Controls on wood and leaf litter incorporation into soil fractions in forests at different successional stages

• Increased litter addition did not cause forest floor accumulation after 5 years.
• Aboveground litter incorporation pattern differed with forest age and litter type.
• Increased leaf litter had no effect on either soil C&N or lignin/SFA concentrations.
• Increased wood litter increased soil C but only in young successional forests.
• Soil lignin increased with wood especially in young forests with high earthworm.

In this study, we examined the response of surface soils to increased leaf and wood litter input within adjacent successional forests recovering from agricultural disturbance at the Smithsonian Environmental Research Center (SERC), Maryland, USA. Previous studies at this site demonstrated an arrested development of O-horizon, even after 130 years of forest growth, and an annual loss of leaf litter in forests with the highest abundance of invasive earthworms. Biogeochemical indices of plant biopolymer dynamics, i.e. extractable lignin and substituted fatty acids (SFAs), were applied to soil physical fractions in order to assess the fate of 5 years of increased Tulip poplar (Liriodendron tulipifera L.) wood and leaf litter into O-horizon and mineral soil particles of purportedly different protection levels in this recovering forest system. Our results showed that in this continuously-disturbed recovering system the pattern of litter incorporation into soil varied with both litter type and forest age. For example, young successional forests, that also contained higher abundances of soil feeding endogeic earthworms, incorporated wood amendments deeper into soils and in a predominantly particulate organic matter (POM) form than older successional systems with predominantly litter and surface dwelling earthworms. Soil lignin concentration increased sharply with wood amendments in both forest stages, but young successional forests exhibited incorporation of fresher lignin into both POM and silt and clay (SC) fractions over 0–5 cm and 5–10 cm depths while old forests only increased in POM in the 0–5 cm depth. We attribute these differences to the higher rates of physical mixing from soil feeding endogeic species and potentially lower fungal activity in young successional forests. However, despite nearly 2.5 times of background annual leaf litter input over 5 years, neither total C content nor SFA concentration in soil fractions increased, a phenomenon we attribute to full decomposition of leaf litter amendments. These results demonstrate how the chemical trajectory of soils and litter layers in recovering forests can be a function of both legacy and current disturbance.