Nitrogen availability is fundamental to plant establishment and growth. Boreal forest soils generally contain large stocks of nitrogen, but this nitrogen is to a large extent present in forms that plants cannot access. Generally, it has been assumed that organic nitrogen must be transformed into inorganic forms in order to be available to plants. Thus, mineralization of nitrogen, i. e. the production of ammonium and nitrate, has been considered to limit plant nitrogen acquisition. Although this assumption has been questioned in a number of studies over the years it is only recently that we have learned that plants in the field actually take up organic nitrogen. The introduction of modern techniques has enabled plant nitrogen capture to be studied directly in the field. This is a prerequisite for correct assessment of such processes. The challenge is now to understand the dynamics of plant N acquisition. Large variations, both quantitative and qualitative, in the availability of nitrogen create a number of niches for plants. We will try to assess the relative importance of different N sources, inorganic and organic, for different plant species. Further goals areto understand how plant acquisition is of organic nitrogen affected by common silvicultural practices and by nitrogen deposition. We are using a number of different techniques in these studies. Dual labeled (13C, 15N) amino acids are used to assess the fraction of nitrogen taken up as intact amino acids by plants. Isotope ratio mass spectrometers are thus used to analyse total labelling of C and N pools. Recently we have also introduced GC-MS techniques to study the fate of labelled substances within plants. HPLC techniques are used in studies of soil N pools and in studies of transport and storage of nitrogen in plants. The initial studies of organic N uptake by plants we performed concerned the amino acid glycine.
Recent investigations have included monitoring a number of different nitrogen compounds and their uptake and metabolism in plants. In close co-operation with soil scientists at the department of Forest Ecology we are considering both soil and plant processes that influence plant-soil interactions and, thus, plant N acquisition.
Plant responses to nitrogen deposition
In several projects we have studied effects of excess N supply on conifers such as Scots Pine and Norway spruce. These studies have shown that excess N is accumulated in these trees in the form of arginine. Recently we have also shown that elevated levels of arginine in conifers are correlated to leakage of nitrate from forest soils. This enables us to study the development of nitrogen enrichment during nitrogen deposition. Currently our main focus is on the effects of nitrogen deposition on field layer vegetation of boreal forests. Studies of uptake and allocation of deposited nitrogen in species like Vaccinium myrtillus and Deschampsia flexuosa are related to growth analyses. Interactions between these species and their natural enemies such as insects and fungal pathogens are studied in co-operation with the Departments of Ecology and Geology at UmeΠUniversity. Changes in plant biochemistry following N deposition are related to attacks of natural enemies. Increased levels of free amino acids in leaves and on leaf surfaces have been shown to promote attacks of pathogenic fungi. Recently, we have also included studies of changes in phenolic defense substances following N additions.
Näsholm, T., Nordin, A., Edfast, A-B. and Högberg, P. () Identification of coniferous stands with incipient nitrogen saturation through analysis of arginine and 15N abundance of foliage. J. Env. Qual. 26: 302-309.
Nordin, A. and Näsholm, T. () Nitrogen storage forms in nine boreal understorey species. Oecologia 110: 487-492
Näsholm, T. () Qualitative and Quantitative changes in plant nitrogen acquisition induced by anthropogenic N deposition. New Phytol.139: 87-90.
Nordin, A., Näsholm, T. and Ericson, L. () Effects of stimulated N deposition on understorey vegetation of a boreal coniferous forest. Functional Ecology 12: 691-699.
Näsholm, T., Ekblad, A., Nordin, A., Giesler, R., Högberg, M. and Högberg, P. () Boreal forest plants take up organic nitrogen. Nature 392: 914-916, .
Näsholm, T., Huss-Danell, K. & Högberg, P. () Uptake of organic nitrogen in the field by four agriculturally important plant species. Ecology in press
