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

Stable macroaggregates are enriched in new C relative to unstable macroaggr egates, 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 co ncentration of new surface residue- and root-derived C in aggregates of dif ferent size and stability during a 1-yr incubation. Two water pretreatments (capillary-wetted and slaked) were applied to the soil samples collected d uring the incubation. The samples were then wet sieved to obtain five aggre gate size classes. Densiometric separations were used to isolate free and r eleased particulate organic matter (frPOM) and intraaggregate POM (iPOM). R oot-derived C-14 was distributed differently in the soil compared to surfac e residue-derived C-14. A comparison of the two water pretreatments indicat ed that root-derived aggregate-C-14 and iPOM-C-14 concentrations were signi ficantly higher in stable (slaking-resistant) small macroaggregates (250-20 00 pm) relative to those in the capillary-wetted pretreatment. In contrast, there were no significant differences in the amount of surface residue-der ived aggregate-C-14 or iPOM-C-14 in small macroaggregates (250-2000 mu m) b etween the two pretreatments. We conclude that in relatively undisturbed sy stems like no-till, new root-derived iPOM-C is more important than surface residue-derived C in the stabilization of small macroaggregates (250-2000 m u m).