Nitrogen fixation by cyanobacteria associated to feathermosses : a comparison between Scots pine and Norway spruce stands

Abstract: The limitations of available nutrients are strong in the boreal forest, especially of nitrogen (N). Due to slow mineralization rate of organic soil nitrogen, caused by low temperatures and acidic soils. Few organisms can fixate atmospheric nitrogen. Among those who have the ability there are some that are common in the boreal forest. Cyanobacteria associated to feather mosses are one example. This study is focused on finding if there is a difference in N2 fixation rate by cyanobacteria associated to feather mosses between forest stands of different tree species. The two tree species that are compared are Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) H. Karst). In a study by Nilsson et al. (2012) the Scots pine stands produced much more volume compared to the Norway spruce. The hypothesis of this study is that N2 fixation by cyanobacteria associated to feather mosses, would enhance N input in particularly Scots pine stands. Thereby contribute to the larger volume production of these stands compared to the Norway spruce stands. The study is carried out on some of the plots used by Nilsson et al. (2012), along with some other similar ones. Samples of the feather moss species Hylocomium splendens (Hedwig) and Pleurozium schreberi (Bridal) was collected from 14 sites on 6 locations, half of which was Scots pine stands and half Norway spruce stands. An acetylene reduction assay was used in combination with a moss biomass inventory, to estimate N2 fixation rates. Results showed that the moss biomass of the two species Hylocomium splendens and Pleurozium schreberi was significantly larger in the Norway spruce stands than in the Scots pine stands. There was also a tendency that estimated N2 fixation rate per unit moss mass was higher in Norway spruce than Scots pine stands. The difference between the stands in estimated N2 fixation rate per hectare and year was not significant. In conclusion the study indicates that the N2 fixation in the moss layer does not have notable effect on the volume growth of the trees in this study.