Do root hairs of barley and maize roots reinforce soil under shear stress?

Highlights

• Barley roots were longer but the thicker maize roots and reinforced the soil more.

• Maize roots had greater tensile strength than barley roots.

• Presence of root hairs did not affect breaking force or tensile strength of root.

• Root hairs are too weak to influence the peak force needed to shear soil.

Abstract

Roots reinforce soil by acting as soil pins, dissipating shear stresses and anchoring the soil in place. By protruding into the soil and binding to soil particles, root hairs increase root-soil contact and aid root anchorage. However, it is not yet known whether this ability to anchor roots affects the root system’s ability to reinforce soil. Using a laboratory box shearing rig, this study explores whether root hairs affect soil shear resistance. The force required to shear soil columns permeated with roots lacking root hairs (barley brb and maize rth3 mutants) are compared to columns permeated with hairy roots (their respective wild types, WT) using unplanted soil columns as controls. Known root traits (e.g. root length density, root surface area density, average diameter, percentage of fine roots, and root tensile strength) were measured to ensure that differences in shear resistance could be attributed to the presence/absence of root hairs. All rooted columns required more force to shear than their respective unplanted columns but the thicker, stronger maize roots were more effective at soil reinforcement than the more numerous but weaker barley roots. After the maximum growth period, root hairs appeared to have a consistent and significant impact on peak shearing force. However, the WT root systems also produced greater root surface area density. As the rate at which peak shearing force increased with increasing root surface area density was similar for roots with and without root hairs, the increased peak shearing force of the WT columns cannot be attributed to resistance supplied by the presence of root hair but rather to a more prolific root system. Therefore, it was concluded that root diameter and root tensile strength most influenced root reinforcement of soil and as such, the relatively minute root hairs had negligible effects compared to their parent roots.

Introduction

Soil structural instability can pose a multitude of socio-economic problems. Small scale erosion can result in loss of fertile soils which has offsite consequences, such as sediment pollution, sedimentation of waterways, and an increase to flood risk (Boardman and Poesen, 2007, Pollen et al., 2013). Larger scale soil instability can result in mass wasting, such as landslides and soil creep, these have the potential to completely alter landscapes, destroy properties, and endanger lives (Petley, 2012). Understanding how to increase soil stability is key to developing methods that mitigate the detrimental effects of soil erosion and mass wasting.

Mass wasting ranges in scale but occurs when the frictional forces holding soil together are overcome by shearing forces caused by gravity. Soils are inherently anisotropic and are weak under shear forces (Al-Karni and Al-Shamrani, 2000). The fault line that occurs when soil fails under shear stress is called the shear plane and a soil’s shear strength is its ability to withstand these shear forces. Some soils are naturally susceptible to shear forces, either due to a layer of weakness referred to as a failure plane or because they have inherently poor particle cohesion. Most mass wasting events occur due to hydraulic pressures resulting from the increased weight of saturated soil or as a result of scouring from running water (Iverson, 2000). With decreasing scale of event, erosion from shear stress can be mitigated with increasing effectiveness by altering soil physical and biological properties.

Plant roots are widely understood to enhance soil shear strength by introducing tensile reinforcement to the soil, countering soil’s natural susceptibility to shear forces (Gyssels et al., 2005, Simon and Collison, 2002, Stokes et al., 2014, Stokes et al., 2009, Wu and Sidle, 1995). Fine roots penetrate laterally through the soil, enmeshing and binding the surface soil, whilst deeper penetrating tap roots cross failure planes, pinning them together as well as anchoring the fine root matting (Fan and Chen, 2010, Simon and Collison, 2002, Stokes et al., 2009). A root’s ability to reinforce the soil depends on its resistance to either being pulled out or breaking. Roots dissipate shearing forces throughout the whole system, increasing the area of soil that is engaged in anchorage until the roots are either broken or pulled out (Bengough et al., 2011, Stokes and Mattheck, 1996). A root remains anchored in the soil when there is sufficient root soil contact to provide friction in excess of the opposing forces (Ennos, 1990). Further, if the root’s tensile strength is greater than the friction of its anchorage roots will slip from the soil; if it is less the root will break (Pollen, 2007). For straight roots, without forks or bends, the length of the root determines how efficiently it is anchored. Forks and bends enables a root to engage more soil and dissipate the shear forces with greater effect. Both root breaking force (Docker and Hubble, 2008, Nilaweera and Nutalaya, 1999, Pollen and Simon, 2005, Tosi, 2007, Yang et al., 2016) and the force required to pull the root from the soil (Nilaweera and Nutalaya, 1999, Norris, 2005, Stokes et al., 2009) increase with root diameter, although, root tensile strength is inversely related to root diameter (Nilaweera and Nutalaya, 1999, Pollen and Simon, 2005, Genet et al., 2005). Therefore, root anchorage is affected by many different root traits.

Since the strength of the root is largely dependent on its diameter, most research in this area has focused on the roots of trees and woody shrubs. Fine roots, associated with annual and perennial species, have frequently been unified into one synonymous category (Hishi, 2007, Pregitzer et al., 2002, Reubens et al., 2007). While the impact of fine roots on shear erosion has been investigated, there are gaps in our understanding of how fine roots mitigate sub-surface shear erosion, and other root traits such as root hairs, have been almost completely disregarded in studies of soil reinforcement.

Root hairs emerge just behind the root elongation zone, protruding laterally to anchor the root and enabling the root tip to penetrate the soil (Bengough et al., 2011, Haling et al., 2013) whilst preventing the growth force from deforming the rest of the root or pushing the plant from the soil (Bengough et al., 2016, Handley and Davy, 2002). Root hairs are considered a key component in root anchorage (Czarnes et al., 1999, Ennos, 1989), to the extent that root anchorage is believed to be a primary function of root hairs (Bengough et al., 2011, Gilroy and Jones, 2000). However, whether this capacity to anchor the root to the soil reinforces soils under shear stress is unknown. This paper aims to address this knowledge gap by assessing the contribution of different root traits (including root hairs) to soil reinforcement. Soil columns permeated by root systems with and without root hairs were subjected to shear force and the resistance of the columns were measured.