Carbon, nitrogen and phosphorus dynamics of ant mounds (Formica rufa group) in managed boreal forests of different successional stages

Wood ants (Formica rufa group) are ubiquitous in European boreal forests and their large long-lived mound nests, which mainly consist of forest litter and resin, accumulate carbon (C) and nutrients. The C and nutrient dynamics of wood ant mounds in response to forest succession have received minor attention in boreal forests. We aimed to study whether the C, nitrogen (N) and phosphorus (P) concentrations and the bulk density of ant mounds differ from those of the surrounding forest soil, to estimate the C, N and P pools in ant mounds, and to test whether the concentrations and pools change with forest age. Norway spruce (Picea abies (L.) Karst.) stands on medium-fertile sites in 5-, 30-, 60- and 100-year stand age classes were studied in eastern Finland. Carbon and P concentrations in the above-ground mound material were higher than those in the surrounding organic layer. The C, N and extractable P concentrations were higher in the soil under the ant mounds than in the surrounding mineral soil (0–21 cm). The low bulk densities in the ant mounds and the soil below them could be a result of the porous structure of ant mounds and the soil-mixing activities of the ants. The C/N ratios were higher in the mounds than in the organic layer. Carbon concentrations in the ant mounds increased slightly with stand age. Carbon, N and P pools in the ant mounds increased considerably with stand age. Carbon, N and P pools in ant mounds were <1% of those in the surrounding forest soil. Nevertheless, the above- and belowground parts of the ant mounds contained more C, N and P per sampled area than the surrounding forest soil. Wood ants therefore increase the spatial heterogeneity in C and nutrient distribution at the ecosystem level.