Mountain Scholar :: Browsing by Author "Stromberger, Mary, committee member"

Browsing by Author "Stromberger, Mary, committee member"

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Open Access
A comprehensive microbiome analysis of wheat and its wild relatives
(Colorado State University. Libraries, 2018) Cantor, Heather, author; Byrne, Patrick F., advisor; Broders, Kirk, committee member; Stromberger, Mary, committee member
Microbiomes are diverse assemblages of endophytic and free-living microorganisms that can confer competitive advantages to their plant hosts such as water acquisition, nutrient mobilization, drought tolerance, salt tolerance, and disease resistance (Chaparro et al., 2012; Sherameti et al., 2008; Zolla et al., 2013). Plant domestication and selective breeding have altered the composition of these plant-microbe interactions in several crops. It is thought that the progenitors of the A, B, and D genomes in modern hexaploid wheat (Triticum aestivum) manage environmental stress in their native environment by establishing symbioses with a consortium of beneficial microbes (Iannucci et al., 2017). However, these microbial communities are not well understood. The goal of this study is to better understand the core community of microbes in wild wheat relatives and how they differ from the microbiome of cultivated wheat. This study compares the bacterial and fungal taxa found in and on the leaves, roots, and rhizosphere of three accessions of hard winter wheat and 14 accessions of eight wild relative species grown in a common soil. These plants and the agricultural soil they inhabit were sampled from a randomized complete block design with two replications, grown in well-watered and water-limited treatments in Fort Collins, Colorado. DNA was extracted and barcoded amplicon sequencing of the 16S-V4 (bacteria/archaea) and ITS2 (fungi) small subunit ribosomal RNA (rRNA) genes was used to describe the diversity of the microbial community associated with the root, rhizosphere and leaves of each accession. The results indicate that while there were limited differences in microbial communities among plant species, plant tissue type appears to be a strong predictor of microbial community structure. Across all plant genotypes, the rhizosphere consistently contained the most diverse and abundant microbiomes, followed by roots, and lastly leaves, which were the least diverse tissue type. When these three tissue types were analyzed independently (PERMANOVA), there was a significant difference in rhizosphere communities between the wet and dry treatments. Wet treatments contained a greater number of facultative anaerobes and bacteria common to cold, saturated soils. The wet treatment received an additional 13 mm of water, applied five days prior to collection. Overall, while plant host genotypes did not differ significantly in their microbiomes, some unique symbioses among different plant accessions indicate evolutionary adaptation. An initial look at the core microbiome shared among representatives of the five plant genomes in this study showed few shared sequence variants (<2% of total microbial SV's). However, this was largely explained by the use of high-resolution SV's that do not necessarily equate to different taxonomic assignment, suggesting an inflated number of actual microbial taxa. Coarser taxonomic overviews depicted a more realistic, and narrow, number of participating taxonomic groups in the phytobiome. Plant tissue type remained a chief driver of microbiome composition. Soil moisture and fertility may have also played a role in determining microbial community structure, but since they were not measured in this study, claims cannot as yet be made. The close genetic relationships among plant species in this study may have reduced the observable differences in microbial community structure. Additionally, common garden experiments limit the pool of potential plant-microbe interactions. Despite the advancement and evolution of modern wheat, the microbiome remains essentially the same as the microbiomes of wild relatives, when grown in the same soil. This indicates that modern winter wheat retains the same ability to recruit and sustain its microbiome as its wild relatives. In the future, microbiome consensus studies in these hosts' centers of origin could broaden our understanding of long-evolved microbial symbioses.
