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Link to original content: https://pubmed.ncbi.nlm.nih.gov/26087130
Soil Penetration by Earthworms and Plant Roots--Mechanical Energetics of Bioturbation of Compacted Soils - PubMed Skip to main page content
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. 2015 Jun 18;10(6):e0128914.
doi: 10.1371/journal.pone.0128914. eCollection 2015.

Soil Penetration by Earthworms and Plant Roots--Mechanical Energetics of Bioturbation of Compacted Soils

Siul Ruiz  1 , Dani Or  1 , Stanislaus J Schymanski  1
Affiliations

Soil Penetration by Earthworms and Plant Roots--Mechanical Energetics of Bioturbation of Compacted Soils

Siul Ruiz et al. PLoS One. .

Erratum in

Abstract

We quantify mechanical processes common to soil penetration by earthworms and growing plant roots, including the energetic requirements for soil plastic displacement. The basic mechanical model considers cavity expansion into a plastic wet soil involving wedging by root tips or earthworms via cone-like penetration followed by cavity expansion due to pressurized earthworm hydroskeleton or root radial growth. The mechanical stresses and resulting soil strains determine the mechanical energy required for bioturbation under different soil hydro-mechanical conditions for a realistic range of root/earthworm geometries. Modeling results suggest that higher soil water content and reduced clay content reduce the strain energy required for soil penetration. The critical earthworm or root pressure increases with increased diameter of root or earthworm, however, results are insensitive to the cone apex (shape of the tip). The invested mechanical energy per unit length increase with increasing earthworm and plant root diameters, whereas mechanical energy per unit of displaced soil volume decreases with larger diameters. The study provides a quantitative framework for estimating energy requirements for soil penetration work done by earthworms and plant roots, and delineates intrinsic and external mechanical limits for bioturbation processes. Estimated energy requirements for earthworm biopore networks are linked to consumption of soil organic matter and suggest that earthworm populations are likely to consume a significant fraction of ecosystem net primary production to sustain their subterranean activities.

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Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Fig 1
Fig 1. A sequence of images of the front segments of an earthworm moving through agar by a series of penetration-expansion steps.
(a) Displays the axial penetration inducing an initial cavity. (b) Illustration of cavity expansion when collecting expanded segments. (c) Further penetration post anchoring processes.
Fig 2
Fig 2. Concept of Elasto-Plastic cavity expansion.
Cavity expansion is based on the assumption of a constant ratio between the initial cavity (r c) and a fixed Elasto-Plastic interface (R) at a distance proportional to the internally applied cavity pressure (P). The stress field propagating into the soil, (σ r, σ θ) depends on the distance from the center (r).
Fig 3
Fig 3. Cylindrical cavity expansion sequentially determines steady state penetration of acute cones.
The conical cross section applies a boundary pressure that opens a cavity to some final steady state cylindrical burrow.
Fig 4
Fig 4. Measured shear modulus values.
(a) Different clay contents for fixed water content of 0.2 kg kg−1 [21]; (b) different water contents for 100% clay content [21, 38, 39].
Fig 5
Fig 5. Fracture toughness vs water content. [–52].
Continuous curve was plotted through the data points in order to approximate fracture toughness values at different water contents.
Fig 6
Fig 6. Analytical cavity expansion model vs. Finite Element cavity expansion model.
(a) Relative cavity pressure vs. relative cavity radius; (b) strain energy density scaled by limit pressure vs. relative cavity radius scaled by initial radius. For both models, changes in soil mechanical parameters only changed the magnitude of P L. Both analytic and numerical models showed close magnitudes of P L. The discrepancy between the strain energy density values was measured as: ‖U0,NUM−U0,ANA‖∞‖U0,NUM‖∞=0.11.
Fig 7
Fig 7. Comparison of analytical cavity expansion based model with an adaptive finite element explicit penetration model (Walker and Yu [48]).
Comparison is drawn between the relative penetration stress vs the radial strain. Penetration stress is scaled by the shear soil strength. The discrepancy between the pressure values over two orders of magnitude never exceeds ‖PNUM−PANA‖∞‖PNUM‖∞<0.2
Fig 8
Fig 8. Soil mechanical impedance to cone penetration for different soil mechanical properties and cone types.
Two replicates of a silty clay were measured to have (a) s u = 65kPa and G = 567 × s u; (b) s u = 40kPa and G = 150 × s u. Experimental data correspond to two tests conducted with duplicate cones of the same geometry but subtle physical design differences [49]: miniature piezocone penetrometer (PCPT4 and PCPT6) and miniature quasi-static cone penetrometer (PCPT3 and PCPT5). Data points were obtained from [49] with the dashed lines denoting the positions when the cone was fully inserted.
Fig 9
Fig 9. Predicted cylindrical cavity pressure as a function of saturation for two clay contents (16 and 50%) and for a range of cavity radii (1 to 5 mm).
The blue and red curves denote soils with clay contents of 16% and 50% respectively, with the hydro-mechanical correlations presented in Table 3. Thick red and blue lines refer to earthworm radius of 2.5 mm, while the enveloping curves represent radii between 1 and 5 mm.
Fig 10
Fig 10. Maximum penetration resistance stress vs. cone apex angles for different normalized water contents with a base radius of 2.5 mm at 16% clay content.
Simulations were conducted for normalized water contents of 0.1, and 1.0 at a soil clay content of 16%. Soils with larger clay content display similar mechanical behavior at larger normalized water contents.
Fig 11
Fig 11. Penetration stress and resistances as a function of base radius.
(a) Penetration stress; (b) penetration resistance. Dashed curves denote soils with clay contents of 50%, while solid curves denote clay contents of 16%. Penetration stress (a) decreases for increasing base radius for fixed soil mechanical properties. In contrast, for the same penetration stresses, penetration resistance (measured as axial force) increases with increasing base radius.
Fig 12
Fig 12. Required soil organic carbon content and maximum strain energy density as a function of normalized water content for clay contents of 16% and 50%.
Analysis is based on the steady state mechanical model to determine strain energy. For given soil organic carbon content, one can determine the range of normalized soil moistures under which the energetic demands of displacing a volume of soil are less than the energy stored in the soil organic carbon in the same volume (range to the right of the respective line in the figure). This neglects any rate dependent effects. Note that mechanical energy is mapped to soil organic carbon content using a factor of 1.2/24.8 kg kJ−1 [7], and the tick marks on the left vertical axis were spaced in order to align with those on the right vertical axis.
Fig 13
Fig 13. Mechanical energy to create a burrow of 1 m length and 1.2 mm radius as a function of normalized water content using a penetration model and a fracture model.
Both models were conducted for a worm of r = 1.2 mm [50] for soils with clay contents ranging from 15–25% [, –52].
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Grants and funding

This work was supported by Swiss National Science Foundation grant number 406840_143061 (http://www.nrp68.ch/E/projects/farming/Pages/default.aspx).

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