Aggregate stability as an indicator of soil susceptibility to runoff and erosion;: validation at several levels

The evaluation of soil susceptibility to runoff and water erosion in the field is often expensive or time-consuming. Several authors have reported that susceptibility is linked to aggregate stability, whose determination is far easier. However, this susceptibility has generally been deduced from rainfall simulation experiments on sieved soil samples, whose behaviour is not always representative of Field-scale phenomena. Our aim was to extend the validity of relationships between soil aggregation and erosion through comparisons of topsoil aggregate stability and field-assessed susceptibility to runoff and erosion. Susceptibility to runoff and erosion was determined at several levels: first, on a southern French Regosol, through measurements of runoff and soil loss from 1-m(2) microplots under simulated rainfall, second, from 100- to 800-m(2) runoff plots on a Nitosol in Benin, a Ferralsol in Cameroon and a Regosol in Mexico (with additional data on Syria from the literature); and finally, soil susceptibility to erosion was determined through semi-quantitative assessment of the frequency of erosion features on vineyard hillsides in southern France. Aggregate stability was determined by immersion in water and wet-sieving of 2-mm sieved, air-dried 0- to 10-cm soil samples, which actually tests aggregate resistance to slaking. Under simulated rainfall, runoff depth and soil loss after 30 min were negatively correlated with topsoil content in stable macroaggregates (>0.2 mm). On runoff plots, 3-year runoff rate and soil loss were negatively correlated with topsoil aggregate stability, especially stable macroaggregate content; these correlations were improved when slope gradient and climate aggressiveness were considered in addition to aggregate stability. On vineyard hillsides, the frequency index of erosion features was negatively correlated with topsoil content in stable macroaggregates, especially in the absence of conservation practices.