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Land use and soil management affect the balance between labile and stable organic matter fractions in surface soils. This study was conducted to quantify the contribution of nonhydrolyzable carbon (NHC) to total soil organic carbon (SOC) and to identify the role of biochemical protection mechanisms in SOC sequestration by comparing the NHC fraction associated with aggregates from 0 to 5-, 5 to 10-, and 10 to 20 cm depths under forest, meadow, no-till (NT) and conventional tillage (CT) treatments. The NHC contribution to SOC declined from 53% under forest to 37% under CT and 39% under NT, implying that the conversion from forest to cultivation led mainly to a reduction in the NHC. Aggregate-associated NHC concentration increased with aggregate size (except for CT treatment). Conversion from CT to NT enriched NHC in all aggregate fractions, but even more so in the >250-μm fraction, underscoring the importance of macroaggregate fractions in encapsulating and thus protecting SOC from microbial processes. The formation of macroaggregates is coupled with the depletion of microaggregates, which can be quantitatively described by functions (for each aggregate class) expressed in terms of the aggregate mass and the proportion NHC/SOC. Both the NHC fraction and the nonhydrolyzable C:N ratio increased with the increase in SOC concentration in soils under meadow and forest, suggesting a large potential for SOC sequestration through biochemical protection mechanisms.

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