The effects of tree species with contrasting litter qualities on soil carbon storage along the soil development gradient

Abstract Previous research indicates that the effects of tree species on carbon (C) storage differ with soil age. It has been hypothesized that in young soils, trees with low carbon to nitrogen ratio (C:N) litter promote C accumulation in mineral-associated organic matter (MAOM) and mineral soil layer, while in mature soils, trees with high C:N litter support C storage in the particulate organic matter (POM) and organic soil layer. To test this, we studied adjacent forest stands of alder (low C:N litter) and spruce (high C:N litter) planted on a 50-year-old post-mining spoil heap (young soil) and in the surrounding landscape (> 1,000 years, mature soil). Spruce stored more C in the organic soil layer across both soil ages. In young soil, alder stored more C in the mineral soil layer and MAOM-containing fractions (silt and clay, microaggregates), while no differences between tree species were observed in mature soil. Contrary to our expectations, no significant differences were found for C storage in POM fraction. Our results indicate that low C:N litter of alder promoted high C stocks in MAOM-containing fractions in young soil, but differences between tree species disappeared as the capacity for further C stabilization declined in mature soil. In contrast, high C:N litter of spruce enhanced C accumulation in the organic layer, contributing to greater C storage in mature soil. These findings show the necessity of considering both tree species and soil age when selecting appropriate C storage strategies and highlight the need for context-specific afforestation and reforestation approaches.