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Title: SURFACE RESIDUE- AND ROOT-DERIVED CARBON IN STABLE AND UNSTABLE AGGREGATES

Author
item GALE, W - IOWA STATE UNIVERSITY
item Cambardella, Cynthia
item BAILEY, T - IOWA STATE UNIVERSITY

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/26/1999
Publication Date: N/A
Citation: N/A

Interpretive Summary: Soil properties like aeration, water infiltration, and resistance to erosion are critical to sustained agricultural production. These attributes are dependent upon the soil having a stable aggregate structure. There is little understanding of the factors that affect the stability of soil aggregates. The better the aggregates can withstand stress, the more stable the soil and the more resistance to various forces. Our study was designed to determine what factors affect the stability of these large soil aggregates. This size of aggregate has been found to increase under no-till farming practices, and our question centered on the role of plant residue on forming these aggregates. Organic matter from the previous crop was a primary constituent of the stable aggregates, and most of this organic matter was from the roots of the crop, not the residues left on the soil surface. Reducing the amount of tillage or disturbance of the soil helps to promote a more stable soil that has better water infiltration and is resistant to erosion. The answers to these questions help to define how scientists and producers can improve soil management practices to increase soil stability and enhance the soil resource. The implications of this research would suggest that differences in tillage, crop varieties, and crop rotations would have a major impact on how rapidly we can improve the soil resource.

Technical Abstract: Stable macroaggregates are enriched in new C relative to unstable macroaggregates, but the origin of this new C, as well as its form, 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 one year incubation. Two water pretreatments (capillary wetted and slaked) were applied to the soil samples collected during the incubation, which were then wet sieved to obtain five aggregate size classes. Densiometric separations were used to isolate free and released POM (frPOM) and intraaggregate POM (iPOM). Root-derived **14C was distributed differently in the soil compared to surface residue-derived **14C. In the capillary wetted pretreatment, large and small macroaggregates (>2000- and 250- to 2000-um) together contained >60% of the root-derived aggregate-**14C in the esoil. Most surface residue-derived **14C was associated with small macroaggregates (250- to 2000-um) or large microaggregates (53- to 250-um). A comparison of the two water pretreatments indicated that root-derived aggregate-**14C and iPOM-**14C concentrations were significantly higher in stable (slaking resistant) compared to relatively less stable (capillary wetted) small macroaggregates (250- to 2000-um). In contrast, there were no significant differences in the amount of surface residue-derived aggregate-**14C or iPOM-**14C in small macroaggregates (250- to 2000-um) 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- to 2000-um).