Impact of root architecture on the erosion-reducing potential of roots during concentrated flow

Many studies focus on the effects of vegetation cover on water erosion rates, whereas little attention has been paid to the effects of the below ground biomass. Recent research indicates that roots can reduce concentrated flow erosion rates significantly. In order to predict this root effect more accurately, this experimental study aims at gaining more insight into the importance of root architecture, soil and flow characteristics to the erosion-reducing potential of roots during concentrated flow. Treatments were (1) bare, (2) grass (representing a fine-branched root system), (3) carrots (representing a tap root system) and (4) carrots and fine-branched weeds (representing both tap and fine-branched roots). The soil types tested were a sandy loam and a silt loam. For each treatment, root density, root length density and mean root diameter (D) were assessed. Relative soil detachment rates and mean bottom flow shear stress were calculated. The results indicate that tap roots reduce the erosion rates to a lesser extent compared with fine-branched roots. Different relationships linking relative soil detachment rate with root density could be established for different root diameter classes. Carrots with very fine roots (D < 5 mm) show a similar negative exponential relationship between root density and relative soil detachment rate to grass roots. With increasing root diameter (5 < D < 15 mm) the erosion-reducing effect of carrot type roots becomes less pronounced. Additionally, an equation estimating the erosion -reducing potential of root systems containing both tap roots and fine-branched roots could be established. Moreover, the erosion-reducing potential of grass roots is less pronounced for a sandy loam soil compared with a silt loam soil and a larger erosion-reducing potential for both grass and carrot roots was found for initially wet soils. For carrots grown on a sandy loam soil, the erosionreducing effect of roots decreases with increasing flow shear stress. For grasses, grown on both soil types, no significant differences could be found according to flow shear stress. The erosion-reducing effect of roots during concentrated flow is much more pronounced than suggested in previous studies dealing with interrill and rill erosion. Root density and root diameter explain the observed erosion rates during concentrated flow well for the different soil types tested. Copyright (c) 2007 John Wiley & Sons, Ltd.