Root traits of crop species contributing to soil shear strength

Roots of annual species would be able to conserve soil properties during traffic induced compaction. The objective of this study was to determine and compare root traits and soil shear strength for three crop species with contrasted root types and morphological traits to see if roots of annual species are able to increase soil shear strength and thus explain the soil properties conservation. The experiment was performed under controlled conditions in steel cylinders containing a repacked loamy sand, with initial dry bulk density of 1.2 g cm−3 and that was kept moist. Soil shear strength parameters, i.e. cohesion and angle of internal friction, were determined from direct shear tests for soil cores at a soil matric potential of −10 kPa (i.e. water content at field capacity). The direct shear tests were performed with six external normal stress levels (25, 34, 44, 63, 83 and 93 kPa) and at a constant shear rate of 3 mm.min−1, and applying Mohr Coulomb equation. The difference of root type, root length density, root density, specific root length and root dry matter content among crop species was not related to a difference in soil shear strength. The root volume density was the main trait involved in both soil cohesion and the angle of internal friction. This study highlights the effect of roots of annual crop species on soil shear strength by comparing their root traits to the apparent cohesion of soil core where they grow. Vicia faba would be a good candidate to improve soil shear strength for soil conservation. This study constitutes a step towards improving the understanding of plants’ effects on soil shear strength with regards to selecting species and designing cropping systems for soil conservation.