Open Access
Abiotic and biotic factors influencing western United States coniferous forests
(Colorado State University. Libraries, 2019) Lalande, Bradley, author; Stewart, Jane, advisor; Stromberger, Mary, committee member; Tinkham, Wade, committee member; Trivedi, Pankaj, committee member
In the next decade, climate models suggest that global temperatures will continue to rise. In the western United States, increases in temperatures and changes in precipitation patterns will escalate the risk of drought conditions. These potentially warmer, drier conditions could induce physiological changes within trees, subsequently increasing stress on coniferous forests that are adapted to cool, wet environments. The abiotic stress accompanied by drought conditions can predispose susceptible hosts to biotic stress of insect and disease populations. In particular, high elevation subalpine fir (Abies lasiocarpa) have encountered higher than average mortality rates throughout the western United States in association with abiotic and biotic agents. Chapter 2 of this thesis investigated the potential drivers of subalpine fir mortality and determined how climatic factors and site and stand characteristics influenced the presence of mortality and biotic agents. The objectives were to identify factors driving subalpine fir mortality in Colorado and included 1) determine abiotic and biotic factors that directly and indirectly affect subalpine fir mortality, 2) determine factors associated with the presence of D. confusus or Armillaria spp., and 3) determine if climate variables were correlated to subalpine fir mortality or the presence of D. confusus and Armillaria spp. I hypothesized that sites with a higher density (i.e. basal area, trees per hectare, or canopy closure) would experience greater mortality due to decreased growth rates from competition and that D. confusus or Armillaria spp. prevalence would be a function of tree stress (i.e. increased density), elevation, slope, and departures from normal precipitation (i.e. drought), and minimum and maximum temperatures. Stand health monitoring plots found that the most relevant factors to subalpine fir mortality are the presence of D. confusus (p = 0.003) and the percent subalpine fir on plot (p = <0.0001). I identified that stand density (p = 0.0038), elevation (p = 0.0581), and Armillaria spp. (p = 0.0006) were the greatest influences on the presence of D. confusus, while the largest influences on the presence of Armillaria spp. are warmer maximum summer temperatures (p = 0.0136) and the presence of D. confusus (p = 0.0289). Results indicated that increased subalpine fir mortality was attributed to high stand density as a predisposing factor, warming temperatures as an inciting factor, and bark beetles (Dryocoetes confusus) and root disease (Armillaria spp.) as contributing factors. The combination of predisposing, inciting, and contributing factors suggests that subalpine mortality can be defined as subalpine fir decline. Management strategies used to reduce the impact of subalpine fir decline will need to address ways to improve stand health, while decreasing populations of both, D. confusus and Armillaria spp. In regards to Armillaria, the inability to successfully manage the disease using current techniques highlights the need to find novel management strategies to minimize its impacts. Since this disease is a root pathogen, soil microbes likely influence its growth and survival. Utilizing soil microbial communities as biocontrols may assist in management of Armillaria. Field sampling within the Priest River Experimental Forest in northern Idaho provided the opportunity to observe how soil microbial communities are associated with two species of Armillaria, A. solidipes (primary pathogen) and A. altimontana (weak pathogen). My research objective for Chapter 3 was to identify the soil fungal communities associated with tree health status (healthy, moderate and dead) and each Armillaria species, A. solidipes and A. altimontana, both of which have differing ecological behaviors (virulent pathogen and non-pathogen, respectively) on western white pine. I hypothesized that soil microbial communities associated with virulent A. solidipes and non-pathogenic A. altimontana would differ in fungal richness and diversity with the latter having a greater richness and diversity due to its beneficial qualities to tree health. While richness and diversity is likely to shift among tree health with a greater diversity and richness for soils associated with healthy trees due to root exudate production near the rhizosphere. Soil samples were collected alongside western white pine (Pinus monticola), while Armillaria rhizomorphs were excavated near the roots. The most abundant fungal taxon was Mortierella spp., which functions as saprophyte decomposing dead and down wood. No significant differences in fungal diversity or richness were found in soils associated with Armillaria species, but, although not significant, there where slight differences between soils associated with moderate and dead trees with a greater diversity and richness in soils with dead trees (p = 0.18). Additionally, soil pH was significantly influenced by soil carbon, nitrogen, and organic matter, while moisture significantly influenced soil carbon, nitrogen, and organic matter, acting as indicators to overall health in the stand. Although not significantly different, more Hypocreaceae (Trichoderma), a known biocontrol for root pathogens, were found within soils associated with A. altimontana and healthy trees. More research is needed to solidify differences, yet these factors give insight into potential beneficial aspects of soil fungal communities in association with Armillaria species and tree health. Changing climates regimes outside of 30-year averages cause increased stress to forests. This stress may predispose trees to a greater abundance biotic agents such as bark beetles and secondary pathogens, such as Armillaria root disease specifically in association with subalpine fir in Colorado. Understanding the role that soil fungal communities play in association to Armillaria root disease and tree health may assist in forest management practices to increase the health of high elevation forests.
