Soil Structural Stability during Rapid Wetting: Influence of Land Use on Some Aggregate Properties

A stable soil structure, resistant to the stresses induced by wetting, is important for maintaining agricultural productivity and reducing erosion. This study was undertaken to assess losses of soil structural stability resulting from long-term, continuous sugarcane (Saccharinum officinarum L.) cultivation (20 yr) in two Brazilian soils (an Oxisol and an Alfisol). When compared with virgin sites, sugarcane cropping decreased the organic C concentration and the wet aggregate stability (WAS) of the soil. The differences for organic C concentration and WAS between land use systems extended to the 1-m depth, in both soils, and were less prononced as depth increased. Cultivation also increased the extent of aggregate slaking relative to virgin sites. This increased sensitivity to structural damage could not be attributed to changes in aggregate cohesion; rather, it was observed that for the Alfisol, the greater slaking and mellowing (slaking without complete fragmentation) could be related to an increase in the rate of water uptake by the aggregate (1.9–3.4 times faster in the cultivated than in the virgin site) for the Alfisol. The pore tortuosity index was also 3.1 to 3.2 times higher in the virgin than in the cultivated site and indicated a pore network less efficient at transporting water within aggregates in the virgin site compared with the cultivated site. This suggests that the lower sensitivity to slaking in the virgin Alfisol was most likely caused by water entry within the aggregates slowed down by a clogged pore network.