Abstract
Nitrogen (N) is a primary limiting nutrient in boreal forests. We have previously shown that N fixation by cyanobacteria associated with feather mosses is a major source of N input to these forests, and that late successional forests support greater N fixation than do early successional forests. However, the reasons for this remain unknown. We plan to investigate the role of three factors that vary during forest succession in influencing N fixation by these cyanobacteria, by using both field and laboratory experiments. These factors are: (1) Levels of available nutrients. During succession nutrient availability declines, and we will investigate whether this decline promotes N fixation. (2) Density of dwarf shrubs in the understory. As succession proceeds the density of dwarf shrubs declines and we will investigate the effects on this on N fixation. (3) Levels of soil moisture. During succession humus depth increases and this increases soil moisture retention; we will determine the response of N fixation to soil moisture status. By investigating these three factors we believe that we will be in a strong position to understand the mechanisms by which N fixation is promoted during long-term succession, and thus develop an improved knowledge on what determines N availability and limitation in boreal forests.
Sweden