Open Access
Alone again, naturally? Loneliness and performance among STEM graduate students
(Colorado State University. Libraries, 2022) Stoa, Rosalyn, author; Fisher, Gwenith, advisor; Dik, Bryan, committee member; Prasad, Joshua, committee member; Stromberger, Mary, committee member
Graduate student mental health is a trending topic of research, and rightfully so considering the growing number of graduate students, high rate of mental health concerns particularly among young adults, and the high rate of attrition from graduate programs. Qualitative research has consistently raised isolation and loneliness as concerns for many doctoral students. Not only is loneliness an issue for mental health and wellbeing, but loneliness may have serious consequences for students' motivation, satisfaction, intentions to stay in school, and the current and future productivity of these scholars. Based on the tenets of self determination theory and the model of workplace loneliness, I hypothesized that perceived loneliness in doctoral students would be negatively related to motivation, satisfaction, productivity and intent to quit. The current study extends previous research by exploring loneliness in doctoral students in a large-scale (N = 1117) quantitative survey to investigate perceptions of loneliness, motivation, engagement, satisfaction, and productivity among doctoral students in STEM fields. Using structural equation modeling (SEM) to test hypotheses, results demonstrated that loneliness was negatively related to satisfaction with program, motivation, and productivity, and explained 49% of the variance of intent to quit. Relationship with advisor mediated the relationship between loneliness and motivation, but not satisfaction with program or productivity. Results may inform graduate programs with students who may be at higher risk of loneliness and/or attrition and may guide future interventions to prevent or reduce loneliness in graduate students.
Open Access
Anaerobic digestion of organic wastes: the impact of operating conditions on hydrolysis efficiency and microbial community composition
(Colorado State University. Libraries, 2012) Griffin, Laura Paige, author; De Long, Susan K., advisor; Sharvelle, Sybil, committee member; Stromberger, Mary, committee member
Anaerobic digestion (AD) is an environmentally sustainable technology to manage organic waste (e.g., food, yard, agricultural, industrial wastes). Economic profitability, however, remains a key barrier to widespread implementation of AD for the conversion of specific feedstocks (e.g., manure, the organic fraction of municipal solid waste (OFMSW), and agricultural residue) to energy. Specifically, high capital and operating costs and reactor instability have continually deterred the use of AD. In order to develop AD systems that are highly efficient and more cost-effective, it is necessary to optimize the microbial activity that mediates the digestion process. Multi-stage AD systems are promising technologies because they allow for separate process optimization of each stage and can enable processing of high-solids content waste. Leachate is recycled through the system, which reduces heating and pumping costs, as well as conserving water. The leachate recycle, however, leads to an increase in ammonia and salinity concentrations. At this time, the impact of reactor conditions (ammonia and salinity concentrations) on hydrolysis is not well understood. As hydrolysis is one rate-limiting step of the process in the conversion of refractory wastes (e.g., lignocellulosic materials), optimization of hydrolysis has the potential to radically improve the economic profitability of AD. The specific objectives of this research were to: 1) determine the effects of operating conditions on hydrolysis efficiency for a variety of solid wastes (manure, food waste, and agricultural residue); 2) determine hydrolysis kinetic parameters as a function of the operating conditions; and 3) identify characteristics of microbial communities that perform well under elevated ammonia and salinity concentrations. To this end, small-scale batch reactors were used to determine hydrolysis efficiency and kinetic rates. Initially, the AD sludge inoculum was exposed directly to the high ammonia and salinity concentrations (1, 2.5, 5 g Total Ammonia Nitrogen (TAN)/L and 3.9, 7.9, 11.8 g sodium/L) as would occur in a reactor treating organic waste with leachate recycle. Results demonstrated a need to acclimate, or adapt, the microorganisms to high concentrations, as methane generation was significantly inhibited with high concentrations. Thus, the organisms were acclimated for two to four months to these testing conditions. The batch studies were repeated, and results demonstrated substantial improvement in hydrolysis efficiency and methane generation. However, although differences in kinetic rates were not statistically significant, general trends in hydrolysis rates suggested that hydrolysis efficiency decreases with increased ammonia and salinity concentrations for a variety of feedstocks (i.e., manure, food waste, agricultural residue). Additionally, results demonstrated that acclimation was necessary to achieve optimal hydrolysis rates. Furthermore, microbial community composition changes in the inocula post-acclimation indicated that reactor inoculation could help improve tolerance to elevated levels of ammonia and salinity to minimize reactor start-up times and improve economic viability.
