Depth-dependent soil mixing persists across climate zones

Abstract

Soil mixing over long (>102 y) timescales enhances nutrient fluxes that support soil ecology, contributes to dispersion of sediment and contaminated material, and modulates fluxes of carbon through Earth’s largest terrestrial carbon reservoir. Despite its foundational importance, we lack robust understanding of the rates and patterns of soil mixing, largely due to a lack of long-timescale data. Here we demonstrate that luminescence, a light-sensitive property of minerals used for geologic dating, can be used as a long-timescale sediment tracer in soils to reveal the structure of soil mixing. We develop a probabilistic model of transport and mixing of tracer particles and associated luminescence in soils and compare with a global compilation of luminescence versus depth in various locations. The model–data comparison reveals that soil mixing rate varies over the soil depth, with this depth dependency persisting across climate and ecological zones. The depth dependency is consistent with a model in which mixing intensity decreases linearly or exponentially with depth, although our data do not resolve between these cases. Our findings support the long-suspected idea that depth-dependent mixing is a spatially and temporally persistent feature of soils. Evidence for a climate control on the patterns and intensities of soil mixing with depth remains elusive and requires the further study of soil mixing processes.

Rarefied particle motions on hillslopes – Part 4: Philosophy

“…For example, the ability to create a small version of a river reach in a laboratory and then measure long time series of bed load flux in order to fully characterize the fluctuations and ensembleaveraged behavior of such series in relation to bedform dynamics (Dhont and Ancey, 2018;Ancey and Pascal, 2020) is simply not a possibility in studies of natural soil creep and its long-term consequences. Indeed, we have only recently achieved the ability to measure small particle motions involved in soil creep (Deshpande et al, 2021), yet the particle residence times of soil mantled hillslopes may be 10 000 years or more, thus requiring indirect measures of particle behavior such as tracer particle mixing (Furbish et al, 2018c;Gray et al, 2020).…”

The perpetual fragility of creeping hillslopes

“…The hypothesized grain motions in Culling's model, however, are inconsistent with known granular mechanics (Supplementary Note 1); moreover, these motions have never been experimentally examined. Researchers have begun to recognize the need to understand grain-scale dynamics, in order to derive physically informed models of soil mixing and transport on hillslopes 18,19 . Decades of tracer measurements have produced coarse profiles of soil displacement on hillslopes 5 that are generally exponential 5,20,21 (Fig.…”

A geomorphic-process-based cellular automata model of colluvial wedge morphology and stratigraphy

“…Richardson et al, 2019) and converting this range into mobile regolith disturbance rate following equations in Tucker et al (2018). To obtain a range for mobile regolith production rate, we note that the Peclet Number, a measure of the relative magnitude of mobile regolith disturbance versus production, appears to fall into a range of 0.1-1 in global compilations, meaning that the orders of magnitude are roughly comparable (Gray et al, 2020). As such, we test the parameter space over a similar range as the mobile regolith disturbance rate.…”

“…For example, the debris facies in the model can form either due to the lack of internal cohesion of the parent material (high rates of lateral collapse), or due to mobile regolith forming processes acting on the exposed free-face, similar to interpretations by Wallace (1977). Such mobile regolith forming/disturbing processes could involve burrowing mammals, root growth, shrink-swell, and freeze-thaw cycles among many others (Gray et al, 2020). Another example is the field-observed effect of microclimate on scarp degradation where aspect-controlled water contents affect the degradation rate (e.g.…”