Changes in Soil Aggregation and Organic Carbon under Corn and Alfalfa

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Abstract
Data on rates of changes in soil structure and organic matter under different cropping systems are necessary for the development of soil and water conservation strategies. Changes in C content and water‐stable aggregation of a Kamouraska clay (fine, mixed, frigid Typic Humaquept) under continuous silage corn (Zea mays L.) and an alfalfa (Medicago sativa L.) stand were monitored monthly during five consecutive growing seasons. A bare soil (fallow) was included as a control. Under alfalfa, the mean weight diameter (MWD) of water‐stable aggregates increased from 1.5 to 2.3 mm during the 5‐yr period. An asymptotic regression explained 69% of the variation in MWD with time under alfalfa. This increase in MWD was largely attributed to an increase in aggregates >2 mm at the expense of the aggregates of 0.25‐ to 1.0‐mm diameter. Also, under alfalfa, the C content increased following a sigmoidal shape from 26 g kg−1 in the first season to 30 g kg−1 in the last year. Changes in MWD were comparatively larger and took place more rapidly than those in C. A correlation coefficient of 0.74** (significant at the 0.01 probability level) was obtained between C and MWD. Under corn and fallow (bare‐soil control), changes in MWD and C content were minimal during the 5 yr. About one‐half of the temporal variation in MWD under corn and fallow could be explained by the variation in soil water content at time of sampling. Conversely, the absence of a significant relationship between water content and MWD under alfalfa suggests that the soil aggregates under this treatment were not subject to slaking.

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