Shearing behaviour of vegetated soils with growing and decaying roots

Plant roots affect the soil shear strength, but the temporal dynamics of roots such as growth and decay due to different mortality means have rarely been considered. This study investigates the effects of root growth and decay on the changes in root biomechanical properties and the shearing behaviour of vegetated soils. Columns of compacted soils were grown with a grass species, Cynodon dactylon. The grass was grown for 6 months, followed by burning or herbicide applications to introduce root decay. The top part of each column was used for direct shear tests, whilst the roots collected from the bottom part were used for the measurements of root tensile and chemical properties. The vegetated soils displayed greater shear strength and larger dilatancy, which were attributable to the growth-induced increase in the root cellulose content, and thus the root tensile strength and modulus. Root decay, upon burning or herbicide use, caused significant declines in the root tensile properties, which translated to the reductions in soil dilatancy and soil shear strength lower than those of the fallow soil. Herbicide use introduced a much faster and significant loss of root reinforcement than burning because of the more severe decay and loss of root biomechanical properties.