In mature forests, gains and losses of nitrogen may be dominated by the gaseous transformations, asymbiotic nitrogen fixation and biological denitrification. Both are reduction reactions and are affected by moisture conditions, temperature, pH, supply of organic carbon and the availability of mineral nitrogen. Gaseous nitrogen inputs, due to asymbiotic nitrogen fixation, and outputs, due to biological denitrification were quantified for a mature coniferous forest in southwestern British Columbia. Forest floor material, mineral soil, decaying wood, foliage and bark were incubated in an atmosphere of 0.1 atm acetylene to allow the simultaneous measurement of N₂0 production by denitrifying bacteria and acetylene reduction by free-living bacteria and blue-green algae. Forest floor material accounted for 80% of a total annual input of 0.8 kg N ha⁻1 a⁻1. Relatively small amounts of nitrogen were fixed in mineral soil, decaying wood and foliage and no indication of nitrogen fixation activity in bark was detected. Traces of denitrification were found, but gaseous output of nitrogen was effectively 0.0 kg N ha⁻1 a⁻1. It is hypothesized that this forest may prevent nitrogen Joss by outcompeting other sinks for mineral nitrogen, thereby allowing a slow accretion of nitrogen by asymbiotic nitrogen fixation and bulk precipitation input. / Forestry, Faculty of / Graduate

The present investigation was undertaken to survey the general features of physiological responses of plants to NO2, and to understand the mechanism of inhibition of gas exchange by NO2. To achieve these objectives, the effects of NO2, on photosynthesis, respiration and transpiration and the rate of NO2, uptake by primary bean (Phaseolus vulgaris. L. cv. 'Pure Gold wax'), leaves were examined in various environmental conditions using an open gas flow system. Apparent photosynthesis, respiration and the evolution of CO2, into CO2- free air, were all inhibited by NO2, concentrations between 1.0 and 7.0 ppm. The degree of inhibition was increased by increasing NO2, concentration and exposure time. A 2 and 5 h exposure to 3.0 ppm NO2, inhibited the gas exchange of bean leaves at all plant ages and in all the environmental conditions examined. Photosynthesis was most inhibited in leaves of intermediate ages, at optimum temperatures, at high light intensities, at relative humidities between 45 and 80% and in leaves of plants grown without any external source of nitrogen. The inhibition was rather less affected by changing C02 or 02 concentration. Maximum inhibition of respiration was observed in the youngest leaves, at high temperatures and in the leaves of nitrogen deficient plants. In most cases, the maximum inhibition of C02 exchange coincided with the maximum control rate in the absence of NO2. The inhibition of transpiration by NO2, was generally small and in a few cases either there was no effect of NO2, on transpiration or it was slightly increased by NO2. This indicated that the primary effects of NO2, were within the leaf mesophyll and not on the stomata. The uptake of N02 was also modified by plant age and environmental conditions. The rate of NO2 uptake increased with increasing concentrations of N02 and decreased with increasing exposure time. It was highest in the leaves of intermediate ages, in the light, at higher temperatures, at low and O2 concentrations, and in nitrogen deficient plants. In most cases, the maximum rate of N02 uptake was correlated with the maximum inhibition of gas exchange, but in several cases, it was not. Although stomatal resistance influenced the rate of N02 uptake to some extent, mesophyll resistance to NO2 was mainly responsible for the regulation of its absorption. In addition to Phaseolus vulgaris L.,10 other angiosperm species were examined. All species absorbed substantial amounts of N02 from an atmosphere of 3.0 ppm NO2, and all experienced a concomitant inhibition of photosynthesis. The rates of N02 uptake and the degree of inhibition varied according to species. The average rate of N02 uptake after a 2 h exposure to 3.0 ppm N02 -2 -1 was 0.391 mg N02 dm h and the average inhibition of photosynthesis with the same dose of N02 was 14.3%. An estimation of N02 uptake on a worldwide basis indicated that a concentration of 0.1 ppm N02 in the world's atmosphere could provide as much as 11% of the total nitrogen requirement of the terrestrial plants. Furthermore, the experiments reveal that the effect of N02 on plant metabolism is not restricted to a particular pathway or process; rather it is generalized. It appears that N02 may inhibit gas exchange by disrupting the structure of cell organelles and/or by interfering with the activities of enzymes. / Land and Food Systems, Faculty of / Graduate

This study aimed to describe the biodiversity of the nitrogen-fixing symbiotic bacteria Frankia, to investigate the occurrence of Frankia in soil and seeds, and finally test the effect of a Frankia symbiosis on growth rate on Sea buckthorn (Hippophae rhamnoides) in an experiment. Data from the high-quality ribosomal gene database SILVA was used to reconstruct phylogenetic trees that revealed a genetic diversity in the genus that includes more strains than 16 described species so far. Strains of Frankia isolated from Sea buckthorn were found in cluster three, which also was the cluster of Frankia strains with the highest diversity in genera of plants possible for the symbiosis (actinorhizal plants).Maps of biogeographical diversity were created from the SILVA gene database and whole genome data from NCBI. Gene sequences of Frankia existed on all continents except Antarctica, which reflects the distribution of actinorhizal plants. From Sweden, only sequences of strains in cluster 1 in symbiosis with Alnus and Myrica were found, although Sea buckthorn is naturally distributed along the Swedish coast. DNA was extracted from seeds of Sea buckthorn and soil, in order to amplify gene sequences affiliated to Frankia, however the preliminary PCR results indicated that there was an issue of inhibition caused e.g. by humic substances – and which therefore would need more optimization.The experiment that was set up to study the relative growth rate (RGR) of Sea buckthorn plants inoculated with Frankia in comparison with controls did not show any difference in RGR during the first month of growth, which is a too short time for any further reliable interpretations.

Patterns and controls of net primary production (NPP) remain a critical question in ecology especially as climate modeling efforts expand. Nutrients, particularly nitrogen (N), can regulate NPP, which couples the N and C cycles. Biological nitrogen fixation (BNF) is the primary natural pathway by which new N enters ecosystems. The magnitude of the natural BNF flux is still not well constrained and the effect of this new N on forest demography and C storage is not well understood. In chapter 1 we use tree census data and two approaches of estimating BNF to make an estimate of the total N fixed by trees across the U.S.: 0.30-0.88 Tg N yr-1 (1.4-3.4 kg N ha-1 yr-1), smaller than previously expected and on par with N inputs from understory or asymbiotic BNF and less than inputs from N deposition. The tree BNF input is dominated by two tree genera: Robinia and Alnus. In chapter 2 we use mixed effect models of forest census data to show that N-fixing trees have no net effect on forest biomass accumulate rate, indicating that though they can fertilize forests on long timescales, during the course of their lives the competitive influences they exert on neighbors balance any fertilization effect they may have. However, the net effect of N-fixing trees on forest development and carbon storage depends on local factors and can be significantly facilitative in contexts where N-fixers are less competitive or when neighbors occupy different forest niches. In chapter 3 we develop a theoretical model which shows lateral leaf litter is a plausible mechanism for observed N-fixer effects, wherein the percent of litter nutrients shared with neighbors can range from almost 0% for small trees to >90% for large isolated trees in low wind, fast decomposition environments. Litter nutrients spread more in windy environments or from trees whose leaf litter falls farther from trees and diffuses more quickly. In sum, N-fixing trees play an important role in temperate forests representing an important N input, however, the flux is smaller than previously expected and the fertilization effect of N-fixing trees is not observed during the census interval.

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