Factors controlling aggregation in a minimum and a conventionally tilled undulating field

Wind and water erosion induce breakdown of soil aggregates and loss of soil organic matter. Whereas most of the relations between aggregation and its driving factors have been established on a plot scale, these relations might be very different within an undulating landscape where both erosion (by wind or water) and deposition occur. The aim of this study was to investigate to what degree spatial patterns in soil variables influence spatial patterns in aggregation under different tillage intensities. We studied an agricultural field of about 3 ha in the silty region of Belgium. The site was split into a conventional tillage (CT) and a minimum tillage (MT) system. Within the field, 396 geo-referenced surface soil samples (0-5 cm) were taken and analyzed for organic matter content, quantity of aggregates and a number of other soil properties. Under CT, 28.5% of the total sample variation was explained by the occurrence of depositional areas, 20.8% by the amount of soil organic matter, and 13.8% by the presence of a clay-rich B horizon which surfaced due to progressive water and tillage erosion. Regression analysis revealed that 27% of the variation in the quantity of macroaggregates (>0.25 mm) was accounted for by these three factors. Under MT, 27.1% of the total sample variation was related to the surface cover of Tertiary sand, 22.6% to the amount of soil organic matter, and 13% to erodibility. These three factors explained 53% of the variation in the quantity of macroaggregates. In the CT system, the correlation between grass- or maize- carbon and the quantity of macroaggregates was strongly linked to erodibility, while this was not the case in the MT system. We concluded that at this site, macroaggregation is dominated by landscape-scale processes (such as water or tillage erosion) rather than determined by the commonly considered local variables (such as small variations in texture or organic matter content).