CHANGES IN SOIL AGGREGATION AND ORGANIC-CARBON UNDER CORN AND ALFALFA

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 m during the 50-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 (bar-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.