Abstract In northernmost Fennoscandia, grazing by reindeer (Rangifer tarandus L.) has a substantial impact on the vegetation of boreal forests and arctic-alpine tundra heaths, which are reflected in below-ground processes, such as nutrient mineralization and soil organic matter decomposition. In the present thesis, the effects of reindeer grazing on soil nutrient cycling were studied by comparing grazed situation with an ungrazed control area in ten boreal forests and six arctic-alpine tundra heaths. In boreal forests, reindeer grazing reduced microbial respiration in both the oligotrophic and mesotrophic study areas, indicating a deficiency of labile substrates for the soil microbes due to reindeer grazing. Simultaneously, there was heterogeneity in the impact on nitrogen mineralization rates as at some sites, mineralization was enhanced by grazing. The fertilization effect of urine and faeces can therefore be strong enough a factor to outweigh a reduction in quality of soil organic matter. In the oligotrophic forests, low soil moisture content in the grazed areas could sometimes limit the mineralization rates even when the potential for mineralization was enhanced by grazing. In the tundra ecosystems, there was spatial variation in the impact of grazing on microbial respiration and nitrogen mineralization. Low grazing intensity occurring outside the growing season had a retarding impact on nutrient cycling in both unfertilized, nutrient-poor and fertilized, nutrient-rich conditions. In contrast, a relatively high grazing intensity enhanced the mineralization rates in two nutrient-poor and two nutrient-rich tundra heaths. When three different grazing intensities were compared in one oceanic, nutrient-rich and one continental, nutrient-poor tundra heath, the strongest positive effect of grazing on soil nutrient cycling occurred in the heavily grazed areas. The data do not support the assumption that soil nutrient availability regulates whether herbivores enhance or retard nutrient cycling in the soil. Instead, the net effect of grazing is determined by the balance between the underlying mechanisms that may work at opposite directions. The most important of these mechanisms are the grazer-mediated impact on the decomposability of the dominant vegetation and fertilization by urine and faeces. The duration, intensity and seasonal timing of the grazing seem to be important factors that regulate whether reindeer grazing enhances or retards soil nutrient cycling in each specific area. Due to the high spatial and temporal variation in the effects of grazing observed in this study, it is not possible to generalize the overall impact of grazing. Further study is required in order to determine the exact conditions under which grazing enhances or it retards soil nutrient cycling.

M. S.

M.S.

Microbial respiration on sticks and leaves, measured as oxygen uptake rate, was compared among four (2nd-4th order) sites along an elevational gradient at Coweeta Hydrologic Laboratory, North Carolina. Rhododendron and birch leaves were placed in mesh bags in the stream at each site on 21 Oct 90. Oxygen uptake rates were measured for both leaf species beginning 15 Dec 90 and continuing monthly until leaves were no longer intact (birch-2 months, rhododendron-7 months). Microbial oxygen uptake rates were also measured monthly for one year on qualitative collections of sticks (1-3 cm diameter) found in the stream at each site. Oxygen uptake rates (mg O₂/hr) were calculated on both a surface area and AFDM basis. Overall mean respiration rate per unit surface area was highest for sticks, followed by rhododendron, and birch. When expressed on an AFDM basis, respiration rates were highest for birch, followed by rhododendron, and sticks. Based on continuous measurements, mean monthly stream temperatures were significantly different among sites, but mean incubation temperatures were not significantly different among sites, Respiration rates were significantly correlated with temperature for both rhododendron leaves and sticks on both an AFDM and surface area basis. Respiration rates on rhododendron also increased with exposure time. Results indicate that differences in respiration rates among sites, for both rhododendron leaves and sticks, can be explained primarily by changes in temperature. Additionally, the high respiration rates per unit surface area on sticks indicate that small woody debris may play a significant role as a substrate for microbial metabolism. / Master of Science

Graduation date: 1993

Thesis (Ph.D.)--Kent State University, 2006. / Title from PDF t.p. (viewed Mar. 5, 2009). Advisor: Robert T Heath. Keywords: bacterioplankton, phosphorus dynamics, bacterial bioenergetics, labile dissolved organic carbon, bacterial productivity, bacterial respiration, bacterial growth efficiency, Lake Erie, hypoxia. Includes bibliographical references (p. 357-397).

