EARLY CROPPING-INDUCED CHANGES IN SOIL AGGREGATION, ORGANIC-MATTER, AND MICROBIAL BIOMASS

Crop management practices influence the amount and quality of organic matter and the state of aggregation in soils. The objective of this study was to determine the early changes in organic and microbial-biomass C contents and in water-stable aggregates of two loamy soils (Typic Fragiaquept and Typic Haplaquept) following plowing of a meadow and subsequent planting of either corn (Zea mays L.) or barley (Hordeum vulgare L.). The effects of 1, 2, and 3 yr of cropping with these two crops were compared with an undisturbed and a recently plowed meadow. Plowing decreased the organic and microbial-biomass C contents of the surface soil (0-6 cm) by an average of 40 to 50%, but, when the whole surface horizon (0-24 cm) was considered, there was no effect. Further cropping with either corn or barley resulted in no net loss of organic matter and microbial biomass but only a redistribution in the surface soil profile. Microbial-biomass C was not preferentially affected by cropping relative to total organic C. The mean-weight diameter by water-stable aggregates decreased not only with plowing but also with length of cropping both in the surface soil (0-6 cm) and throughout the top 24 cm. There was little difference in the specific effects of the two crops on organic C, microbial-biomass C, or aggregation.