MINERALOGICAL MAGAZINE,VOLUME 62A Interactions between organic matter and soil structure and their impact on organic matter preservation in soils

A significant fraction of soil organic matter (SOM) would be physically protected from biodegradation by virtue its location within the soil mineral matrix. As organic matter contributes to the formation and stability of soil aggregates, trong feedback effects are expected between soil structure dynamics and soil organic matter dynamics. The aims of this study were to get insigths into these relationships through an analysis of the spatial distribution of SOM within soil aggregate fractions. Surface horizons of silty soils, cultivated or under forest vegetation, were studied. Continuous maize cultivation since several years allowed to distinguish this recently incorporated carbon from older one using 13C natural abundance. In the same field experiments we traced organic nitrogen using lSN labelling. Organic matter (C and N) with different locations in the soil structure were separated, quantified and their 6J3C and 815N were measured. We separated organic fractions (i) associated with different particle size fractions 2 and located (ii) in dry-sieved aggregates of different sizes 1,5 (iii) in wet-sieved aggregates of different sizes and stabi-lities 5 or (iv) located inside vs outside of stable aggregates 3'4. Organic matter with different locations in the soil structure were found to have different apparent turnover rates. C and N were found to turnover more rapidly in macroaggregates than in microag-gregates ~'6 and outside of aggregates than inside aggregates 3'6. These results support the idea of