Soil production in heath and forest, Blue Mountains, Australia: influence of lithology and palaeoclimate

Abstract

Recent determinations of soil production from in situ cosmogenic nuclides indicate that production decreases exponentially with soil depth. This contrasts with a long-held assumption that maximum soil production occurs under a soil cover of finite depth. Sites in the Blue Mountains, Australia, show a sharp decrease of soil depth where vegetation changes from forested plateau surfaces to heath-covered spurs, and bands of bare rock in the heath suggest that soil production depends on presence of a finite depth of soil. The substrate varies from hard ferruginized sandstone to soft saprolite. In situ 10 Be determinations indicate that apparent rates of erosion and soil production are greater under the relatively thin heath soil than under the thicker forest soil but, in contrast to other studies, these sites do not show significant depth-dependence of apparent soil production. The pattern reflects both hardness variation in the rock substrate and the effect of Late Quaternary climatic change. Optically stimulated luminescence (OSL) dating indicates that soil ≤ ≤ ≤ ≤ ≤30 cm depth is of Holocene age whereas the deeper soil is substantially older. The age-break coincides with a stone line interpreted as a former surface lag deposit. Assuming that pre-Holocene soil depths were 30 cm less than today, recalculated soil production tends to decrease with increasing depth. Soil production at this site requires soil cover but bare rock patches and vegetation comprise a shifting mosaic. In the long term, average rates of erosion and soil production decrease with increasing soil depth.

Toward process-based modeling of geochemical soil formation across diverse landforms: A new mathematical framework

“…Several published works, using in-situ and meteoritic cosmogenic nuclides, determined the physical production rate of the PDZ (Heimsath et al, 1997;Small et al, 1999;Heimsath et al, 2000Heimsath et al, , 2001aWilkinson et al, 2005) and chemical production rate of saprolite (Pavich, 1986(Pavich, , 1989), but the rates of both processes at two distinct interfaces within a single soil profile have not been considered or determined.…”

Tree-root control of shallow landslides

“…In the long term, the presence of vegetation (i) increases soil production rates through mechanical and chemical processes (Wilkinson et al, 2005;Phillips et al, 2008) (100-1000 years); (ii) increases soil residence time on hillslopes due to root reinforcement and protects against runoff erosion (Istanbulluoglu and Bras, 2005) (10-100 years; note that in the case of natural or human driven disturbances, the response time of the system (i.e., root decay) is of the order of a few years (Vergani et al, 2016)); and (iii) enhances soil diffusion rates on hillslopes due to tree wind throw (Pawlik, 2013;Roering et al, 2010), root mounds (Hoffman and Anderson, 2014), and biological activity (Gabet and Mudd, 2010) (100-1000 years).…”

Solving a conundrum of a steady-state hilltop with variable soil depths and production rates, Bodmin Moor, UK

“…McKean et al, 1993;Heimsath et al, 1997Heimsath et al, , 2000Small et al, 1999;Wilkinson, 2005). The approach is predicated on an assumption that the morphology of the study site does not change in time (i.e., the site is in geomorphic steady state, see section 2) where soil thickness (shielding CRN exposure), surface lowering rate, and soil production rate from underlying bedrock have remained constant for timescales commensurate with the residence time of the particles analyzed (Lal, 1991).…”