Biological Nitrogen Fixation in Coniferous Forest Watershed Areas in Central Sweden

An investigation on biological nitrogen fixation in the forest floor was carried out in five watershed areas in Central Sweden during the summers 1978 and 1979. Five different types of bottom layers with underlying soil horizons were investigated for nitrogen fixation. An inventory on the occurrence of potential nitrogen fixing lichens and alder trees was also carried out. Low fixation rates were determined in mesic-soil mosses, F/H,layers and mineral soil. Though two orders of magnitude higher than in mesic-soil mosses, the nitrogen fixation in the investigated Sphagnum mosses must be considered as low, probably due to absence of blue-green algal associations. The contribution to total nitrogen fixation by lichens is low, but Alnus incana may play an important role. The estimated biological nitrogen fixation calculated as a weighted mean over the five watersheds is in the order of 0.5 kg${\rm N}\ {\rm ha}^{-1}\ {\rm yr}^{-1}$.