Aggregates (1–2mm) were subjected to shaking, increasing intensities of ultrasonification, or a peroxide treatment and then physically fractionated into sand-, silt- and clay-size fractions. CO2 evolution was measured during a 20-day incubation of the sand-, silt- and clay-size fractions and was used to assess the decomposability of the organic matter within aggregates and associated with these size fractions.
All of the size fractions showed a large increase in the amount of readily decomposable C when the ultrasonic energy input increased from 300 to 500 J ml−1 and disruption of microaggregates occurred. The data suggest that some readily decomposable organic matter is sequestered within microaggregates and protected from microbial attack.
Following complete dispersion, the C mineralized (mg C g−’C) upon incubation was greatest in the sand particles and least in the clay. The levels of potentially mineralizable C (C0) in the sand-size fraction increased with increased dispersion energy whereas the mineralization rate (k) remained about the same. The levels of C0 in the clay-size fraction decreased and the estimates of k increased abruptly upon the disruption of micro-aggregates.