Temporal changes in dry aggregate size and stability: tillage and crop effects on a silty loam Mollisol in Illinois

Soil aggregation is one of the main factors controlling the chemical, physical, and biological processes that contribute to soil productivity and agricultural sustainability. Little is known about the relationship between dry-sieved aggregate size and aggregate stability. Temporal changes in the size and stability of dry-sieved aggregates were studied in a Thorp silt loam(US Taxonomy: fine-silty, mixed, mesic Argiaquic Argialboll; FAO Classification: Orthic Greyzems) in a long-term (1986-present) tillage experiment established in Urbana, IL, US. Tillage treatments included no-tillage (NT), disk/field cultivation (DT), and moldboard/chisel plow (MP) in which corn (Zea mays L.) and soybeans (Glycine max L.) were rotated annually. Soil samples were collected in 1995 before and after fall field operations and again before planting in 1996. The dry mean weight diameter (DMWD), wet mean weight diameter (WMWD), and wet aggregate stability (WAS) of surface soil (0-5 cm), and the WAS of dry-aggregate size classes were analyzed. Aggregate DMWD was greater in NT (8.8 mm) than in DT (7.1 mm) or MP (7.2 mm) soils. The DMWD increased regardless of tillage or crop from October (7.3 mm) to December (9.4 mm), and had decreased sharply by April (6.3 mm). Soil water content at the time of sampling did not significantly influence DMWD and WAS. Crop alone had no significant influence on soil aggregation but interacted with sampling time and tillage. Dry aggregate size, WMWD, and WAS were positively correlated. Aggregate size increased with soil organic carbon (SOC) and the large (8-25 mm) dry aggregates, which were most affected by tillage, were the most water stable. These findings suggest that large dry-sieved structural units function like true aggregates and should be studied further. (C) 1998 Published by Elsevier Science B.V. All rights reserved.