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DIVISION S-3-SOIL BIOLOGY & BIOCHEMISTRY

Surface Residue– and Root-derived Carbon in Stable and Unstable Aggregates

^a USDA-ARS National Soil Tilth Lab., 2150 Pammel Dr., Ames, IA 50011 USA
^b Dep. of Statistics, Iowa St. Univ., Ames, IA 50011 USA

cindyc{at}nstl.gov

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 ¹⁴C 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 ¹⁴C was distributed differently in the soil compared to surface residue–derived ¹⁴C. A comparison of the two water pretreatments indicated that root-derived aggregate-¹⁴C and iPOM-¹⁴C concentrations were significantly higher in stable (slaking-resistant) small macroaggregates (250–2000 µm) relative to those in the capillary-wetted pretreatment. In contrast, there were no significant differences in the amount of surface residue–derived aggregate-¹⁴C or iPOM-¹⁴C in small macroaggregates (250–2000 µm) 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 µm).

Abbreviations: frPOM, free and released particulate organic matter • iPOM, intraaggregate particulate organic matter • LSD, least significant difference • POM, particulate organic matter

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