Mechanics and Energetics of Soil Penetration by Earthworms and Plant Roots: Higher Rates Cost More

Abstract

Core Ideas We quantified mechanical differences between earthworms and roots using penetration rates. Mechanical modeling was justified by direct imaging of both plant roots and earthworms. We validated model predictions with rate‐controlled miniature cone penetrometer experiments. Earthworm burrows are dominantly formed through mechanical processes, not ingestion. We outline mechanical and energetic limitations for a range of water contents. We quantified the mechanics and energetics of soil penetration by burrowing earthworms and growing plant roots considering different penetration rates and soil mechanical properties. The mechanical model considers cavity expansion by cone‐like penetration into a viscoelastic soil material in which penetration rates affect the resulting forces and hence the mechanical energy required. To test the predicted penetration rate effects on forces and energetics, we conducted rate‐controlled cone penetration experiments across rates ranging from 1 to 200 μm s−1 to determine the mechanical resistance forces for cone geometries similar to plant roots and earthworms. These measurements also enabled inverse estimation of soil rheological parameters that were in good agreement with literature values for similar soils and water contents. The results suggest that higher soil penetration rates typical for earthworm activity (about 200 μm s−1) may significantly increase resistance forces and energy expenditure by up to threefold relative to slower penetration rates of plant roots (0.2 μm s−1) for similar soil properties and geometries. Another important mechanical difference between earthworms and roots is the radial pressures that earthworms' hydro‐skeleton exerts (<230 kPa), whereas plant roots may exert radial pressures exceeding 1 MPa. These inherent differences in burrowing rates and expansion pressures may significantly extend the range of conditions suitable for root growth in drier and compacted soil compared to earthworm activity. Results suggest that the mechanical energy costs of soil bioturbation under agricultural intensification and drier climate could greatly increase the energetic costs of these ecologically important soil structure‐forming bioprocesses.

Continuum cavity expansion and discrete micromechanical models for inferring macroscopic snow mechanical properties from cone penetration data

“…Alternatively, the continuum mechanical model (CEM) infers snow bulk mechanical properties considering a remote elastic influence that dense snow would have during cone penetration and also allows the model to predict the higher elasticity of snow while characterizing the penetration force profile (details in supporting information section S3). Furthermore, the CEM is capable of considering effects of different cone penetration rates and potential of snow viscous deformation [Mellor, 1974] by considering different rheological formulations [Ruiz et al, 2017], which also manages scale dependency. However, unlike the MMM, estimates of [Mellor, 1974] (orange shaded region in the plots).…”

“…Ladjal and Wu [2011] later derived the scale effect from a hardening parameter, which is a function of second-order strain gradient. The scale effect can be managed without assuming higher-order gradient terms when considering rate dependency [Ruiz et al, 2017]. The scale effect can be managed without assuming higher-order gradient terms when considering rate dependency [Ruiz et al, 2017].…”

“…Furthermore, the CEM is capable of considering effects of different cone penetration rates and potential of snow viscous deformation [Mellor, 1974] by considering different rheological formulations [Ruiz et al, 2017], which also manages scale dependency. Furthermore, the CEM is capable of considering effects of different cone penetration rates and potential of snow viscous deformation [Mellor, 1974] by considering different rheological formulations [Ruiz et al, 2017], which also manages scale dependency.…”

“…The gradient-based approach was originally used by Vardoulakis and Aifantis [1991] to approximate the rate-dependent flow law of the yielding material. The scale effect can be managed without assuming higher-order gradient terms when considering rate dependency [Ruiz et al, 2017]. For the rate-independent penetration model considered in this study, we approximate of the scale dependency with the radial stress equation [Ruiz et al, 2016].…”

Influence of Pore Characteristics on the Fate and Distribution of Newly Added Carbon

“…Plant root diameters are typically >40 µm and, therefore, roots can only access and/or create pores exceeding that size (Wiersum, 1957;Cannell, 1977). Root pores are formed by compressing the soil matrix radially as the root pushes through the soil and then their walls are stabilized through mucilage (Gray and Lissmann, 1938;Greacen and Oh, 1972;Greacen and Sands, 1980;Czarnes et al, 2000;Ruiz et al, 2017). Fungal hyphae are known to create pores of 20-30 µm size by pushing aside silt particles and exuding binding agents to buttress the pores (Dorioz et al, 1993;Bearden, 2001;Emerson and McGarry, 2003).…”