Soil and residue carbon mineralization as affected by soil aggregate size

The nature of the contact between fresh organic matter and soil depends mainly on the characteristics of the plant residues and on the physical properties of the soil. In a cultivated cropping system, changes in soil organic C cannot be entirely attributed to changes in organic matter input. Breakdown of aggregates caused by cultivation not only affects soil organic matter but also influences the rate of mineralization of added organic matter. Many models simulating organic matter decomposition in the field are calibrated with laboratory data from experiments where crop residues are ground and mixed homogeneously with soil aggregates. In the present study, soil aggregate size was used as a means of varying the contact between crop residue and the soil. The results demonstrated that cumulative soil carbon mineralization from different aggregates had a significant (r=0.60, p=0.05) and positive relationship with their oxidizable soil carbon content. Residue carbon mineralization in different aggregate size classes was inversely related to aggregate oxidizable soil carbon content (r=−0.95, p=0.01), cumulative soil carbon mineralization (r=−0.89, p=0.01) and resistant soil carbon pool (r=−0.80, p=0.01). Residue carbon mineralization in different aggregate size classes was also inversely (r=−0.61, p=0.05) related to the active carbon content (KMnO₄ oxidizable carbon) of the aggregates. There was no significant difference in soil active carbon pool in different aggregate size classes. Determination of size and turnover of a slow pool showed significant difference in different aggregate size classes. The slow carbon pool in different aggregate size classes ranged from 13.7 to 25.5% with mean residence time of 1.8 to 5.4 years. Water soluble carbon and active carbon (alkaline KMnO₄ oxidizable C) were significantly higher in macro-aggregates than in micro-aggregates.