Soil carbon accumulation and nitrogen retention traits of four tree species grown in common gardens

Tree species effects on soil carbon (C) accumulation are uncertain, especially with respect to the mineral soil C, and the consistency of such effects across soil types is not known. The interaction between C accumulation and nitrogen (N) retention among common tree species has also been little explored. Effects of four tree species on soil C and N stocks and soil water nitrate concentration below the root zone were evaluated in a common garden design replicated at eight sites in Denmark. The tree species were beech (Fagus sylvatica L.), oak (Quercus robur L.), larch (Larix leptolepis Kaempf), and Norway spruce (Picea abies (L) Karst.). After four decades, there were significant differences in forest floor C stocks among all four species, and C stocks increased consistently in the order oak < beech ≪ Norway spruce < larch along the soil texture gradient of the sites. Forest floor N stocks only differed significantly between conifers and broadleaves. The observed differences in forest floor C and N stocks were attributed to differences in litter turnover rates among the tree species. Mineral soil C stocks were significantly higher in stands of Norway spruce than in stands of oak and beech while mineral soil N stocks did not differ significantly. Total soil C stocks were thus significantly higher under conifers than under broadleaves. Even though forest floor C and N stocks decreased with finer texture, the C–N stoichiometry in forest floors was quite similar across sites. A significant difference in soil water nitrate concentration at 70–90 cm depth was found only between beech and oak. Nitrate concentrations were highest on fine-textured soils with high pH and low C/N ratio as well as on a site with high local N emissions. Nitrate concentrations were negligible on sandy acidic soils with high C/N ratio. We conclude that tree species may establish consistent differences in soil C and N stocks and N retention traits even along soil texture gradients.

Figure optionsDownload as PowerPoint slideHighlights
► We evaluated C and N stocks and N retention in four common European tree species.
► Tree species affected SOC stocks consistently along a soil fertility gradient.
► Conifers stored more C and N than broadleaves in forest soil.
► Mineral soil C and N stocks were most affected by site-specific factors.
► Larch and spruce stands sequestered most SOC but oak stands were best at retaining N.