Open Access
Cheatgrass (Bromus tectorum L.) interactions with arbuscular mycorrhizal fungi in the North American steppe: prevalence and diversity of associations, and divergence from native vegetation
(Colorado State University. Libraries, 2011) Busby, Ryan Ray, author; Paschke, Mark, advisor; Beck, George, committee member; Gebhart, Dick, committee member; Meiman, Paul, committee member; Stromberger, Mary, committee member
To view the abstract, please see the full text of the document.
Open Access
Effect of graywater irrigation on soil quality and fate and transport of surfactants in soil
(Colorado State University. Libraries, 2012) Negahban Azar, Masoud, author; Sharvelle, Sybil, advisor; Carlson, Kenneth, committee member; Stromberger, Mary, committee member; Durnford, Deanna, committee member
While interest in and adoption of graywater reuse for irrigation has rapidly grown in recent years, little is known about the long-term effects of graywater irrigation. Concerns exist in relation to the presence of pathogenic organisms, fate of personal care products, and accumulation of salts. The purpose of this research was to evaluate the long-term effects of graywater irrigation to soil quality. The specific objectives were to evaluate the effects of graywater application on physical and chemical quality of soil, including surfactants, salts and boron accumulation, organic matter leaching and soil hydrodynamic properties in real environment in the field, in controlled environment in the greenhouse and column studies. In addition, fate and transport of surfactants in soil were investigated including how surfactant characteristics impacts mobility in soil of varying types. Graywater irrigation was found to significantly increase sodium in soil at households with graywater systems in place for more than five years; however SAR was not high enough in any of the sampling locations to raise concern about soil quality or plant health. There is a potential for salts, N, and B to leach through soil when graywater is applied for irrigation. A portion of the applied N is assimilated by plants, but leaching of N was observed. Graywater irrigation was also found to significantly increase surfactants in soil. Surfactants mainly accumulated in surface soil (0-15 cm) compared to depth soil. While surfactants have high sorption capacity due to their hydrophobic characteristics, they can be transported through soil if a large amount of water is applied. Among the surfactants measured in this study, AS and AES had the highest mobility. Mobility of surfactants in soil decreased when their number of ethoxylated groups increased. Adding organic matter to the soil increased sorption capacity of soil, as a result, more surfactants retained in the soil columns. Antimicrobials, including triclosan and triclocarban were detected in graywater irrigated areas only in surface soil samples, but not freshwater irrigated areas.
