Role of the underlying massive soil structure on surface platy structure development under no-tillage in silty soils of the argentinean humid pampas

摘要: The generalization of no-tillage (NT) in the silty soils of the Argentinean Humid Pampas has led to widespread soil densification (massive structure) and the frequent development of a platy structure layer near the soil surface. Previous findings have shown that exposure of a massive soil structure to wet-dry cycles results in the rapid extension of the platy structure, which has negative consequences on water infiltration and vertical transfer. We used a 25-year NT trial with two contrasted crop sequences (a soybean monoculture and a Maize-Wheat/Soybean succession), which resulted in highly different proportions of platy structure in the A horizon (>50% vs. <20%), with the aim to identify potential causes of such large differences in platy structure development. We compared pore size distribution, aggregate stability, shear strength and rate of water transfer of the massive soil structure of each of the two situations, assuming that the differences in either the strain associated with wet-dry cycles or resistance of the soil to this strain may explain the observed differences in platy structure development. The main differences between the two situations were that the Maize-Wheat/Soybean succession showed a higher proportion of large macropores (85% > 300 µm), a higher water transfer rate and a higher aggregate stability. In addition, the most sensitive mechanism of aggregate breakdown was differential swelling during the slow wetting of the soil. The combination of a faster water movement in the Maize-Wheat/Soybean succession and the main role of differential swelling during the slow wetting of the soil for aggregate breakdown suggest that …