Stream ecosystems can be influenced by cattle grazing in the riparian zone due to sediment input, nutrient loading, and soil compaction, which lead to alterations of macroinvertebrate and microbial activity. Recently government programs, such as the Conservation Reserve Enhancement Program (CREP), have provided funding for farmers to exclude cattle from streams and riparian zones. Funding for CREP is limited and does not allow for post exclusion assessment. The objectives for this study were; 1) to explore whether CREP and other cattle exclusion initiatives help restore functional integrity to streams; 2) and if they do, to evaluate the time required for integrity to be restored. I predicted leaf processing (a fundamental ecosystem level function) in streams would be influenced by excluding cattle from the riparian zone due to changes in nutrient availability, sediment abundance, shredding macroinvertebrates, and microbial activity. I tested this prediction by measuring leaf processing at sites that had cattle excluded for <1 to 15 years. Breakdown rates did not correspond linearly to time since cattle exclusion. This was probably due to the opposing effects of elevated sediment versus nutrients on leaf breakdown at recently grazed sites. Leaf breakdown and shredder density were strongly correlated with riparian vegetation density. This study suggests that in addition to cattle exclusion, reforestation of woody riparian vegetation may be essential to restore functional integrity to agricultural streams. / Master of Science

Bradyrhizobium japonicum USDA 143 grew chemoorganotrophically when supplied with exogenous nitrous oxide as the terminal electron acceptor, or as the alternate terminal electron acceptor to nitrate under anoxic conditions. Cell growth and dissimilatory N₂O reduction were significantly inhibited by acetylene when either N₂O or N₂O plus nitrate served as terminal electron acceptor(s). Reduction of nitrous oxide accounted for 20% of the energy for cell growth in cultures supplied with nitrate as the terminal electron acceptor. Nitrous oxide was produced stoichiometrically in cultures supplied with nitrate and acetylene and growth was proportionately reduced compared to cultures supplied with an equal amount of nitrate. Exogenous nitrous oxide delayed the reduction of nitrate in cultures supplied with both electron acceptors. The final cell yield and/or growth rate of the cells were reduced when N₂O was ≥ 15% of the culture flask headspace. Direct amperometric monitoring of nitrous oxide respiration indicated a specific activity of 0.082 ± 0.004 µmoles N₂O/min/mg cell-protein. The respiration was inhibited by azide. A Clark-type electrode with a platinum cathode, and the instrumentation for monitoring hydrogen uptake amperometrically were used to monitor the reduction of N₂O during anaerobic respiration. / Master of Science

Living (biotic) and non-living (abiotic) factors drive the function of ecosystems across a variety of scales from the root-soil interface to the watershed. Biotic and abiotic global change pressures such as increasing temperature and invasive species are shifting how ecosystems function. Thus, exploring and understanding how these factors shape function across the landscape is an important research area. For example, climate change both directly and indirectly affects soil microbial functions – such as carbon mineralization and nitrogen transformations – through increasing activity under warming and altering inputs to the soil through species composition changes. Mountains provide a useful tool for studying relationships among biotic and abiotic factors because climate and species diversity shift along gradients. Here, I measured carbon and nitrogen soil processes as well as microbial extracellular enzyme activity along an elevational gradient to explore how changes in climate, edaphic properties, and biotic composition affects ecosystem function. As expected, climate and species composition varied in predictable ways along the gradient – actual evapotranspiration declined, and conifer dominance increased. Soil functions also shifted along the gradient. Potential carbon mineralization increased with elevation and with conifer dominance. Potential nitrogen mineralization rates increased with elevation and with conifer dominance. Surprisingly, there were few predictors for potential soil nitrification, which increased only with soil functional diversity. While temperature and moisture availability drive ecosystem function at broad scales and biotic factors typically drive function at the regional scale, we saw that function of soils at the mountain watershed scale was best explained by a combination of both abiotic and biotic factors.

Biochar has been heralded a mechanism for carbon sequestration and an ideal amendment for improving soil quality. Melaleuca quinquenervia is an aggressive and wide-spread invasive species in Florida. The purpose of this research was to convert M. quinquenervia biomass into biochar and measure how application at two rates (2% or 5% wt/wt) impacts soil quality, plant growth, and microbial gas flux in a greenhouse experiment using Phaseolus vulgaris L. and local soil. Plant growth was measured using height, biomass weight, specific leaf area, and root-shoot ratio. Soil quality was evaluated according to nutrient content and water holding capacity. Microbial respiration, as carbon dioxide (CO2), was measured using gas chromatography. Biochar addition at 5% significantly reduced available soil nutrients, while 2% biochar application increased almost all nutrients. Plant biomass was highest in the control group, p2 flux decreased significantly in both biochar groups, but reductions were not long term.