Deadwood effects on dissolved organic carbon in forest soils depend on bedrock type, tree species, and microclimate

Deadwood plays an important role in the forest carbon cycle by supplying dissolved organic carbon to the underlying soils. Yet, the extent to which this effect varies across different site conditions remains insufficiently understood. We monitored dissolved organic carbon at multiple soil depths beneath European beech and Norway spruce logs at sites with contrasting bedrock types over 2.5 years, comparing them with adjacent control plots. Concentrations were consistently elevated beneath deadwood, but the magnitude of the increase varied with bedrock type, tree species, and depth, and was influenced by soil temperature and moisture. Differences between deadwood and control were minor beneath the forest floor but pronounced in the upper mineral soil (15 cm), with the strongest increase (+ 90%) observed at the silicate site. When separating by tree species, this increase was driven by beech deadwood, while spruce showed little effect. At the silicate site, concentrations declined markedly between 15 and 30 cm, suggesting enhanced retention or processing in deeper layers. The effect of soil moisture on DOC was similar between deadwood and control plots, but temperature effects differed. These results indicate that the contribution of deadwood to soil carbon inputs is context-dependent, varying with tree species and soil properties.