Relationship between soil erodibility and topsoil aggregate stability or carbon content in a cultivated Mediterranean highland (Aveyron, France)

In the Rougiers de Camares area (in the south of France), hillslopes are very susceptible to water erosion. This is the result of physical features (steep slopes, soft bedrocks, thin soils), climatic aggressiveness (frost, storms), as well as farming systems (intensive tillage, short crop cycles, land consolidation). The objective of this work was to study the relationships between soil erodibility, macroaggregate stability, and carbon content of surface samples (0-10 cm), in a Rougiers Entisol (Lithic Udorthent) under various management practices (flat or raised moldboard ploughing, superficial tillage, direct drilling, with inputs in the form of mineral fertilizers or sheep manure). The soil erodibility was assessed by field rainfall simulation (60 mm h-1) on manually retilled bare dry soil; water-stable macroaggregation (>0.2 mm) was assessed by wet-sieving, after immersion in water. Runoff, turbidity and soil losses were linked to water-stable macroaggregation and carbon content in the 0-10 cm layer. During the first 30 minutes of rainfall, runoff and soil losses were closely correlated with topsoil initial water-stable macroaggregation, but not with topsoil carbon content (although there was a correlation between water-stable macroaggregation and carbon content). At the end of the rain (runoff steady state), turbidity and soil losses were closely correlated with topsoil carbon content, and to a lesser extent, with waterstable macroaggregation. Water-stable macroaggregation (which prevents crusting) and carbon content (which has an effect upon liquidity limit, among others) were thus important determining factors of erodibility for the studied soil. The influence of management practices on soil erodibility was therefore dependent upon their effects on these factors.