Organo-mineral associations largely contribute to the stabilization of century-old pyrogenic organic matter in cropland soils

International audience; Understanding the processes underlying carbon (C) stability in soils is of utmost importance in the context of climate change. In this setting, biochar is often studied for its persistence in soils and reported to have positive impacts on soil fertility. Whilst recent research has mostly focused on the short-term effects of biochar amendments to soil, a better understanding of its long-term persistence in soils is needed. Our study focuses on agricultural soils enriched in charcoal residues, produced in preindustrial kiln sites ca. 220 years ago, as a proxy for aged biochar. Our aim is to better understand the processes governing the long-term persistence of pyrogenic organic matter (PyOM) in cultivated soils. To achieve this, we focus on i) the effect of PyOM on soil aggregation, ii) its distribution amongst soil fractions and iii) its chemical and thermal properties. For this purpose, we combined a soil particle size-density fractionation with elemental and thermal analyses on topsoil samples collected in charcoal enriched (CHAR) and adjacent reference (REF) soils in a conventionally cropped field in Wallonia (Belgium). The presence of charcoal in soils resulted in a 91 ± 34% higher C content in CHAR soils than REF soils, of which 84 to 94% was PyOM and 6 to 16% was additional non-PyOM. In CHAR soils, macroaggregation was promoted at the expense of microaggregation (CHARMACRO = 48.9 ± 12.8; CHARMICRO = 25.5 ± 8.3 g 100 g−1 soil) whereas these were similar in REF soils (REFMACRO = 41.3 ± 9.5; REFMICRO = 36.0 ± 8.7 g 100 g−1 soil). Elemental and thermal analyses revealed that PyOM did not only occur as free light fractions but was also occluded in aggregates or sorbed onto mineral phases (56.4 ± 22.9% of total PyOM) suggesting PyOM may be further stabilized through organo-mineral associations. Century-old PyOM showed increased H:C and O:C atomic ratios compared to recently pyrolyzed OM, particularly in occluded as opposed to free light fractions, vouching for the functionalization of its surfaces. Furthermore, regardless of high C contents in CHAR soils, similar C:N ratios between studied soils in mineral dominated fractions suggested increased amounts of N-rich non-PyOM in charcoal rich soils. Our results demonstrate that, over centuries in cultivated soils, PyOM plays an active role in aggregation patterns. We conclude that century old PyOM is broken down from coarse to fine particles, and through functionalization of its surfaces contributes to organo-mineral associations. Beyond its intrinsic chemical recalcitrance, the long-term persistence of biochar is enhanced by occlusion and sorption processes.