Dynamics of soil organic matter associated with particle-size fractions of water-stable aggregates

Stable macroaggregates (> 200 mum) of cultivated soils are reported in the literature to be richer in organic carbon, and in young organic carbon in particular, than microaggregates (< 200 mum). However, the nature of this additional carbon is not yet known. To determine it, we compared the composition of organic matter in stable macroaggregates with that in unstable ones. Macroaggregates 2-3 mm in diameter were separated from two silty cultivated soils from the Paris basin. They were slaked, and the primary particle composition of the resulting fractions was analysed. We used the natural abundance of C-13 to quantify the amount of young carbon, derived from a maize crop, in the various size fractions. The stable macroaggregates were richer in total C and in young C (younger than 6 and 23 years respectively in the two soils studied) than the unstable ones. This young C comprised 50% particulate organic matter, 20% associated with silt and 30% with clay particles. We propose a schematic composition of aggregates in these soils in which stable aggregates are formed by the binding of microaggregates by additional young organic matter, predominantly plant debris. Young organic matter is preferentially incorporated and is responsible for aggregation, though it is eventually redistributed among aggregate classes through the destruction and re-formation of the aggregates. We have developed a model to simulate this redistribution. The model shows that stable macroaggregates have a life of a few years, but that microaggregates may exist for decades. We suggest that the stabilization and de-stabilization of macroaggregates in soils is linked to the incorporation and biodegradation of plant debris.