Now showing items 1999-2018 of 3447

• #### Neural Network Approach To Classification Of Infrasound Signals

As part of the International Monitoring Systems of the Preparatory Commissions for the Comprehensive Nuclear Test-Ban Treaty Organization, the Infrasound Group at the University of Alaska Fairbanks maintains and operates two infrasound stations to monitor global nuclear activity. In addition, the group specializes in detecting and classifying the man-made and naturally produced signals recorded at both stations by computing various characterization parameters (e.g. mean of the cross correlation maxima, trace velocity, direction of arrival, and planarity values) using the in-house developed weighted least-squares algorithm. Classifying commonly observed low-frequency (0.015--0.1 Hz) signals at out stations, namely mountain associated waves and high trace-velocity signals, using traditional approach (e.g. analysis of power spectral density) presents a problem. Such signals can be separated statistically by setting a window to the trace-velocity estimate for each signal types, and the feasibility of such technique is demonstrated by displaying and comparing various summary plots (e.g. universal, seasonal and azimuthal variations) produced by analyzing infrasound data (2004--2007) from the Fairbanks and Antarctic arrays. Such plots with the availability of magnetic activity information (from the College International Geophysical Observatory located at Fairbanks, Alaska) leads to possible physical sources of the two signal types. Throughout this thesis a newly developed robust algorithm (sum of squares of variance ratios) with improved detection quality (under low signal to noise ratios) over two well-known detection algorithms (mean of the cross correlation maxima and Fisher Statistics) are investigated for its efficacy as a new detector. A neural network is examined for its ability to automatically classify the two signals described above against clutter (spurious signals with common characteristics). Four identical perceptron networks are trained and validated (with >92% classification rates) using eight independent datasets; each dataset consists of three-element (each element being a characterization parameter) feature vectors. The validated networks are tested against an expert, Prof. Charles R. Wilson, who has been studying those signals for decades. From the graphical comparisons, we conclude that such networks are excellent candidate for substituting the expert. Advantages to such networks include robustness and resistance to errors and the bias of a human operator.
• #### Neural-Network Modeling Of Placer Ore Grade Spatial Variability

Traditional geostatistical methods have been used in ore reserve estimation for decades. Research in the last two decades or so has added a number of other statistical methodologies for ore reserve estimation procedures. Recent advances in neural networks have provided a new approach to solve this problem. This thesis is focused on the Neural-network modeling for the estimation of placer ore reserve. Due to the spatial variability, multiple dimensional inputs and very noisy drill hole sample data from the selected region, it requires that the neural-network be organized in a multiple-layers to handle the non-linearity and hidden slabs for smoothing the predicted results. Various neural-network architectures are investigated and the Back-propagation is selected for modeling the ore reserve estimation problem. Sensitivity analysis is performed for the following parameters: the type of neural-network architecture, number of hidden layers and hidden neurons, type of activation functions, learning rate and momentum factors, input pattern schedule, weight updated, and so on. The influences of these parameters on the predicted output are analyzed in details and the optimal parameters are determined. To investigate the accuracy and promise of neural network modeling as a tool for ore reserve estimation, the ore grade and tonnage of Neural-network output is compared with those estimated by geostatistical methods under various cut-off grades. In addition, the overall performance is also validated by the analysis of R-squared (R2), Root-Mean-Squared (RMS), and the comparison between predicted values and 'actual' values. As the final part of this study, the optimized Neural Network was used to estimate the distribution of placer gold grade and volume of gold resource in offshore Nome. The predicted results for all the mining blocks in the lease area are validated by checking the values of RMS, R2, and Scatter plots. The estimated gold grades are also presented as contour maps for visualization.
• #### Neuroendocrine control of song in the dark-eyed junco (Junco hyemalis)

This dissertation includes three discrete projects addressing various aspects of the neuroendocrine control of song in the Dark-eyed Junco (Junco hyemalis), a migratory songbird. Specifically, the roles of testosterone, photoperiodic condition, opioids, and age were investigated with respect to song production and neural plasticity in the regions of the brain that control song (vocal control regions, VCRs)." The author "found that, in males photoperiodic condition and testosterone interact to regulate seasonal VCR volume plasticity, whereas testosterone alone controls song production. The opioid system is probably not involved in VCR plasticity or song production, but is indicated to play a role in song learning or auditory processing. Finally, VCR volumes and song production do not differ with age in photostimulated adult male juncos.
• #### Neuroplasticity And Neurotoxicology: Central Breathing Control Following Developmental Nicotine Or Ethanol Exposure