Open Access
Effects of cyanobacterial fertilizers compared to commonly-used organic fertilizers on nitrogen availability, lettuce growth, and nitrogen use efficiency on different soil textures
(Colorado State University. Libraries, 2013) Sukor, Arina, author; Davis, Jessica, advisor; Stonaker, Frank, committee member; Storteboom, Heather, committee member; Stromberger, Mary, committee member
Nitrogen plays a crucial role in synthesis of amino acids and proteins, plant growth, chlorophyll formation, leaf photosynthesis, and yield development of lettuce. Generally, organic farmers use composted manure, legume cover crops, and off-farm fertilizers such as fish emulsion to meet the nitrogen (N) demand of crops. However, the nutrient composition of off-farm fertilizers such as composted manure and fish emulsion varies widely depending on animal species and often have higher transportation costs. Therefore, an evaluation of the application of cyanobacteria in comparison to the commonly-used organic fertilizers was conducted as an alternative potential N biofertilizer. The laboratory soil incubation and greenhouse studies were conducted to evaluate the effect of N availability from potentially mineralizable N on different types of soil textures. Then, a greenhouse study was conducted to assess the effect of N availability from cyanobacterial fertilizers compared to the commonly-used organic fertilizers on lettuce growth, fertilizer recovery and lettuce root response on N use efficiency. Lettuce (Lactuca sativa) is a shallow-rooted crop and requires an extensive amount of N fertilizer to produce yield. The aims of the soil incubation study were to determine the rates of mineralization for different organic fertilizers, influence of soil texture on N mineralization, and to evaluate changes in soil microbial biomass from fertilizer application to sandy and clayey soils. In this study, N mineralization potential of cyanobacterial fertilizers were compared with traditional organic fertilizers in two soils with contrasting textures in a laboratory incubation study at constant temperature (25 degrees C) and moisture content (60% water-filled pore space) for 140 days. Soils were destructively sampled over the course of 140 days and analyzed for NH4+-N, and NO3--N, soil microbial biomass C, soil organic C, and soil total C and N. In both soils, soil NH4+-N was the highest at day 56 and decreased from day 56 to 140 due to its conversion to soil NO3--N. Compost treatment significantly increased soil microbial biomass C (207.5 mg C kg-1 soil) compared to fish emulsion (115.42 mg C kg-1 soil) in sandy soil. The N availability was 9% greater from fish emulsion than liquid cyanobacteria, and 6% greater from solid cyanobacteria than compost in sandy soil. The fish emulsion treatment showed 5% higher N availability compared to the solid and liquid cyanobacterial fertilizers. In the greenhouse study, percentage fertilizer recovery (PFR) was quantified to assess the efficiency of N uptake by lettuce to produce yield. A greenhouse study was conducted for 63 days to evaluate cyanobacterial and traditional organic fertilizers application on lettuce N response. Total leaf area, fresh yield, leaf dry weight, and leaf total N content were measured at the end of the greenhouse study. Total N uptake in lettuce tissue and PFR were calculated based on the analyses results. Soil applied fish emulsion recorded significantly higher fresh yield at 112 kg N ha-1 (147 g) compared to 56 kg N ha-1 (117 g) in clayey soil relative to sandy soil. Soil-applied liquid cyanobacteria recorded significantly higher yield compared to composted manure by 58%. Solid cyanobacteria recorded significantly higher total N uptake at 56 kg N ha-1 compared to 112 kg N ha-1 in clayey soil. In conclusion, soil applied fish emulsion treatment recorded higher PFR (99%) than soil applied composted manure (44%) at 56 kg N ha-1 on clayey soil. Soil applied fish emulsion has significantly higher PFR (57%) compared to the combination soil and foliar fertilizer (FFCom and FLScyb) at 56 kg N ha-1 in sandy soil. Nitrogen is also acquired from the soil by the plant roots. In the greenhouse study, root response to N fertilization was assessed to determine the efficiency of N uptake by lettuce to produce yield. A greenhouse study was conducted for 63 days to evaluate cyanobacterial and traditional organic fertilizers application on lettuce root response. Root: shoot ratio, root dry weight, root surface area, and root length density were measured at the end of the greenhouse study. Nitrogen use efficiency (NUE) was calculated based on the analyses results. There was no significant difference observed in root dry weight. The composted manure (Com) treatment recorded significantly higher root: shoot ratio at 56 kg N ha-1 while foliar and soil applied liquid cyanobacteria (FLScyb) treatment recorded lower root: shoot ratio at 112 kg N ha-1. The foliar applied fish emulsion and soil applied composted manure (FFCom) treatment recorded the highest root surface area compared to other treatments at 112 kg N ha-1 on clayey soil . The FLScyb treatment recorded higher root surface area compared to the Com treatment at 112 kg N ha-1 on sandy soil. The fish emulsion (Fish) treatment recorded higher root length density at 112 kg N ha-1 on clayey soil while FLSCyb recorded higher root length density on sandy soil at 112 kg N ha-1 compared to the Fish and solid cyanobacteria (Scyb) treatments. In conclusion, the Fish treatment recorded 35 % higher NUE at 56 kg N ha-1 on clayey soil while Scyb treatment has significantly 24% higher NUE compared to Com treatment at 56 kg N ha-1 in sandy soil. Overall, the soil applied fish emulsion treatment recorded higher percentage fertilizer recovery and NUE compared to the solid and liquid cyanobacterial fertilizers at 56 kg N ha-1 on clayey soil. However, the combined soil and foliar cyanobacterial fertilizer and soil applied solid and liquid cyanobacterial fertilizers recorded higher percentage fertilizer recovery and NUE at 56 kg N ha-1 compared to the composted manure which correspond to lettuce yield component which was higher in fish emulsion compared to the composted manure.
