Soil organic matter stabilization at the pluri-decadal scale: Insight from bare fallow soils with contrasting physicochemical properties and macrostructures

•Physical protection of C was studied in soils from a long-term bare fallow experiment.•Soils with a more stable macrostructure had higher organic matter contents and stocks.•Higher SOM contents are not due to better physical protection at the mm scale.•Differences are due to physicochemical protection in silt- and clay-size fractions.

Physical protection is recognized as one of the main processes governing soil organic matter stabilization, but its importance over long time scales (decades to century) is being discussed. In this work, we used data from a long-term bare fallow experiment initiated in 1928 in Versailles (France) to investigate such process: given that no organic carbon inputs occurred since 1928, all the organic matter present had pluri-decadal to centennial mean residence times. The annual addition of mineral amendments (CaCO3 or KCl) produced contrasting superficial soil structures in some plots compared to non-amended plots. We hypothesized that a more aggregated and stable soil structure would likely be associated with higher organic carbon stocks. Stability of soil structure was quantified using three complementary tests: slow wetting to study macroaggregation, moderate dispersion compared to strong dispersion to study microaggregation. The CaCO3 plots had the highest macroaggregate stability, and KCl plots the lowest but microaggregation at the silt-size scale was similar for the three soils. After 79 years of bare fallow soil organic carbon stocks in the bulk soil were about 25% higher in the CaCO3 plots than in the KCl and reference plots. This relative increment was due to a higher amount of silt and clay-associated soil organic matter in CaCO3 plots, as a consequence of better physical or physicochemical protection at these scales, with a less important role of physical protection within stable macroaggregates.