling along profiles positioned centrally on those slopes. The extent to which the inferred geomorphic sequence is paralleled by progressive changes in site geochemistry is considered.</p><p> Crusts and subsoils differ significantly in terms of pH and Sodium Adsorption Ratio (SAR), but not calcium content. The crust is thus distinctive but not calcic, rather the pH differences suggest a process involving an organic agent, probably algae. Slope position was an important discriminant; taking SAR alone, differences between crusts and subsoils were not significant at the slope base but increased in significance with distance up the profile. Considering differences between the three profiles, the 'middle' profile had significantly higher SAR values both in the crust and at depth, than the other profiles, whereas 'late' slope subsoils and crusts taken alone are not dispersive (SAR< 10), nor are the soils at the base of the other two profiles. Whilst pH increases systematically with SAR up all the profiles, crust pH vs. SAR 'signatures' progressively bunch when compared to those of the subsoils. It is inferred that surfaces loose dispersive status first, with subsequent subsoil stabilisation occurring at the slope base. Although the poor statistical fit on some of the pH/SAR relationships rules out a conclusive proof that these three profiles represent a progressive or 'ergodic' geochemical stabilisation sequence, the progressive changes in calcium content of crust and subsurface from profile to profile is statistically significant. From this perspective, calcium may be involved over the longer term in the reduction of surface dispersivity. </p><p> The potential 'stabilisation' of this type of badland soil and surface is finally considered. Whereas Harvey [Harvey, A.M., 1982. The role of piping in the development of badlands and gully systems in southeast Spain. In: Bryan, R., Yair, A. (Eds.), Badland Geomorphology and Piping. Geobooks, Norwich, pp. 317–335] argues that site morphology suggests that geomorphological instability increases through time, the soil sequence suggests decreases in material dispersivity and consequently, surface autostabilisation. It is concluded that the concept of badland 'stabilisation' needs further clarification.</p>