Similarity of differently sized macro-aggregates in arable soils of different texture

The concept of structural and functional hierarchy prevails in studies on the turnover of organic matter (OM) in soil. We examined the degree of structural hierarchy in arable soils sampled at six locations along a naturally occurring texture gradient (11 to 45% clay, 12.9 to 15.8 mg C g− 1 soil). One set of samples was incubated with 14C labeled ryegrass for 8 months while another set remained unamended. Constituent aggregates were isolated from the soils by fragmentation along natural planes of weakness. Eight aggregate size-classes (< 0.063, 0.063–0.125, 0.125–0.25, 0.25–0.50, 0.5–1.0, 1–2, 2–4 and 4–8 mm) were isolated and analysed for textural composition and contents of total-C (and 14C). The evolution of CO2 (and of 14CO2) during a 15-week laboratory incubation was determined for four aggregate size-classes in the range 0.125 to 2 mm. The aggregate size distributions reflected differences in whole soil texture with decreasing proportions of < 1 mm aggregates and increases in > 2 mm aggregates with increasing whole soil clay content. While the texture of aggregates > 1 mm was similar to that of the whole soil, aggregates within the range 0.125 to 1 mm were enriched and those < 0.125 mm depleted in sand. Macro-aggregates (> 0.25 mm) showed a similar distribution of native and of 14C labelled OM. For micro-aggregates, concentrations of native and labelled OM increased with decreasing size. When corrected for sand particles > 63 μm, the only consistent trend in native and labelled OM across aggregate size-classes was a higher content in micro- than in macro-aggregates. While native OM showed no consistent trend in lability, the 14C labeled OM in the 0.125–0.25 mm size-class was more labile than that in macro-aggregates. The similarity of whole soils and differently sized macro-aggregates suggests that macro-aggregates in our soils experience a rapid turnover and merely represent fragments of larger soil units (peds and clods) that constitute the soil in situ.