Open Access
Evaluating soil microbial community assembly to understand plant-soil diversity feedbacks
(Colorado State University. Libraries, 2022) Hoosein, Shabana, author; Paschke, Mark W., advisor; Trivedi, Pankaj, advisor; Stromberger, Mary, committee member; Busby, Ryan R., committee member; Egan, Cameron, committee member
To view the abstract, please see the full text of the document.
Open Access
Experimental restoration treatments for burn pile fire scars in conifer forests of the Front Range, Colorado
(Colorado State University. Libraries, 2014) Shanklin, Amber, author; Paschke, Mark, advisor; Rhoades, Charles, advisor; Stromberger, Mary, committee member
Drastic changes in soil physical, chemical, and biotic properties following slash pile burning and their lasting effects on vegetation cover have been well documented in ecosystems worldwide. However, processes that inhibit burn scar recovery are poorly understood as are the means for their rehabilitation. This study compared plant and soil responses to a number of surface treatments designed to alter microclimate, moisture infiltration, and nutrient status of recently burned slash piles along the Front Range of Colorado. Hand-applied surface manipulation treatments including: scarification, woodchip mulch, and tree branch mulch were compared with untreated burn scars, both with and without addition of a native species seed mix at 19 sites. Pile burning effects were observed by comparing fire scar centers with unburned reference areas while restoration treatment effectiveness was observed by comparing treated scar centers with untreated scar centers. I found surface manipulations had little effect on vegetation recovery while seeding scars increased total plant biomass significantly. Woodchip mulch consistently increased soil moisture, decreased inorganic nitrogen availability, and inhibited plant regrowth in scars. Branch mulch and soil scarification showed no effect on plant regrowth and little effect on soil physical and chemical properties. Non-native species did not have a significant presence within slash scars and were no more prevalent in fire scar centers than reference conditions (unburned areas). Recommendations based upon results of this study include seeding native species in fire scars to promote native species reestablishment.
Open Access
Fate and transport of surfactants in graywater when applied to soil
(Colorado State University. Libraries, 2013) Huang, Zhaohua, author; Sharvelle, Sybil, advisor; Carlson, Kenneth, committee member; Stromberger, Mary, committee member
Graywater reuse for irrigation has been considered an efficient way to reduce demand on water supply. Concerns, however exist regarding the potential impacts that graywater pose to soil quality. In particular, the fate of surfactants, the primary component in personal care and cleaning products, is not well understood. The objective for this study was to gain a better understanding of the adsorption behavior of surfactant onto soils, with particular attention on the effect of the organic matter and soil texture, then provide a suggestion about the kind of surfactants and soil be reused during graywater irrigation. Surfactants linear alkylbenzene sulfonates (LAS) (anionic), alcohol ethoxysulfates (AES) (anionic) and alcohol ethoxylates (AE) (nonionic) were applied to three different soils with varying organic matter (OM) and clay fraction column studies. Adsorption results were obtained from leachate and soil samples. The fraction ranges of leached surfactants to sorbed of LAS, AES and AE were 0.10-0.42, 0.42-2.35, 0.06-0.77 respectively. The results indicated that AES had the most potential leaching capacity, which mean they could reach deeper soil layer even groundwater systems. On the other hand, from soil properties, OM played an important role in the adsorption of surfactants, both anionics and nonionics, whereas, the clay fraction content had a negative effect on anionic surfactants sorption (p=0.006, 0.002 for LAS and AES), possibly due to an increase in negative charge, repulsion forces as clay content increasing, but not significant on nonionic surfactants with clay content increased from 33% - 46% (p=0.986 for AE). Meanwhile, AES homologues which contained different number of ethylene oxide (EO) groups were studied. Results indicated that adsorption increased as EO chain increased. Based on the results above, AE were recommended for graywater irrigation in terms of surfactants with relative high OM.
