A review of the mechanical effects of plant roots on concentrated flow erosion rates

Abstract

Living plant roots modify both mechanical and hydrological characteristics of the soil matrix (e.g. soil aggregate stability by root exudates, soil cohesion, infiltration rate, soil moisture content, soil organic matter) and negatively influence the soil erodibility. During the last two decades several studies reported on the effects of plant roots in controlling concentrated flow erosion rates. However a global analysis of the now available data on root effects is still lacking. Yet, a meta-data analysis will contribute to a better understanding of the soil-root interactions as our capability to assess the effectiveness of roots in reducing soil erosion rates due to concentrated flow in different environments remains difficult. The objectives of this study are therefore: i) to provide a state of the art on studies quantifying the effectiveness of roots in reducing soil erosion rates due to concentrated flow; and ii) to explore the overall trends in erosion reduction as a function of the root (length) density, root architecture and soil texture, based on an integrated analysis of published data. We therefore compiled a dataset of measured soil detachment ratios (SDR) for the root density (RD; 822 observations) as well as for the root length density (RLD; 274 observations). A Hill curve model best describes the decrease in SDR as a function of R(L)D. An important finding of our meta-analysis is that RLD is a much more suitable variable to estimate SDR compared to RD as it is linked to root architecture. However, a large proportion of the variability in SDR could not be attributed to RD or RLD, resulting in a low predictive accuracy of these Hill curve models with a model efficiency of 0.11 and 0.17 for RD and RLD respectively. Considering root architecture and soil texture did yield a better predictive model for RLD with a model efficiency of 0.37 for fibrous roots in non-sandy soils while no improvement was found for RD. The unexplained variance is attributed to differences in experimental set-ups and measuring errors which could not be explicitly accounted for due to a lack of additional data. Based on those results, it remains difficult to predict the effects of roots on soil erosion rates. However, by using a Monte Carlo simulation approach, we were able to establish relationships that allow assessing the likely erosion-reducing effects of plant roots, while taking these uncertainties into account. Overall, this study demonstrates that plant roots can be very effective in reducing soil erosion rates due to concentrated flow.