The importance of mycelial connection at the soil–litter interface for nutrient translocation, enzyme activity and litter decomposition

► The addition of easily available C strongly decreases decomposition rates of plant litter. ► Possible mechanisms of negative priming are preferential utilization and catabolic repression. ► Disrupting of the mycelial connection decreases decomposition rates of N-poor litter. ► Disruption of mycelial connection does not necessary decrease the net N input into the litter. The translocation of nutrients by networks of fungal hyphae is an important mechanism controlling decomposition rates of plant residues. We performed a 15-month long field experiment aiming to assess the effects of the C and N availability and the role of the mycelial connection between litter and underlying mineral soil on the decomposition rates and levels of enzyme activities in plant litter. Litterbags containing either N-rich litter of alder ( Alnus glutinosa) or N-poor litter of aspen ( Populus tremula) were exposed on top of A-horizon of forest soil. The experiment was set up in a full-factorial design with litter species, C and N availability, and presence of mycelial connection as factors. Labile C and N were added as sucrose and ammonium nitrate solutions in four combinations: −C−N, +C−N, −C+N and +C+N. To disrupt the mycelial connection between litter and soil half of the litterbags were slightly moved, whereas others were left untouched. All experimental manipulations were performed every ten days during snow-free period. At the natural levels of C and N availability, N-poor aspen litter, but not N-rich alder litter, accumulated significant amounts of N. The addition of easily available C decreased the activity of xylanase and phenoloxidase and reduced litter decomposition rates of aspen litter; in alder litter similar but less pronounced trends were also observed. The severance of the mycelial connection between plant litter and underlying soil generally decreased xylanase activity and decomposition rates of the N-limited litter, likely due to the reduced presence of basidiomycete fungi in the litterbags. This effect was not observed, if the N-limitation was artificially relieved by the addition of easily available N, likely due to the increased activity of bacteria and microfungi.