Browsing by Subject "typpi, typensidonta, boreaalinen metsä, metsäpalo, sukkessio"

Nitrogen is an important nutrient for forest growth. The sources of nitrogen in boreal forests are underground processes, biological nitrogen fixations and depositions. The availability of nitrogen in boreal forests is limited, which is why biological nitrogen fixation is a crucial thing. In natural forests, nitrogen is fixed in bottom layer vegetation and in the bacteria that live on the ground, especially in cyanobacteria. The fixing activity of bacteria depends on many environmental factors, of which the temperature, moisture and species of the bottom layer are the most important. It’s been predicted that forest fires will increase in the boreal zone, due to climate change. This will increase the number of different successional stages. Forest fires are the most important disturbance in boreal forests, since they can initiate the succession from the beginning and affect the circulation of nutrients. Fires also affect the activity of biological nitrogen fixation by releasing nutrients and, partly or completely, destroying the bottom layer vegetation. The results of fires at different successional stages are poorly known and there’s no research concerning the whole bottom layer. The purpose of the study was to find out the effects to biological nitrogen fixation when a forest fire has started succession at the beginning. The material was collected at Värriö nature reserve in Northern Finland (67⁰46´ N, 29⁰35´ E). Four forests were chosen for the research. In these areas, fires had occurred 5, 45, 62 and 156 years ago. The forest type was sub-xeric heath forest, dominated by Scots pine (Pinus sylvestris L.). To measure the nitrogen fixation, a sample plot was built in each forest. Ten samples of the ground vegetation and humus layer were collected four times during the summer from each plot. The biological nitrogen fixation was defined with acetylene reduction assay. The samples were analyzed for both the field moisture and saturation humidity at the different moisture stages. The inventory of the vegetation was performed with a 1 m2 area during the summer. In this study, it was discovered that the successional stage, growing season, vegetation and moisture affect the nitrogen fixation. Along with the succession process, the amount of bottom layer mosses increases, which enables a larger growing area for bacteria. The diversity of mosses can be linked to the moisture status, which is an essential thing for nitrogen fixation. From the results, it can be concluded that biological nitrogen fixation becomes important when mosses are recovered during the late succession stages. Climate change is supposed to increase the temperature and precipitation, which can slightly raise the level of the biological nitrogen fixation. In this study, the nitrogen fixation of free living bacteria was found, which might be of interest for future research.