Anthropogenic nitrogen enrichment increased the efficiency of belowground biomass production in a boreal forest

Anthropogenic nitrogen (N) enrichment in boreal forests has been shown to enhance aboveground net primary production and downregulate soil respiration, but it is not well understood if these effects are driven by reduced belowground C allocation or shifts between biomass production and respiration in fine-roots and ectomycorrhizal fungi (EMF). We utilized an experiment in a Pinus sylvestris (L.) forest simulating anthropogenic N enrichment with additions of low (3, 6, and 12 kg N ha−1 yr−1) and high (50 kg N ha−1 yr−1 × 12 yr) doses of N (n = 6) and measured the production of needles, fine-roots, and EMF mycelium during the 12th and 13th year of the experiment. We created a biomass production efficiency index by relating the biomass production rate to root-associated respiration, including both root and EMF respiration. The high N treatment enhanced the production of both needles and fine-roots, with a relatively larger increase in fine-roots, and strongly increased fine-root biomass production efficiency but had no effect on the fungal biomass in fine-roots or the production of EMF mycelium. The low N treatments had no effect on any of the measured variables. These results show that high levels of N enrichment drive shifts in the use of C allocated below ground, with less C going towards metabolic functions that result in rapid C emissions, and more C going towards the production of new tissues. •High N addition doubled Pinus sylvestris fine-root production.•High N addition reduced autotrophic soil respiration by one third.•Low N additions had no effect on fine-roots or soil respiration.•Neither low or high N addition had any effect on EMF biomass or production.•High levels of N deposition enhance fine-root production efficiency.