Open Access
Free water surface and horizontal subsurface flow constructed wetlands: a comparison of performance in treating domestic graywater
(Colorado State University. Libraries, 2012) Hollowed, Margaret Ellen, author; Sharvelle, Sybil E., advisor; Roesner, Larry A., committee member; Stromberger, Mary, committee member
Communities throughout the United States and abroad are seeking innovative approaches to sustaining their freshwater resources. Graywater reuse for non-potable demands is gaining popularity because it allows for the reuse of minimally contaminated wash water, generated and treated on site. Graywater is defined as any wastewater generated at the home or office including wastewater from the laundry, shower, and bathroom sinks but excluding water from the toilets, kitchen sinks, and dishwasher. When compared to other wastewater generated in the home, graywater is contaminated with lower concentrations of organics, solids, nutrients, and pathogens. These characteristics make the water suitable for reuse with negligible treatment when compared to other domestic wastewater sources. Graywater reuse for non-potable demands reduces the demand for treated water and preserves source waters. One method of treating graywater at a community scale for irrigation reuse is constructed wetlands. Despite widespread interest in this innovative approach, limited guidance is available on the design and operation of constructed wetlands specific to graywater treatment. The foremost objective of this research was to compare the performance of a free water surface constructed wetland (FWS) to a horizontal subsurface constructed wetland (SF) for graywater treatment and to assess their ability to meet water quality standards for surface discharge and reuse. This was done by comparison of percent (%) mass removal rates and requisite surface areas (SA) based on determined removal rates ( k ). Aerial loading rates were compared to EPA suggested aerial loading rates in an attempt to provide recommendations for target effluent concentrations. Determining contaminant removal rates is important for creating wetland design standards for graywater treatment and reuse. Contaminant removal rates were evaluated over the summer and fall of 2010 and 2011 for a SF wetland. These removal rates were compared to the removal rates evaluated over a two year period (2008-2010) for a FWS wetland. Another objective was to determine the % mass removal of three common anionic surfactants in constructed wetlands (both FWS and SF) and finally, the possibility of incorporating constructed wetlands into greenhouse community garden centers as an option to reduce the losses resulting from evapotranspiration (ET) in arid climates was explored briefly. The results indicate that SF wetlands provide relatively stable and more efficient treatment year round when compared to FWS wetlands. In particular, the SF wetland showed statistically significant higher mass removal of both biological oxygen demand (BOD5 ) and total nitrogen (TN) than the FWS wetland during winter months (P=0.1 and 0.005; α=0.1). When all the seasons were compared for each wetland individually there was a statistically significant degree of removal for BOD5 and TN between the seasons in the FWS wetland (P=0.09 and 0.04; α=0.1) while there was none in the SF wetland (P=1.0 and 0.9; α=0.1). These results are consistent with other findings in the literature. When mass removals were compared to HLRs, the trends support the ability of SF wetlands to function across a wide range of HLRs and climatic conditions, whereas FWS wetlands are less capable of performing well under less than ideal conditions. Results of the k-C* and SA analyses, though limited in their completeness, suggest once again that SF wetlands are capable of increased rates of removal not only during the warm summer months but also during the winter and transition months. Specifically, nitrification and denitrification processes may be contributing to TN removal in the SF wetland, particularly during senescent periods. Surfactant removal was also consistent with findings in the literature, with 50% removal of LAS and greater than 70% removal of AES/AS, suggesting that LAS is more persistent.
