Macroaggregate characteristics in cultivated soils after 25 annual manure applications

Agricultural management practices that after the soil organic matter (SOM) content are expected to cause changes in soil stability and aggregation. Animal manure is a source of organic matter (OM) that has been demonstrated to increase macroaggregate formation and stability. The objectives of this study were to determine how long term cattle manure applications to a calcareous Haploboroll clay loam (Lethbridge, AB, Canada) affected aggregate size distribution, the total C, N, and P content of aggregate size fractions, and water-stable aggregates. Beef cattle manure applied at rates >30 Mg ha(-1) yr(-1) under dryland production and >60 Mg ha(-1) yr(-1) to soils under irrigation resulted in fewer dry-sieved aggregates >7.1 mm and more dry-sieved aggregates between 0.47 and 1.2 mm in the 0- to 5-cm depth, compared with unamended soils. The dry-sieved aggregate fractions between 0.47 and 1.2 mm include the <0.84-mm fraction that has been associated with increased susceptibility to wind erosion in the Canadian semiarid prairies. There was more total C, N, and P in all dry-sieved aggregate fractions of soils receiving >30 Mg manure ha(-1) yr(-1) than unamended soils, and dry-sieved aggregates between 0.47 and 2.0 mm tended to have the highest C, N, and P contents. Water aggregate stability was higher in irrigated than dryland soils, but did not improve with increasing manure application rates. Dispersing agents in the cattle manure appear to have destabilized the larger soil macro aggregates.