Pore occlusion by sugars and lipids as a possible mechanism of aggregate stability in amended soils

Understanding the underlying mechanisms of structural stability and the contribution of specific organic fractions to such mechanisms is critical in designing new soil and water conservation strategies relying on organic amendments. The objective of this work was to study the role of neutral and uronic sugars and lipids in affecting key mechanisms (swelling rate, pressure evolution) involved in the stabilization of individual aggregates. A 48-wk incubation study was performed on a clay loam and a silty clay loam amended with either deinking-secondary sludges, primary-secondary sludges, or composted deinking sludges at rates ranging from 8 to 24 Mg dry matter ha^-1. Different structural stability indices were measured during the incubation, along with CO₂ evolved, neutral and uronic sugar, and lipid contents. Significant increases in all stability indices were measured for both soil types. These improvements were linked to a very intense phase of C mineralization and highly correlated with neutral and uronic sugars as well as lipid contents. Paper sludge amendments also resulted in significant decreases in maximum internal pressure of aggregates and aggregate swelling following immersion in water, two mechanisms affecting structural stability. Overall, the results suggest that reduction in maximum internal pressure induced by organic amendments probably resulted from increases in pore surface roughness and pore occlusion rather than an increase in surface wetting angles. This study also supports the view of a nonspecific action of the lipids and neutral and uronic sugars on aggregate stability to rapid wetting.