| Soil nitrogen (N) is a major factor limiting forest growth. While inorganic N forms such as ammonium (NH4+) and nitrate (NO3-) have been the focus of plant N nutrition studies, a growing body of research has shown direct uptake of organic N forms (including amino acids) is common across the plant kingdom. However, there are few examinations of the contribution amino acid N makes to plant nutrition in temperate forest regions. The main goals of this thesis were to determine (i) potential contributions amino acid-N and inorganic-N could make to N-nutrition of some temperate forest plants, (ii) N-form adaptations among species that could lead to N-niche partitioning among them, and (iii) the influence of mycorrhizae on plant growth and N-form uptake. I determined the relative availabilities of amino acid N and inorganic N in three sites near Jordan River, British Columbia, biomass production of four species of forest plants, some associated with mycorrhizae, when given inorganic (NH 4+, NO3-) or organic (glycine, glutamic acid) N, and short-term uptake (24 hours) of labelled NH4 +, NO3-, glycine, and glutamic acid using Sitka spruce (Picea sitchensis), Douglas-fir (Pseudotsuga menziesii), blueberry (Vaccinium ovalifolium) and salmonberry (Rubus spectabilis). All species performed well when supplied with N primarily as NH4+, and NH 4+ tended to be the most available N-source in all soils tested. Soils dominated by salmonberry tended to have greater NO3 - production and lower amino acid-N: inorganic-N ratios compared to soils dominated by blueberry. This corresponded to the good growth and short-term 15NO3- uptake of salmonberry when grown with N supplied primarily as NO3-, compared to other N-form treatments. Blueberry grew significantly more when given NH4 + and NO3- compared to amino acids. Thus, while blueberry has the ability to take up and use NO3- for growth, sites on which it was dominant had low NO3 - availability. Mycorrhization did not improve biomass production, and in some cases actually decreased biomass. However, 15N uptake tended to be improved in mycorrhizal plants compared to non-mycorrhizal plants. The comparable rates of short-term 15N uptake from amino acids and inorganic-N by all plants, in addition to the comparable amino acid-N and inorganic-N availability in soils, suggested that amino acids may be significant contributors to the N-nutrition of these temperate species. However, biomass production was often reduced in amino acid treatments, which this was not always reflected by short-term 15N uptake. |
