Surface residue- and root-derived carbon in stable and unstable aggregates

Stable macroaggregates are enriched in new C relative to unstable macroaggregates, but the origin and form of this new C is not known. Under simulated no-till (NT) conditions, we used a C-14 label to monitor changes in the concentration of new surface residue- and root-derived C in aggregates of different size and stability during a 1-yr incubation. Two water pretreatments (capillary-wetted and slaked) were applied to the soil samples collected during the incubation. The samples were then wet sieved to obtain five aggregate size classes. Densiometric separations were used to isolate free and released particulate organic matter (frPOM) and intraaggregate POM (iPOM). Root-derived C-14 was distributed differently in the soil compared to surface residue-derived C-14. A comparison of the two water pretreatments indicated that root-derived aggregate-C-14 and iPOM-C-14 concentrations were significantly higher in stable (slaking-resistant) small macroaggregates (250-2000 pm) relative to those in the capillary-wetted pretreatment. In contrast, there were no significant differences in the amount of surface residue-derived aggregate-C-14 or iPOM-C-14 in small macroaggregates (250-2000 mu m) between the two pretreatments. We conclude that in relatively undisturbed systems like no-till, new root-derived iPOM-C is more important than surface residue-derived C in the stabilization of small macroaggregates (250-2000 mu m).