Aggregation and carbon storage in silty soil using physical fractionation techniques

Soil texture and degree of aggregation affect the stabilization of organic matter. We studied their influences in silty soils using samples from two field experiments with contrasting long-term use (cropped versus bare fallow). The cropped soil had a larger organic C content than the bare fallow, and allowed us to compare a soil with pools of organic C differing in turnover time with a soil dominated by the passive organic C pool. Increasingly dispersive treatments applied to the soils yielded aggregates of various sizes, stabilities, and organic matter contents. We found an intimate interaction between soil structure and organic matter by demonstrating that aggregation is hierarchical and that active pools of organic matter are responsible for this hierarchy. Microaggregates were found to consist of a constant ratio of clay to silt particle-size fractions. We propose that such a property be used to estimate true micro aggregation and aggregate stability by estimating the amount of soil material dispersed by a given treatment. Organic matter associated with clay is confirmed as an important sink of long-term stabilized C, and it appears to have been increasingly preserved when in increasingly larger aggregates. However, most of the soil mass and associated organic C is in smaller aggregates. We hypothesize that the physical protection within macroaggregates does not directly control long-term stabilization of organic C in the soil, but rather contributes indirectly through the time and local conditions it offers for organic matter to gain chemical or physico-chemical protection by interacting with the soil environment.