Graduate Theses & Dissertations

Carbon Exchange along a Natural Gradient of Deciduous Shrub Coverage in the Low-Arctic
Arctic terrestrial ecosystems have experienced substantial structural and compositional changes in response to warming climate in recent decades, especially the expansion of shrub species in Arctic tundra. Climatic and vegetation changes could feedback to the global climate by changing the carbon balance of Arctic tundra. The objective of this thesis was to investigate the influence of increased shrub coverage on carbon exchange processes between atmosphere and the Arctic tundra ecosystem. In this study a space-for-time substitution was used, referred to as a shrub expansion “chronosequence”, with three sites along a natural gradient of deciduous shrub coverage in the Canadian low Arctic. Leaf-level photosynthetic capacity (Amax) of dominating birch shrub Betula glandulosa (Michx.) was significantly higher (P<0.05) at the site where shrubs were more abundant and taller than at the other sites. For all sites, mean Amax in 2014 was significantly lower than in 2013, in part potentially due to differences in precipitation distribution. Bulk soil respiration (RS) rate was significantly higher (P<0.05) at the site with more shrubs compared with the other sites. The differences in RS across sites appeared to be driven by differences in soil physiochemical properties, such as soil nitrogen and soil bulk density rather than soil microclimate factors (e.g. soil temperature, moisture). The three sites were either annual CO2 sources (NEP<0) to the atmosphere or CO2 neutral, with strongest annual CO2 sources (-44.1±7.0 g C m-2) at the site with most shrubs. Overall this study suggests that shrubs tundra carbon balance will change with shrub expansion and that shrub ecosystems in the Arctic currently act as annual carbon sources or neutral to the atmospheric CO2 and further shrub expansion might strengthen the CO2 emissions, causing a positive feedback to the warming climate. Author Keywords: arctic tundra, carbon exchange, climate change, photosynthetic capacity, shrub expansion, soil respiration
Size and fluorescence properties of allochthonous dissolved organic matter
Dissolved organic matter (DOM) is a mixture of molecules with dynamic structure and composition that are ubiquitous in aquatic systems. DOM has several important functions in both natural and engineered systems, such as supporting microorganisms, governing the toxicity of metals and other pollutants, and controlling the fate of dissolved carbon. The structure and composition of DOM determine its reactivity, and hence its effectiveness in these ecosystem functions. While the structure, composition, and reactivity of riverine and marine DOM have been previously investigated, those of allochthonous DOM collected prior to exposure to microbes and sunlight have received scant attention. The following dissertation constitutes the first in-depth study of the structure, composition, and reactivity of allochthonous DOM at its point of origin (i.e. leaf leachates, LLDOM), as detected by measuring its size and optical properties. Concomitantly, novel chemometric methods were developed to interpret size-resolved data obtained using asymmetrical flow field-flow fractionation, including spectral deconvolution and the application of machine learning algorithms such as self-organizing maps to fluorescence data using a dataset of more than 1000 fluorescence excitation-emission matrices. The size and fluorescence properties of LLDOM are highly distinct. Indeed, LLDOM was correctly classified as one of 13 species/sources with 92.5% accuracy based on its fluorescence composition, and LLDOM was distinguished from riverine DOM sampled from eight different rivers with 98.3% accuracy. Additionally, both fluorescence and size properties were effective conservative tracers of DOC contribution in pH-controlled mixtures of leaf leachates and riverine DOM over two weeks. However, the structure of LLDOM responded differently to pH changes for leaves/needles from different tree species, and for older needles. Structural changes were non-reversible. Copper-binding strength (log K) differed for the different fluorescent components of DOM in a single allochthonous source by more than an order of magnitude (4.73 compared to 6.11). Biotransformation preferentially removed protein/polyphenol-like fluorescence and altered copper-binding parameters: log K increased from 4.7 to 5.5 for one fluorescent component measured by fluorescence quenching, but decreased from 7.2 to 5.8 for the overall DOM, as measured using voltammetry. The complexing capacity of DOM increased in response to biotransformation for both fluorescent and total DOM. The relationship between fluorescence and size properties was consistent for fresh allochthonous DOM, but differed in aged material. Since the size and fluorescence properties of LLDOM are strikingly different from those of riverine DOM, deeper investigation into transformative pathways and mixing processes is required to elucidate the contribution of riparian plant species to DOM signatures in rivers. Author Keywords: Analytical chemistry, Chemometrics, Dissolved organic matter (DOM), Field-flow fractionation, Fluorescence spectroscopy, Parallel factor analysis (PARAFAC)
