Losses of biomechanical properties and soil reinforcement upon the decomposition of the roots of Cynodon dactylon

Plant roots provide mechanical reinforcement to soil and improve soil shear strength. How root decay upon mortality may affect the root biomechanical properties and the subsequent changes in root reinforcement to soil have rarely been systematically studied. The aim of this study is to provide new experimental evidence and quantify the influences of root growth and decomposition on the temporary variations in root breakage strength, root Young’s modulus and the shearing behaviour including dilatancy of compacted soils vegetated with a grass species, Cynodon dactylon L. In this study, C. dactylon was cultivated for six months in 33 columns of compacted lateritic soils (90 mm diameter and 115 mm height), and then either burned or treated with herbicide to introduce root mortality and decay. At different durations of plant growth (60, 120 and 180 days), decay after plant burning (30, 60, 120, 180 and 360 days) and after herbicide application (15, 30 and 60 days), each column was split into two parts; the top part was used for direct-shear tests, whilst root samples were collected from the bottom part for the measurements of root diameter as well as root biomechanical and chemical properties (including the cellulose and lignin contents) (n = 303). Our results showed that all the tensile strength-diameter relations of the roots of C. dactylon followed a negative power law relation (R² > 0.6) regardless of the treatment applied. Growth effects had significant effects on the increase in median tensile strength, which was consistent with the increase in cellulose and lignin contents. As a result, 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. The predominant root failure mode at all growth durations was pull-out (rather than breakage); thus, the soil shear strength was better explained by root modulus (36.0%; which defines root extension) than root strength (25.8%; which defines root breakage capacity) and root biomass (1.6%; which defines root content). On the other hand, root decay due to burning or herbicide application caused significant reductions in cellulose and lignin contents, accompanied by a drop in root tensile strength. This explained the significant reductions in soil dilatancy and soil shear strength. The predominant root failure mode switched from pull-out to breakage. Compared with burning, herbicide application introduced a greater and faster degradation of cellulose and lignin con-tents, which explained the more significant and faster root weakening. Reducing the shear strength of the vegetated soils to the level of fallow soil took approximately 2 months of herbicide application and 4 months of burning.