How nitrogen deposition affects the flow of carbon from trees to fungi
The tree delivers carbon from the photosynthesis in exchange for nutrients taken up from the soil by the fungi. This mutualistic relationship is called ectomycorrhiza. The aboveground mushrooms are the fruit bodies of the fungi that live as a network of thin branching structures in the soil (figure 1). This network, called mycelium, constitutes about 80% of the fungal biomass. The fungal mycelium is attached to the tree roots and grows around the fine root tips of the tree like a glove and this is where the exchange of nutrients occurs.
Up to half of the carbon that the trees obtain from the air by photosynthesis can be transported to the fungal mycelium in the soil, which makes the fungi a potentially important carbon sink. However if there are a lot of available nutrients (such as nitrogen) in the soil, the allocation of carbon should be an unnecessary energy cost for the trees since they can take up more nutrients by themselves and thereby need less help from the fungi.
The purpose of this ongoing project is to analyse how nitrogen deposition, mainly caused by fossil fuel combustion, affects the growth of ectomycorrhizal fungi and thereby the allocation of carbon from the atmosphere, through the tree, to the soil. This large scale field study was set up at 30 spruce forest locations in southern Sweden, that are monitored for about 50 different environmental parameters regarding deposition, soil water chemistry, needle chemistry and vegetation (figure 1). The monitoring is done by the Swedish environmental research institute and the Swedish forest agency. The growth of ectomycorrhizal fungi is measured in small, sand filled, bags (figure 1) that was placed in the soil during the two years of the experiment. The bags have a fine mesh that allows ingrowth of the fungal mycelium while the coarser tree roots can't grow through the small pores of the mesh.
According to preliminary results, from the first of two years, nitrogen deposition reduced the growth of ectomycorrhizal fungi. This even seemed to be the most important factor affecting fungal growth when all the other monitored environmental parameters as well as geographic location were included into the analyses. This could be an important factor to take into account when modeling the role of boreal forests as a carbon sink that counteracts the global warming.