Aggregate stability in range sandy loam soils Relationships with runoff and erosion

The spatial variability of soil aggregate stability and its relationship to runoff and soil erosion were examined in a catena of soils and vegetation in a semiarid environment at the Rambla Honda field site (Tabernas, Almería, SE Spain) to evaluate the validity of structural stability as a soil erosion indicator in sandy loam range soils. The influence of soil properties and topography on the variability of aggregate stability was also examined. Methods include: 1) aggregate stability assessment at 12 sites (3 repetitions per site) on the hillslope by two methods: a) aggregate size distribution by dry sieving b) water drop test; 2) soil organic carbon content; 3) particle size distribution determination; 4) terrain attributes derived from a digital elevation model (1-m resolution); 5) monitoring runoff and erosion for nearly 3 years in eight (10 × 2 m) plots distributed over the hillslope. Results: 41% of the average soil mass is formed by > 2-mm aggregates. However, wet aggregate stability is poor, with a mean (of a total of 1440 aggregates) of only 26 drop impacts necessary to break up a wet aggregate (pF = 1). Significant relationships were found in the number of water drops required for aggregate breakdown and runoff and erosion rates. However, no significant relationships between the mean weight diameter of aggregates under dry conditions and runoff or erosion rates were observed. The relationships of aggregates with other soil properties, hillslope position and proximity to plants are also analysed. The most significant correlation found was between the number of drop impacts and soil organic matter content. The stability of topsoil aggregates seems to be a valuable indicator of field-assessed runoff and inter-rill erosion of sandy loam range soils under semiarid conditions.