Turnover of intra- and extra-aggregate organic matter at the silt-size scale

Temperate silty soils are especially sensitive to organic matter losses associated to some agricultural management systems. Long-term preservation of organic C in these soils has been demonstrated to occur mainly in the silt- and clay-size fractions, although our knowledge about the mechanisms through which it happens remains unclear. Although organic matter in such fractions is usually assumed to be present in the form of organo–mineral complexes, a large percentage of this fraction appears to be micro-aggregated. In this study we hypothesized that slaking-resistant silt-size aggregates should be preferential sites of long-term organic matter preservation in soils, by physical entrapment that would allow for stabilization by interaction with mineral surfaces. To verify this, we studied the organic C stocks and turnover within and outside slaking-resistant aggregates in the silt-size fraction (2–50 μm) after complete dispersion of larger aggregates (> 50 μm) in a wheat–maize (C3–C4) chronosequence of a silty soil in Northern France. Changes in the natural abundance of 13C from C3 to C4 plants were used to assess the turnover time of organic C in the different isolated fractions. Slaking-resistant silt-size aggregates stored almost half of the soil organic C, both as entrapped silt-size light (< 1.6 g cm− 3) organic particles (μLF) and as organic C in the mineral fraction. We found that overall, organic C stored in the silt-size fraction had longer turnover times than sand-size particulate organic matter. However, we found similar turnover rates within and outside slaking-resistant aggregates of the silt-size fraction, both for μLF and for the organic C recovered with minerals. Moreover, the apparent turnover rates of μLF (167–330 years) were slower than that of C in the clay-size fraction (100 years). We postulate that a rapid turnover rate of silt-size aggregates is needed to explain preservation of this μLF by physical entrapment. Our results also demonstrated the heterogeneity of organic matter kinetics in the silt- and clay-size fraction, which should be accounted for when modelling organic matter dynamics in silty temperate soils.