Water-stable aggregation and organic matter in four soils under conventional and zero tillage

Zero tillage management reduces soil exposure and disturbance and, therefore, may improve soil aggregation and organic matter sequestration under some environments. We determined the distribution and soil organic C (SOC) content of five water-stable aggregate (WSA) classes at depths of 0-50, 50-125, and 125-200 mm in a loam, a silt loam, a clay loam, and a clay soil managed for 4-16 yr under conventional shallow tillage (CT) and zero tillage (ZT) in the Peace River region of northern Alberta and British Columbia. Macroaggregation (> 0.25 mm) and mean weight diameter (MWD) were greater under ZT than under CT in coarse-textured soils at a depth of 0-125 mm. Under CT, macroaggregation and MWD increased with increasing clay content, thereby reducing the potential of ZT to improve these properties in soils with high clay content. Concentration of SOC tended to be greatest in macroaggregates and lowest in microaggregates of coarse-textured soils, but was not different among WSA classes of fine-textured soils. Soil organic C content of macroaggregates under ZT was 0.34, 0.40, 0.62, and 0.16 kg m(-2) greater than under CT at a depth of 0-200 mm in the loam, silt loam, clay loam, and clay soil, respectively. Our results suggest that implementation of ZT in this cold semiarid climate can quickly improve WSA of coarse-textured soils and potentially increase SOC sequestration, albeit more slowly than in warmer more humid climates, when macroaggregation is improved.