Smucker, A. J. and E. J. Park. 2006. Soil biophysical responses by macroaggregates to tillage of two soil types. Pages 455-460 in Advances in GeoEcology. Cantena Verlag Publishers, Reiskirchen, Germany.

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Strength and function of soil macroaggregates are modified by tillage, crop residues, frequent drying and rewetting (DW) cycles, and in some regions, repeated freezing and thawing. Climate and management environments control the internal porosities of soil aggregates and facilitate the movement of soluble plant carbon © compounds into their interiors. Greater C and nitrogen (N) associations with mineral surfaces within aggregate pores control their formation, stability, and function. It is well known that SOM contents within aggregates are different among aggregate size fractions, however, C gradients that control flux rates of microbial substrates into different sized soil aggregates need further investigation. Soil aggregate erosion (SAE) chambers were used to remove concentric layers from aggregates (1-15 mm across). Gradients of total and isotopic C and N contents and bacterial populations and communities have been quantified within aggregates from tilled and nontilled soils subjected to different crop rotations, grasslands and forests. Two thirds of the total C in exterior regions of macroaggregates contain new C originating from a two yearold alfalfa field. Continuous moldboard tillage reduces intra-aggregate porosity by at least 17%. DW cycling increases C movement into aggregate interiors. Unless these soluble organic carbon (SOC) compounds diffuse more deeply into aggregate interiors, added C that remains on aggregate surfaces contributes little more than to accelerate microbial respiration. Continuous additions of SOC onto aggregate surfaces, during DW cycling, increase C flux rates into aggregate interiors and sequester more C away from microbial degradation. Maintenance of greater C and N gradients between surface regions and within soil aggregates increase their bacterial biomass and polysaccharide biofilms that contribute to aggregate formation and the maintenance of their stability.

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