Rich litter admixture effects on belowground carbon dynamics in temperate forests: From leaf litter input to molecular composition of soil organic matter

Soil acidification is a common threat to forests in sandy areas of Western Europe, resulting from past land-use and atmospheric deposition. Converting pine monocultures to mixed forests is a management strategy to enhance biodiversity and overall forest resilience that allows to counteract acidification through admixture of tree species with high litter quality. While positive effects of “rich litter” admixture on soil nutrient availability have been evidenced, the implications for soil organic carbon (SOC) dynamics remain poorly understood. Here, we present an in-depth, small-scale study that uniquely combines data on tree composition, forest floor properties, decomposer communities, SOC stocks, carbon fractionation and soil organic matter molecular composition to assess the effects of converting Pinus sylvestris monocultures to mixed broadleaved forest. We investigated five twin-plots on sandy soils in Belgium and the Netherlands, each consisting of a pine monoculture and an adjacent mixed forest enriched with broadleaved species producing nutrient-rich litter. We examined how changes in tree composition cascaded through the forest carbon cycle, from tree litter inputs up to SOC at the molecular level. Our findings indicate that admixture of rich-litter species strongly alters soil pH, forest floors and belowground communities. Total soil carbon (C) changes were more subtle, suggesting a longer response time to overstory changes. Nonetheless, despite the limited spatial-scale, a change in belowground processes was clearly observed in the SOC fractions and molecular composition: pine monocultures store proportionally more C in labile fractions and molecular changes indicating a stronger microbial reworking in mixed broadleaved forests, promoting increased C storage in stable fractions over time.