Long-term effect of fresh and composted cattle manure on the size and nutrient composition of dry-sieved soil aggregates

Miller, J. J., Bremer, E., Beasley, B. W., Drury, C. F., Zebarth, B. J. and Larney, F. J. 2012. Long-term effect of fresh and composted cattle manure on the size and nutrient composition of dry-sieved soil aggregates. Can. J. Soil Sci. 92: 673–683. Application of feedlot manure to cropland may impact the size distribution and nutrient content of soil aggregates. The objective of this study was to determine the effects of fresh or composted feedlot manure containing straw or wood-chip bedding on dry-sieved aggregate size distribution and nutrient contents. Surface (0–15 cm) soil samples were obtained from a long-term field experiment where treatments were fresh (FM) or composted manure (CM) containing straw or wood-chips applied annually at 0 (control) or 77 Mg ha⁻¹ yr⁻¹ for 11 yr. Air-dried soil samples were separated using a rotary sieve into six aggregate size fractions ranging from <0.47 mm to >12.7 mm. Total C, total N, C:N ratio, soil mineralizable N (41-d incubation), total P, soil test P, and P saturation index were determined on the six aggregate fractions. The amendments significantly (P≤0.05) increased the proportion of smaller (<0.47 mm) aggregates and decreased the proportion of the larger (>12.7 mm) aggregates relative to the unamended control. The geometric mean diameter (GMD) was also lower and wind erodible fraction (WEF) was greater for the amended treatments than unamended control. We attributed this manure effect to increased organic matter content in the soil making the aggregates more friable and susceptible to breakdown by tillage. Carbon, N, and P concentrations were not shifted to smaller aggregate sizes where root growth and nutrient uptake are generally greater. The exception was mineralizable N, which tended to be greater in the finer<0.47 mm fraction. Mineralizable soil N in all aggregate sizes ≥0.47 mm was reduced for wood-chip compared with straw bedding, and resulted in net N immobilization in aggregate sizes ≥1.2 mm. Phosphorus sorption was lower in soil amended with wood-chips compared with straw bedding for aggregates ≥0.47 mm. Long-term manure application may shift soil aggregates from larger to finer fractions because of greater friability and suggests that these soils should be managed to avoid the greater risk of wind erosion.