Nicotine or ethanol exposure early in development are both risk factors for Sudden Infant Death Syndrome (SIDS). I tested the hypothesis that both nicotine and ethanol may be linked to SIDS by impairing central breathing control responses to low oxygen (hypoxia) and high carbon dioxide (hypercapnia) stressors. Experiments were conducted in bullfrog tadpoles, a model system for respiratory neurotoxicology research. I addressed three specific aims: to characterize the effect of chronic ethanol on central responses to hypercapnia and hypoxia, to characterize the effect of chronic nicotine on central hypoxic responses, and to determine the persistence of hypercapnic impairments following 10-wk exposure to either nicotine or ethanol. 10-wk nicotine exposure resulted in neuroplastic changes that eliminated the central hypoxic responses of early but not late metamorphic tadpoles. Thus, central responses to both hypoxia and hypercapnia were impaired following nicotine exposure. The attenuated central hypercapnic response of nicotine-exposed tadpoles persisted for 1 - 3 wk. Following 10-wk chronic ethanol exposure central responses to hypercapnia and hypoxia were lost regardless of the developmental timing of exposure. Impairments in central hypercapnic responses persisted for 3 - 6 wk after ethanol exposure ended. The recovery of central hypercapnic responses in nicotine- and ethanol-exposed tadpoles may be an example of recuperative neuroplasticity resulting in either a reinstatement of network components and functions or an accommodation to deleterious nicotine- and ethanol-evoked neuroplastic changes. Collectively these data suggest that both nicotine and ethanol may target adaptive and compensatory mechanisms in central breathing control. The teratogen-induced impairments were developmentally dependent in the case of nicotine, and they persisted longer following ethanol exposure. The overall result of exposure to either neuroteratogen was an inability to respond to central breathing stressors, supporting the possible link to SIDS.
• #### Neuroprotection in hippocampal slices from the hibernating species Arctic ground squirrel, Spermophilus parryii

Stroke is the third leading cause of death in the U.S. and the leading cause of adult onset disability worldwide. Despite tremendous efforts to find therapeutics, only one currently approved treatment for stroke exists which is indicated for use in less than 5% of stroke victims. During a stroke, the brain experiences oxygen and nutrient deprivation due to lack of blood flow (i.e., ischemia) and tissue destruction ensues. Hibernating Arctic ground squirrels (AGS), Spermophilus parryii, are able to survive profound decreases in blood flow and cerebral perfusion during torpor, and return of blood flow (i.e., reperfusion) during intermittent euthermic periods without neurological damage. Hibernating species are a natural model of tolerance to insults, such as ischemia, that would be injurious to non-hibernating species, and are a novel model for investigating much needed therapeutics for pathologies such as stroke. Tolerance to traumatic brain injury demonstrated in hibernating AGS in vivo could be due to tissue properties, circulating factors or hypothermia. To investigate mechanisms of tolerance in brain of hibernating animals, the current project established a chronic culture system for hippocampal slices from AGS at 37°C. By using this in vitro approach, tissue properties of AGS brain could be assessed without effects of circulating factors or the protective nature of hypothermia. This project determined whether an intrinsic tissue tolerance to oxygen and nutrient deprivation, an in vitro model of ischemia-reperfusion, persists in chronic AGS slice culture and addressed associated mechanisms. Here, for the first time, slices from hibernating AGS were shown to possess a persistent tolerance to oxygen and nutrient deprivation. Thus, intrinsic tissue properties in hippocampus of hibernating AGS confer tolerance to oxygen and nutrient deprivation in addition to hypothermia. Evidence in the literature supports that neuroprotective factors are present in serum and tissue of hibernating animals, and here a preliminary investigation suggests that factors in AGS serum may play a role in protection in brain of hibernating AGS. Finally, a model is proposed that incorporates these findings, which suggests that mimicking properties of tissue and serum from hibernating animals in non-hibernating species may yield success in developing efficacious stroke therapeutics.
• #### Neuroprotection in the hibernating brain: tissue trauma and glutamate studied by microdialysis

