Cropping rotations: Effect on aggregate stability and biological activity

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Abstract
Growing crops and crop sequences over a period of 14 years leads to changes in soil properties. The effect of nine cropping rotations (wheat/soybean-maize, wheat/soybean-soybean, wheat/soybean, wheat-wheat soybean-soybean, maize-maize, sunflower-soybean, sunflower-sunflower and soybean-maize) on bulk density, structural stability, biological activity, and organic carbon (C) was investigated in experimental plots subject to conventional tillage. Type of soil used was a well-drained Typic Argiudoll, with a silty loam texture and an easily alterable structure by cultivation and rainfall. Bulk density, structural stability and biological activity showed statistically significant differences between treatments, a high correlation between biological activity and stable aggregates was also found. Four of the crop sequences (wheat/ soybean-maize, soybean-maize, sunflower-soybean, soybean-soybean) were chosen because they represented extreme behaviour patterns as regards the effects of water. For these soil samples, the following fractions of organic C: fulvic acids, humic acids, humins, and light C and water soluble C were determined. The results indicated that maintenance of structure was stable in those sequences which included maize and wheat as opposed to soybean monoculture or soybean-sunflower combinations.