Tree species affect the vertical distribution of soil organic carbon and total nitrogen

Background Forest soils are considered sinks for atmospheric C. Many studies revealed that tree species and their mycorrhizal association affect forest floor and topsoil organic C (OC) and total N, while the knowledge of their effect on subsoil OC and N is still scarce. Aims We aimed to identify (1) tree species and mycorrhizal association effects on forest floor, topsoil (0-30 cm) and subsoil (30-80 cm) OC and N stocks and vertical distribution and (2) drivers for soil OC and N distribution. Methods We sampled forest floor, topsoil and subsoil under Fagus sylvatica L., Quercus robur L., Acer pseudoplatanus L. and Fraxinus excelsior L. in four Danish common garden experiments along a gradient in soil texture and determined OC and N stocks. Results Total N (forest floor + soil) was higher under oak than beech, while total OC was unaffected by species. Forest floor C and N were higher under oak and beech, both ectomycorrhizal species (ECM), compared to under maple and ash, which are both arbuscular mycorrhizal species (AM). Relatively more OC and N were transferred to the topsoil under AM than ECM species, and this could be explained by greater endogeic earthworm biomass in AM species. In contrast, a higher proportion of OC was stored under ECM than AM species in the subsoil, and here OC correlated negatively with anecic earthworms. Subsoil N was highest under oak. Conclusions Tree species and in particular their mycorrhizal association affected the vertical distribution of soil OC and N. Tree species differences in topsoil OC and N were not mirrored in the subsoil, and this highlights the need to address the subsoil in future studies on AM- versus ECM-mediated soil OC and N stocks.

Automated summary

Tree species and mycorrhizal association affect the vertical distribution of soil organic carbon and nitrogen, with ectomycorrhizal species transferring more carbon and nitrogen to the topsoil, and arbuscular mycorrhizal species storing more carbon in the subsoil. Oak trees have the highest nitrogen content in the subsoil.