Hibernation, a natural model of tolerance to 'cerebral ischemia', represents a state of pronounced fluctuation in cerebral blood flow where no brain damage occurs. This study systematically investigates the brain tissue response of hibernating and euthermic arctic ground squirrels to CNS trauma, modeled by insertion of microdialysis probes. The effect of glutamate, an excitatory amino acid neurotransmitter, on the cellular response and the origin of the significant amount of gltuamate were determined by quantitative microdialysis study. The present results indicate in euthermic brain tissue a typical inflammatory tissue response evidenced by the presence of activated microglia and astrocytes and the oxidative stress response. However, this response was profoundly suppressed in hibernating animals. Importantly, the progressive increase in [glu]dia is not necessarily associated with the enhanced tissue response observed in euthermic animals and could be avoided by using sterile microdialysis technique, which suggests a microbial origin of glutamate.
• #### New 3-d video methods reveal novel territorial drift-feeding behaviors that help explain environmental correlates of Chena River chinook salmon productivity

Chinook salmon (Oncorhynchus tshawytscha) are critical to subsistence and commerce in the Yukon River basin, but several recent years of low abundance have forced devastating fishery closures and raised urgent questions about causes of the decline. The Chena River subpopulation in interior Alaska has experienced a decline similar to that of the broader population. To evaluate possible factors affecting Chena River Chinook salmon productivity, I analyzed both population data and the behavior of individual fish during the summer they spend as fry drift feeding in the river. Using a stereo pair of high definition video cameras, I recorded the fine-scale behavior of schools of juvenile Chinook salmon associated with woody debris along the margins of the Chena River. I developed a software program called VidSync that recorded 3-D measurements with sub-millimeter accuracy and provided a streamlined workflow for the measurement of several thousand 3-D points of behavioral data (Chapter 1). Juvenile Chinook salmon spent 91% of their foraging attempts investigating and rejecting debris rather than capturing prey, which affects their energy intake rate and makes foraging attempt rate an unreliable indicator of foraging success (Chapter 2). Even though Chinook salmon were schooling, some were highly territorial within their 3-D school configurations, and many others maintained exclusive space-use behaviors consistent with the population regulatory effects of territoriality observed in other salmonids (Chapter 3). Finally, a twenty-year population time series from the Chena River and neighboring Salcha River contained evidence for negative density dependence and a strong negative effect of sustained high summer stream discharge on productivity (Chapter 4). The observed territoriality may explain the population's density dependence, and the effect of debris on foraging efficiency represents one of many potential mechanisms behind the negative effect of high stream discharge. In combination, these findings contribute to a statistically and mechanistically plausible explanation for the recent decline in Chena River Chinook salmon. If they are, in fact, major causes of the decline (other causes cannot be ruled out), then we can be tentatively hopeful that the population may be experiencing a natural lull in abundance from which a recovery is possible.
• #### A new affordability indicator for rural Alaskan water utilities

The Alaska Department of Environmental Conservation (DEC) administers funding for the construction of new water utilities in rural parts of the state. Funding allocation is partially based on whether the recipient community can cover the annual operation, maintenance, repair, equipment and capital replacement costs of the utility. Currently, the DEC deems a project affordable if the annual costs account for 5% or less of the community's median household income (MHI). In rural Alaska MHI is an inaccurate affordability indicator. This is partially because MHI fails to reflect the cost burden experienced by below median income households, it is a static snapshot of income, it does not account for living costs, nor does it account for the demographic composition of a community or the distribution of income. An alternative indicator was developed. The new indicator is composed of a Residential Indicator (RI) and a Financial Capability Index (FCI). RI is obtained by dividing the community's annual user fee by each income quintile value. FCI is composed of socioeconomic indicators chosen for their ability to detail the situation in rural Alaska. The FCI value is obtained by calculating the average of score assigned to the indicators based on pre-established thresholds. The new indicator was found to be more accurate than the MHI indicator. The new indicator was retroactively applied to Akiachak and found to more accurately assess affordability. The new indicator was also used to assess the current situation in communities with water utilities. The MHI indicator was found to have underestimated the price burden of user fees in numerous communities, and to have overestimated the burden in one community.
• #### New algorithms for the compilation of glacier inventories

