Stabilization of soil aggregates on roadside embankments along a 70 years-old vegetation successional gradient

The stabilization of roadside embankments is a major challenge for land managers worldwide. While most studies focused on the short-term influence of revegetation measures (such as planting or hydroseeding) on soil stabilization, little is known about the long-term effect of successional dynamics occurring along roadsides. Our aim is to explore the influence of a vegetation successional dynamic on the stabilization of soil aggregates, a proxy for soil stability, along a 70-years roadside chronosequence. We selected 24 plots (16 x 4 m) on embankments along roadsides in the Mediterranean region (South France), spread into 5 age-classes (0-10; 11-20; 21-30; 31-40 and >40 years-old, Fig.1).We measured soil aggregate stability and several soil (soil organic carbon, soil nitrogen, soil texture, pH, CEC) and vegetation (root morphology, root mass density, plant community composition) characteristics. We found that soil aggregate stability varied significantly along the successional gradient, from unstable in early-successional plots (0-10 years-old) to very stable in late-successional plots (>40 years-old). More precisely, soil aggregates first appeared stable in the 31-40 years-old age class, reflecting that efficient stabilization of soil embankments by vegetation dynamics required about 3 decades (Fig.2). We notice that the most critical period for embankment stability is restricted to the 0-10 years-old class, characterized by unstable aggregates and thus high erosive risk, while moderate stability is reached from 11-20 years-old class.This highlights the importance to invest in revegetation measures in the first decade after roadside construction. Along the gradient, the accumulation of soil organic carbon related to plant community dynamics appeared as the major factor driving the stabilization of soil aggregates. The increase in root density also enhanced soil aggregate stability. Remarkably, the replacement of herbs species by shrubs and tree species as the succession proceeded was not related to any destabilization of the soils, even though the direct soil coverage is usually reduced by encroachment. To conclude, vegetation successional dynamics are responsible for the long-term stabilization soil aggregates on roadside embankments.