Structure Stability and Carbon Distribution in Silty Loam Soil Aggregates as Affected Tillage and Corn-Soybean Crop Rotation.

Different tillage intensities may influence soil physical and chemical properties, distribution of nutrients and organic carbon between and within the aggregates. We studied the effect of long term (25 years) conservation tillage on structure stability and the total C and N distribution in Miami silt loam aggregates sampled from the upper 0-5 cm layer of field plots located at the Throckmorton Purdue Agriculture Center. Aggregates with the sizes of 20-12, 12-8, 8-6, 6-4, 4-2, 2-1, 1-0.5, 0.5-0.25, 0.25-0.05, and < 0.05 mm were studied. The plots were cultivated by ridge (RT), no-till (NT), moldboard plow/disk (MB) and chisel plow/disk (CP) for 15 years, after that, all plots were tilled by MB and planted with soybeans for one year and then for the next 10 years the plots were managed with either a CP or no till (NT) again under continuous corn, soybeans, or corn-soybean rotation. The structure stability of the treatments were:  NT corn > NT soybean-corn > NT soybeans > CP corn > CP soybean-corn > CP soybeans. The carbon content on a mass basis was greater: (i) in the larger aggregates (8-0.25 mm; 20-4 mm) for the ridge and no-till plots, and in the smaller aggregates (0.5-0.05 mm) from the soil managed with the moldboard and chisel plows and (ii) also for the soybean-corn rotation over the corn-soybean rotation for tilled soils whereas, in the RT and NT treatments, the carbon content differed to a lesser degree among the rotations. For the same tillage practice, soil samples having similar carbon content under different crop rotations had a different aggregate stability, showing that carbon content alone is not the only factor determining aggregate stability. Soil structural stability, distribution of total carbon and nitrogen in the aggregates depended on tillage history, crop rotation and aggregate size.