The objective of this thesis was to assess the influence of anthropogenic sulphur (S) and nitrogen (N) deposition, and harvesting on soil acidity and calcium (Ca2+), magnesium (Mg2+), potassium (K+) and N soil pools in plantation forest soils in Ireland. The response to reductions in anthropogenic S deposition was assessed using temporal trends in soil solution chemistry at two long-term monitoring plots--one on a blanket peat, the other on a peaty podzol. At the peat site, there was little evidence of a response to reductions in throughfall non marine sulphate (nmSO42-) and acidity; soil water acidity was determined by organic acids. In addition, temporal variation in soil water did not respond to that in throughfall. In the podzol, reductions in anthropogenic S and H+ deposition led to a significant improvement in soil water chemistry at 75 cm; pH increased and total aluminum (Altot) concentrations declined. The impact of harvest scenarios on exchangeable Ca2+, Mg2+ and K+ pools was assessed using input-output budgets at 40 sites (30 spruce, 10 pine). Harvest scenarios were stem-only harvest (SOH), stem plus branch harvest (SBH) and stem, branch and needle harvest (whole-tree harvesting; WTH). Average K+ and Mg2+ budgets were positive under these scenarios. However, exchangeable K+ pools were small and due to uncertainty in K+ budgets, could be depleted within one rotation. Average Ca2+ budgets for spruce were balanced under SOH, but negative under SBH and WTH. Nitrogen deposition was high, between 5 and 19 kg N ha-1 yr-1, but was balanced by N removal in SOH. However, N budgets were under SBH and WTH, indicating that these harvesting methods would lead to depletion of soil N over the long-term. Finally, monitoring of N cycling at a spruce plot indicated that N deposition was contributing to large NO3- leaching, and as such the site was N saturated. However, N cycling did not fit the criteria of the N saturation hypothesis; instead leaching was directly related to N deposition and supported the model of kinetic N saturation. Author Keywords: acidic deposition, base cations, input-output budgets, Ireland, nitrogen, whole-tree harvesting
Hydroclimatic and spatial controls on stream nutrient export from forested catchments
Winter nutrient export from forested catchments is extremely variable from year-to-year and across the landscape of south-central Ontario. Understanding the controls on this variability is critical, as what happens during the winter sets up the timing and nature of the spring snowmelt, the major period of export for water and nutrients from seasonally snow-covered forests. Furthermore, winter processes are especially vulnerable to changes in climate, particularly to shifts in precipitation from snow to rain as air temperatures rise. The objective of this thesis was to assess climatic and topographic controls on variability in stream nutrient export from a series of forested catchments in south-central Ontario. The impacts of climate on the timing and magnitude of winter stream nutrient export, with particular focus on the impact of winter rain-on-snow (ROS) events was investigated through a) analysis of long-term hydrological, chemical and meteorological records and b) high frequency chemical and isotopic measurements of stream and snow samples over two winters. The relationship between topography and variability in stream chemistry among catchments was investigated through a) a series of field and laboratory incubations to measure rates and discern controls on nitrogen mineralization and nitrification and b) analysis of high resolution spatial data to assess relationships between topographic metrics and seasonal stream chemistry. Warmer winters with more ROS events were shown to shift the bulk of nitrate (NO3-N) export earlier in the winter at the expense of spring export; this pattern was not observed in other nutrients [i.e. dissolved organic carbon (DOC), total phosphorus (TP), sulphate (SO4), calcium (Ca)]. Hydrograph separation revealed the majority of ROS flow came from baseflow, but the NO3-N concentrations in rainfall and melting snow were so high that the majority of NO3-N export was due to these two sources. Other nutrient concentrations did not show such a great separation between sources, and thus event export of these nutrients was not as great. Proportionally, catchments with varying topography responded similarly to ROS events, but the absolute magnitude of export varied substantially, due to differences in baseflow NO3-N concentrations. Field and laboratory incubations revealed differences in rates of net NO3-N production between wetland soils and upland soils, suggesting that topographic differences amongst catchments may be responsible for differences in baseflow NO3-N. Spatial analysis of digital elevation models revealed strong relationships between wetland coverage and DOC and dissolved organic nitrogen (DON) concentrations in all seasons, but relationships between topography and NO3-N were often improved by considering only the area within 50 or 100m of the stream channel. This suggests nutrient cycling processes occurring near the stream channel may exert a stronger control over NO3-N stream outflow chemistry. Overall, topography and climate exert strong controls over spatial and temporal variability in stream chemistry at forested catchments; it is important to consider the interaction of these two factors when predicting the effects of future changes in climate or deposition. Author Keywords: biogeochemistry, forest, nitrate, south-central Ontario, stream chemistry, winter

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