Organic nitrogen uptake by boreal forest plants

Organic nitrogen (N) is becoming increasingly recognized as a potentially important N source for plants. Such a role have important consequences for our understanding of key processes in the nitrogen cycle, but also have implications for the way we should consider important phenomena such as global warming and nitrogen fertilization. Despite an intense research within the area of organic N, no clear picture has emerged of its importance, neither on a plant physiological, nor on an ecosystem level. The work presented in this thesis has taken a broad approach aiming to learn more of the importance of organic N in general, and amino acids in particular for plants in boreal forest ecosystems. Thus, it includes physiological studies of the regulation of plant amino acid uptake, as well as ecophysiological studies examining the relative importance of different N sources for various plants in the field. Briefly, the results suggest that amino acid uptake is a wide-spread ability among boreal forest plants. Amino acid uptake was found to be regulated by a number of factors, including N- and C-status of the plant and substrate access. Moreover, although mycorrhiza may have important functions for amino acid acquisition in field situations, it appears as if plant uptake capacity of organic N is unrelated to the mycorrhizal association. These results further emphasize the potential importance of organic N for the nitrogen nutrition of plants in boreal forests.