Glacier inventories are used for many applications in glaciology, however, their manual compilation is time-consuming. Here, we present two new algorithms for the automatic compilation of glacier inventories. The first approach is based on hydrological modeling tools and separates glacier complexes into individual glaciers, requiring a digital elevation model (DEM) and glacier complex outlines as input. Its application to > 60,000 km² of ice in Alaska (~98% success rate) and southern Arctic Canada (~97% success rate) indicates the method is robust if DEMs and glacier complex outlines of good quality are available. The second algorithm relies on glacier outlines and a DEM and derives centerlines in a three-step 'cost grid -- least cost route' procedure. First, termini and heads are determined for every glacier. Second, centerlines are derived by determining the least cost route on a previously determined cost grid. Third, the centerlines are split into branches, followed by the attribution of a branch order. Application to > 21,000 Alaska glaciers shows that ~5.5% of the glacier heads and ~3.5% of the termini require manual correction. With corrected heads and termini, ~1.5% of the actual derived centerlines need edits. Comparison with alternative approaches reveals that the centerlines vary significantly depending on the algorithm used.
• #### A new elasmosaurid (Sauropterygia: plesiosauria) from the Bearpaw Formation (late Cretaceous, Maastrichtian) of Montana and the evolution of neck length in elasmosauridae

Plesiosauria is a diverse clade of marine reptiles that have been studied since the early 19th century. However, phylogenetic relationships within the group have been contentious due to limited taxon sampling and a misunderstanding of how ontogeny, interspecific and intraspecific variation affect character states. This is particularly true for elasmosaurids, a clade of long-necked plesiosaurians known from the Cretaceous. In 2010, a new, nearly complete skeleton, MOR 3072, was collected from the Late Cretaceous (Campanian-Maastrichtian) Bearpaw Shale of northeast Montana, and it provides morphological information rarely observed within Elasmosauridae. MOR 3072 consists of a complete skull, the anterior 23 cervical vertebrae, a partial dorsal and caudal vertebral column, incomplete pectoral and pelvic girdles, elements of both fore- and hindlimbs, ribs, and gastralia. Here, I present a detailed description of the specimen and conduct the most complete phylogenetic analysis of Elasmosauridae to date. A new taxon is recognized on the basis of the following suite of autapomorphies and unique character combinations: a chordate bilobed external naris, a squared-off posteroventral margin of maxilla, the presence of a maxilla-squamosal contact, a deep anteroposterior-oriented cleft in the articular posterior to the glenoid, a reduced number of cervical vertebrae, proximal caudal vertebrae that are wider than dorsoventrally tall, and small facets for forelimb and hindlimb preaxial accessory ossicles. A phylogenetic analysis places MOR 3072 as the sister taxon to the long-necked, Western Interior elasmosaurids Hydralmosaurus serpentinus + Styxosaurus snowii. Being early Maastrichtian in age, MOR 3072 is the stratigraphically youngest elasmosaurid yet known from the Western Interior Seaway. It is also one of the smallest adult elasmosaurids ever recovered (4.5-5 m) and exhibits a reduced neck length due to a reduction in both the number of cervical vertebrae and centrum length, which is convergent with another clade of Maastrichtian elasmosaurids, Aristonectinae.
• #### New instrumentation for the detection of sulfur dioxide in the remote atmosphere

