Macroaggregates persist during wetting-drying cycles after glucose addition to a mineral soil

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Soil with persistent macroaggregates should maintain its integrity following wetting disturbance. This study determined how wetting-drying cycles affect two pre-sieved macroaggregate size fractions (2–4 mm and 0.6–2 mm) of clay and organic soils pretreated with organic carbon substrates. Soil aggregates were unamended or treated with organic carbon (4 g C from soluble glucose or insoluble wheat straw per kg soil) before being exposed, during three months of incubation, to nine sequential wettings at three wetting intensities (no wetting, slow wetting at the rate of 1 g of water per second or fast wetting with 2 g water per second). The glucose-treated macroaggregates from the clay soil were minimally disturbed by wetting-drying cycles, maintained their shape (shown by high aggregate circularity and low major/minor axis ratio) and were more cohesive, possibly because of a higher microbial activity provided by the glucose treatment. In contrast, the clay soil macroaggregates mixed with wheat straw, which had the same carbon content as glucose, did not persist when exposed to wetting-drying cycles. Macroaggregates from organic soil resisted nine wetting events with or without glucose addition, implying a distinct aggregate binding mechanism in organic soil compared to the clay soil. The persistence of clay soil macroaggregates treated with glucose may be related to microbial processes that promote macroaggregate stability, with glucose physical adhesion expected to substantially improve aggregate stabilization in this mineral soil as well. Glucose-mediated aggregate stabilization can apparently persist for many weeks in an environment with recurring wetting-drying cycles.