Freeze-Thaw Cycles Increase Near-Surface Aggregate Stability

Soils with stable surface aggregates resist water and wind erosion better than soils with unstable aggregates. From earlier studies, we had preliminary evidence that one to three freeze-thaw cycles (FTCs) increased soil aggregate stability when measured by wet sieving field-moist aggregates. In this study, we measured the stability of aggregates vapor-wetted to field capacity from the Ap horizons of four soils after undergoing either zero, one, two, or four FTCs, and we determined the number of FTCs at which aggregate stability would be greatest for each soil. Moist soil was packed to a dry bulk density of 1.15 Mg m−3 by tapping it into 28-mm-diameter, 50-mm-tall brass cylinders. Each cylinder was then sealed in a polyethylene bag and inserted into a polystyrene foam tray. The soil in each cylinder was frozen convectively at −5°C for 48 h and then thawed at +6°C for 48 h for each FTC. Aggregate stability increased with the first one to two FTCs but changed little thereafter. Trend analysis revealed that aggregate stability would be greatest after two or three FTCs. When averaged across the four soils, FTCs stabilized aggregates more at 0 to 15 mm than at 15 to 30 mm. Near the surface of wet soils, two or three FTCs may be beneficial rather than detrimental to soil structure.