Sulfur gases are an important chemical component of the atmosphere. Gaseous sulfur compounds effect the acidity of rainwater and are important precursors to aerosol particles which affect public health, climate and visibility of scenic vistas such as the Grand Canyon. Sulfate aerosols are also known to participate in ozone catalysis in the stratosphere. A vast majority of the gaseous sulfur cycling through the atmosphere will exist as sulfur dioxide (SO2) at some time during its atmospheric lifetime. Since SO 2 is a primary component of the atmospheric sulfur cycle, quality measurements of this gas are important to understanding the cycling of sulfur through the atmosphere. The mixing ratio of SO2 in the atmosphere can be as low as a few 10's of parts-per-trillion by volume (pptv) in unpolluted areas and as high as 100's of parts-per-billion by volume (ppbv) near industrial centers. Obtaining SO2 measurements with mixing ratios that can differ by 105 in magnitude is a difficult task, especially for mixing ratios less than a few hundred pptv. The Diffusion Denuder/Sulfur Chemiluminescence Detector (DD/SCD) was developed further and tested in a rigorously blind comparison under controlled laboratory conditions. The DD/SCD exhibited excellent sensitivity and little-to-no interference from other trace gases. The DD/SCD performance was comparable to that of other state-of-the-art instruments developed for measuring SO 2 in the remote atmosphere. The Continuous SO2 Detector was developed to overcome the limitation of long sampling times (4 to 90 minutes) inherent in the DD/SCD and other state-of-the-art techniques. The Continuous SO2 Detector (CSD) was developed based on the design of the DD/SCD, but has been optimized for sensitive, high-time resolved measurements of SO2 in air. Sensitive, high-time resolved measurements would be beneficial for studying atmospheric SO2 over large geographical areas from a moving sampling platform such as an aircraft. The current prototype of the CSD is capable of measuring SO2 at mixing ratios of less than 100 pptv on the order of seconds. The DD/SCD, CSD and an automated, computer controlled dynamic dilution system described in this thesis represent a suite of instruments for the measurement of SO2 in the remote atmosphere.
• #### New microfabrication method for prototyping integrated microfluidic modules with SR-3000 and polydimethylsiloxane (PDMS)

This thesis presents the first work on the fabrication of microfluidic modules with SR-3000 Rayzist photoresist paper and polydimethylsiloxane (PDMS). Chapter 1 of the thesis is on the analysis of elemental composition of SR-3000. By using the X-Ray Fluorescence spectrometer we found the SR-3000 sheet is enriched with silicon, the key element for forming covalent bonding to PDMS. Chapters 2, 3,and 4 of the thesis is focused on the characterization of both the hydrophilicity of the plasma-treated SR-3000 surface and the bonding strength between SR-3000 and PDMS. Unfiltered air was used as the process gas for plasma-assisted bonding of SR-3000 to PDMS. Pressure rupture tests were conducted to measure the strength at the bonding interface, which can be as high as 57.7 psi, strong enough to hold the fluid pressure for typical microfluidics applications. The hydrophilicity of SR-3000 is mainly governed by the plasma treatment time. Chapter 5 demonstrates how to use the developed microfabrication method to prototype microfluidic modules for typical microfluidic applications, which include manipulation of laminar flow, mixing of miscible fluids, and production of oil droplets in a stream of water flow.
• #### A new model for the substorm growth phase

The physics of geomagnetic substorms has been under debate for a long time. In particular, the formation of a thin current sheet (CS) is a central unresolved problem because it provides the magnetotail conditions for the expansion phase onset. This dissertation presents a new CS thinning mechanism based on midnight magnetic flux depletion (MFD), which is caused by sunward convection to balance dayside reconnection during periods of southward interplanetary magnetic field. The results demonstrate that MFD is a highly efficient mechanism to generate a very thin CS in the near-Earth tail. This study also examines CS formation under the influence of adiabatic lobe compression in combination with MFD and proposes a double-current sheet evolution at distinct locations in the near-Earth region and mid-tail region. The results suggest that substorm expansion onset is associated only with near-Earth onset of magnetic reconnection, while mid-tail reconnection causes bursty bulk flows. In addition, this dissertation investigates the changes of the auroral morphology associated with the magnetotail evolution. An ionospheric map is constructed based on Tsyganenko 96 magnetic field model corrected by magnetic flux conservation. By employing MFD, the mapping results such as the equatorward expansion of the open/closed field boundary, the convergent motion of strong field-aligned currents, and the location of electron and ion isotropy boundaries are consistent with typical ionospheric observations. These results demonstrate that MFD is the first model that can consistently explain and predict the typical magnetotail and ionospheric evolution during the substorm growth phase and shed light on the physics of the growth phase aurora.

