Effect of nitrogen input on carbon accumulation of boreal forest soils and ground vegetation

The effects of nitrogen deposition on the carbon balance of forest soils and ground vegetation were estimated in long-term fertilization experiments. The experiments were established during 1958-1962. They were situated on 36- to 63-year-old Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies Karst.) stands of different levels of fertility. The experiments received nitrogen fertilization five to seven times over a 30-year period, and the total input of nitrogen was 596-926 kg ha(-1). Nitrogen input increased the amount of carbon in the humus layer and mineral soil. With one exception, the carbon storage of the humus layer was increased by 14-87%, and carbon storage of the mineral soil by 15-167%. The humus layer was thicker on the fertilized plots due to accumulation of organic matter during the study period. In the changing environment, nitrogen deposition reinforces the accumulation of organic matter in forest soils, and may enhance carbon accumulation in boreal forests. On the other hand, both the biomass and the amount of carbon in the ground vegetation were decreased by the addition of nitrogen. The ground vegetation under a closed canopy does not have the potential to respond positively to addition of nitrogen, because the vegetation is dominated by slow-growing mosses and dwarf shrubs. Thus, the ground vegetation of the boreal forests is not a sink for carbon in the changing environment.