Carbon mineralization in soil size fractions after various amounts of aggregate disruption

SUMMARY 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. CO 2 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 ( C 0 ) in the sand‐size fraction increased with increased dispersion energy whereas the mineralization rate (k) remained about the same. The levels of C 0 in the clay‐size fraction decreased and the estimates of k increased abruptly upon the disruption of micro‐aggregates.