• #### A new sensitivity analysis and solution method for scintillometer measurements of area-average turbulent fluxes

Scintillometer measurements of the turbulence inner-scale length l₀ and refractive index structure function C²n allow for the retrieval of large-scale area-averaged turbulent fluxes in the atmospheric surface layer. This retrieval involves the solution of the non-linear set of equations defined by the Monin-Obukhov similarity hypothesis. A new method that uses an analytic solution to the set of equations is presented, which leads to a stable and efficient numerical method of computation that has the potential of eliminating computational error. Mathematical expressions are derived that map out the sensitivity of the turbulent flux measurements to uncertainties in source measurements such as l₀. These sensitivity functions differ from results in the previous literature; the reasons for the differences are explored.
• #### Nicotine exposure in the developing bullfrog: influences on neuroventilatory responses to CO₂

Developmental exposure to the neuroteratogen nicotine may affect ventilatory responses to hypercapnia. Developmental changes in normocapnic and hypercapnic neuroventilation of the isolated bullfrog brainstem preparation have been previously characterized. I investigated the effect of 3- and 10-wk chronic nicotine (30 [mu]g/L) exposure on lung burst frequency exhibited by early and late metamorphic bullfrog tadpoles during normocapnia (1.5 % CO₂) and hypercapnia (5.0 % CO₂). Chronic nicotine exposure impairs the hypercapnic neuroventilatory response of early metamorphic tadpoles following both 3- and 10-wk exposure. Late metamorphic tadpoles demonstrated an impaired hypercapnic neuroventilatory response only after 10-wk exposure. Chronic nicotine exposure had no effect on normocapnic neuroventilation. Brainstem preparations from early and late metamorphic tadpoles and juvenile bullfrogs were exposed acutely to 18 [mu]g/L nicotine. Acute nicotine had no effect on normocapnic or hypercapnic neuroventilation of early metamorphic tadpoles. Late metamorphic tadpoles and juvenile bullfrogs demonstrated depressed normocapnic neuroventilation in response to acute nicotine exposure, while late metamorphic tadpole brainstems responded significantly to hypercapnia during acute exposure. This suggests that bullfrogs have a differential response to acute nicotine exposure that increases with development. Collectively these data suggest that the consequences of developmental nicotine exposure differ between acute and chronic exposure and throughout bullfrog development.
• #### Nietzsche in a train station

Although Nietzsche in a Train Station is referential, the poems are not written for the scholar with a pen in hand, but are for the average reader with a laptop by his or her side. The "Mr. Everidge" poems attempt to capture the frenetic pace and fragmented consciousness of the information age, while maintaining a core identity. Although some poems directly take the narrative aspect of the short story and compress it as much as possible, using the sound of the language as much as the literal meaning to tell the story, every poem is a self-contained narrative. Many of the poems in this collection follow traditional forms and subjects, such as Shakespearian sonnets that explore kinds of love; however, a majority of the poems explore less traditional forms and rhythms. Stylistically, the poems tend to use long, complex sentences, with the occasional willful rebellion against traditional grammar. The poems drive toward a resolution, and everything structurally serves this purpose. Each poem is in essence a question about the things existing outside of everyday experience, the constructions created for and by the individual, constructions that both sustain and limit a life.
• #### Nitrogen cycling at treeline: latitudinal and elevational patterns across the boreal landscape

We studied spatial and temporal patterns of soil nitrogen pools and fluxes at treeline and forested sites within three Alaskan mountain ranges along a latitudinal transect of 785 km during 2001- 2002. We measured soil temperatures, pools of soil mineral (ammonium and nitrate) and organic (amino acid and microbial biomass) nitrogen, in situ rates of net mineralization, net nitrification, and net amino acid production, conducted a decomposition experiment at all sites using common litter, and studied soil carbon turnover in a laboratory incubation experiment. Soils at treeline were colder than forested soils, particularly during fall and over winter, and had reduced rates of nitrogen cycling and litter decomposition relative to soils in forested stands. During incubation, treeline soils had lower respiration rates per unit carbon, suggesting lower soil organic matter quality relative to forested soils. 70% of annual net nitrogen mineralization occurred from August-May, suggesting that fall and winter are critical periods for soil nitrogen transformations in forested and treeline ecosystems. Among mountain ranges, nitrogen pools and fluxes were similar, despite variation in growing season length and mean annual temperatures. Soil moisture and organic matter quality may have stronger effects on variation in nitrogen cycling than temperature at our sites.
• #### Nitrogen dynamics in primary successional soils on the Tanana River of interior Alaska

