Aggregate size and the state of soil aggregation may be influenced by different soil management and cultural practices which lead to changes in soil organic matter content and biological activity. The objective of this study was to verify the relationship between organic carbon and the size and stability of soil aggregates in two planting systems (conventional and direct), three crop rotations (corn/wheat/corn, soybean/wheat/corn and soybean/wheat/soybean), under two soil sample preparation processes: use of 4 and 8 mm sieves before the wet sieving process. The study was carried out in a Typic Haplorthox in 1990, using soil samples from an experiment which had extended over a 14 year period, located at IAPAR's experiment station in Londrina, Paraná, Brazil. Soil samples were collected on April 5, 1990 at two depths: 0-10 and 10-20 cm. The aggregation indices determined were: the mean weight diameter (MWD), the geometric mean diameter (GMD) and the aggregate stability index (AS%). The results showed that the no-tillage system improved the state of soil aggregation due to an increase in organic carbon, especially at the depth 0-10 cm where the MWD and GMD had significantly higher values. Soil aggregation had a tendency to increase when crop rotation included a species with higher C/N ratio (corn). The increase in organic carbon resulted in higher AS% since aggregate class < 0.25 mm decreased and the other classes increased. The methods for soil sample preparation showed differences in aggregation: the 8 mm sieve method increased the sensibility to detect differences in the state of aggregation as a function of soil management.
aggregation indices; aggregate size; aggregation mechanisms