| Summary: | The main objective of this research was to determine the short-term effects of cropping on the aggregation and the carbohydrate fraction of a marine clay soil. When compared to a fallow (bare soil) control, cropping to barley and alfalfa for two seasons resulted in significant increases in the size and stability of soil aggregates. The increase in water-stable aggregates $>$2.0 mm was at the expense of aggregates $<$1.0 mm. Under corn, soil aggregation was not different than under fallow. After two seasons, soil under barley or alfalfa contained 15 to 25% more carbohydrates than the fallow or corn treatments. The partial correlation (r = 0.63, P = 0.001) which was observed between carbohydrate content and mean weight diameter of water-stable aggregates, and periodate oxidation tests strongly suggested that carbohydrates were at least partly responsible for the increase in macro-aggregate stability. The remaining cropping effects were removed by tetraborate which suggest the participation of more-humified though ill-defined organic matter. The nature of the changes in organic matter was further investigated using particle size fractionation. Compared to the fallow control, cropping to barley and alfalfa resulted in an enrichment in carbon, nitrogen, and carbohydrates in the sand fraction. The carbohydrate composition of the soil and of its size fractions, determined by liquid chromatography, suggested that cropping treatments had only little effect on the origin of the soil carbohydrates.
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