As succession determines the patterns of ecosystem development, it also provides a temporal framework in which to investigate the controls of nitrogen-cycling. The object of this study was to examine patterns of nitrogen-cycling within primary successional soils. Estimates of N mineralization, nitrogen fixation, and denitrification were made within open shrub, alder, and white spruce stands, representing early, mid-, and late successional stages, respectively. Net NH$\sb4\sp+$ mineralization and net nitrification, measured using the polyethylene bag technique, were significantly different between sites and among forest floors and mineral soils. The alder forest floor had the highest observed mineralization rates, $<$1-21 ug N $\cdot$ g dry soil$\sp{-1}$ $\cdot$ d$\sp{-1}$, with the white spruce forest floor exhibiting rates of $<$1-2 and mineral soils $<$1. Seasonal patterns of N mineralization were more pronounced in the alder forest floor: high net NH$\sb4\sp+$ mineralization in early summer and high net nitrification in late summer. Immobilization of nitrogen was observed at all sites. Laboratory studies indicate temperature as a limiting factor of N mineralization in early and mid-successional stages, while both temperature and moisture were limiting in later succession. Laboratory studies suggest heterotrophic nitrification may be important in the alder forest floor. Denitrification activity was low to undetectable at all sites, the highest observed rate was in alder forest floor samples, 220 g N $\cdot$ ha$\sp{-1}$ $\cdot$ d$\sp{-1}$. Potential denitrification was low, the alder forest floor exhibited the highest rate, 3.4 Kg N $\cdot$ ha$\sp{-1}$ $\cdot$ d$\sp{-1}$. Laboratory studies indicated denitrification was more limited by carbon and NO$\sb3\sp-$ than temperature or moisture. Nonsymbiotic nitrogen fixation was low to undetectable and highly variable. The highest observed nitrogenase activity was associated with alder root nodules, 163 Kg N $\cdot$ ha$\sp{-1}$ $\cdot$ yr$\sp{-1}$.
• #### Nitrogen Dynamics Through The Forest Floor Of Two Interior Alaska Black Spruce Ecosystems

Nitrogen flow in the forest floor of two interior Alaska black spruce (Picea mariana (Mill.) B.S.P.) ecosystems was investigated and related to environmental constraints unique to the area, specifically temperature, moisture, and organic matter quality (C/N ratio). Pools examined were NH(,4)-N, NO(,3)-N, soluble organic N, total (Kjeldahl) and residual organic N. Low addition levels of high enrichment isotope (< 1% of the total nitrogen pool wth 99 atom percent excess ('15)N) were used to describe nitrogen dynamics through pools of selected forest floor components of permafrost-free and permafrost-dominated black spruce sites. A thick carpet of mosses, made up primarily of the feather moss species Hylocomium splendens (Hedw.) B.S.G. and Pleurozium schreberi (B.S.C.) Mitt. played a vital role in the nitrogen economy of the forest floor. Nitrogen, quickly immobilized in the moss layers (green, brown) and retained there, was released very slowly and sequestered in the fermentation and humus layers (021 + 022) where most of the vascular plant roots were located. Vascular understory ('15)N uptake was minimal as was ('15)N export via the soil solution. Periodic mineralization episodes, more frequent and dynamic at the permafrost-free site (where C/N ratios were lower), were largely restricted to the moss layers since available N pools in deeper forest floor layers incorporated little label over the three year period. It proved difficult to separate the effects of rainfall events from that of forest floor temperature fluctuations upon seasonal nitrogen dynamics. In the lower layers of the forest floor temperature and/or moisture rather than organic matter quality appeared to be the overriding factor controlling N flow. The dominance in pool size of NH(,4)-N over NO(,3)-N is discussed with reference to current theories of ecosystem strategy.