Organic matter in size‐density fractions after 16–50 years of grass ley,cereal cropping and organic amendments

Size‐density fractionation, which was originally developed to examine short‐term decomposition of added material in sandy soil, was highly sensitive to medium‐ to long‐term changes in loam and clay soils. Materials from different size classes (>1 mm, 0.1–1 mm, 0.05–0.1 mm and <0.05 mm) were separated by density into light (ρ < 1.0 g cm⁻³), medium (1.0 < ρ < 1.85 g cm⁻³) and heavy (ρ > 1.85 g cm⁻³) fractions. In 16–18‐year cropping experiments the 0.1–1‐mm heavy fraction contained 17–19% of total carbon in ley soils compared with 7% in continuously cropped soils. Greater N‐mineralization rates after sieving of ley cropped soils could not be related to differences in C:N ratios of fractions, but this was assumed to be related to exposure of aggregate‐binding agents. In a 50‐year trial 40% of total soil carbon was contained in the 0.1–1‐mm heavy fraction in soil treated with sewage sludge compared with 7–9% in the fallow and ‘zero N’ treatments. Thus, the soils studied expressed an aggregate hierarchy dependent on organic carbon dynamics. The relative abundance of carbon in heavy organo‐mineral fractions >0.1 mm was inversely related to the relative abundance of C in black‐brown medium density material <0.1 mm, defined as uncomplexed particulate organic matter that was presumably released during ultrasonic disruption deterioration of finer (<0.1 mm) aggregated organo‐mineral particles. The size density fractionation allowed identification of materials of contrasting visual appearance, chemical qualities and, by inference, biological turnover times. However, they were found to be predominantly composite fractions and may be too complex to be represented by unique model pools.