Dynamics of soil organic matter associated with particle-size fractions ofwater-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 p articular, than microaggregates (< 200 mum). However, the nature of this ad ditional carbon is not yet known. To determine it, we compared the composit ion of organic matter in stable macroaggregates with that in unstable ones. Macroaggregates 2-3 mm in diameter were separated from two silty cultivate d soils from the Paris basin. They were slaked, and the primary particle co mposition of the resulting fractions was analysed. We used the natural abun dance 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 t han 6 and 23 years respectively in the two soils studied) than the unstable ones. This young C comprised 50% particulate organic matter, 20% associate d 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, predominantl y plant debris. Young organic matter is preferentially incorporated and is responsible for aggregation, though it is eventually redistributed among ag gregate 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 microaggr egates may exist for decades. We suggest that the stabilization and de-stab ilization of macroaggregates in soils is linked to the incorporation and bi odegradation of plant debris.