Open Access
Geospatial analysis of water and nutrient transport in two northern Colorado mixed-landuse watersheds
(Colorado State University. Libraries, 2011) Cowley, Cortney A., author; Arabi, Mazdak, advisor; Carlson, Ken, advisor; Bledsoe, Brian, committee member; Stromberger, Mary, committee member
This study examines the effect of different sources, transport pathways, and hydrologic regimes on phosphorus concentrations along a pristine-urban-agricultural gradient. A total of 48 sampling locations were monitored to characterize total phosphorus concentrations in the Cache la Poudre River Watershed in Northern Colorado. The comprehensive design of sampling locations aimed to capture the influence of anthropogenic activities and geospatial heterogeneity. Samples were collected at seven points in time with distinct climatic and hydrologic characteristics from April 2010 to February 2011. A geographic information system (GIS) was used to measure the overland, irrigation ditch, and stream/river distances from the sources to sampling locations. Analysis of variance, non-linear regression, and multiple linear regression models were used in combination to explore the co-variation of phosphorus concentrations with capacities of upstream WWTPs and CAFOs, along with other geospatial factors. It was evident, under all hydrologic conditions, that phosphorus concentrations downstream from WWTPs were significantly higher than the concentrations upstream of the facilities. Transport from WWTPs governed phosphorus concentrations in surface water during dry and low flow conditions, whereas contribution of CAFOs was significant during rainfall events. The total flow distance (a function of overland, irrigation ditch, and stream/river distances) from CAFOs to the sample locations was strongly associated with phosphorus concentrations during precipitation events. The results of this study provide the foundation for creating a decision support system for water quality analysis, monitoring, and management in the Poudre River basin and other similar mixed-land use watersheds. After examining the Poudre River watershed, a thorough investigation of Boxelder Creek basin was executed. The objectives were to gain an understanding of the geospatial heterogeneity and hydrologic complexity of the watershed using available data, aerial photography, and ground truthing and to develop a model that could accurately simulate the hydrology and nutrient routing in the watershed. Modeling the system using a simplified method for irrigation produced simulated results that were inconsistent with observed flow measurements. These results seem to indicate that irrigation ditches play a vital role in the hydrologic cycle of the basin. Previous studies indicate that watersheds in the study region can be accurately modeled; and although stream flow was not adequately simulated, the model did perform better when estimating total phosphorus concentrations. Therefore, future studies attempting to model basins containing irrigation ditches, like Boxelder Creek basin, should incorporate methods for representing the channels and their various interactions with the natural system. Routing irrigation canals through the watershed, along with irrigation and manure application methods described in this study, should improve the feasibility of modeling the heterogeneity of mixed landuse watersheds.
Open Access
Increasing bud cold hardiness through foliar application of abscisic acid and urea on four cultivars of V. vinifera in western Colorado
(Colorado State University. Libraries, 2015) Kearney, Anne A., author; Caspari, Horst, advisor; Menke, Stephen, advisor; Stromberger, Mary, committee member; Wallner, Stephen, committee member
There is a lack of economic sustainability on the increasingly popular cold-sensitive Vitis vinifera cultivars due to cold damage, resulting in very low crop yields. Recent research to improve the cold hardiness of these cultivars to keep up with demands has shown that cold hardiness of grapevine buds can be increased through foliar applications of abscisic acid (ABA) and urea. Therefore, five different ABA treatments at 400 mg L⁻¹ each and one treatment of 40 g L⁻¹ urea were evaluated on Chardonnay and Syrah vines growing at the Western Colorado Research Center in Grand Junction, CO. The treatments were: veraison (V) which was applied at 50-75% veraison, 20 days post-veraison (V20), 40 days post-veraison (V40), double treatments at veraison plus 20 or 40 days post-veraison (V + V20 and V + V40, respectively) and a late season urea treatment. The treatments were evaluated against a control of 0.05% surfactant and water in Chardonnay and Syrah grapes. V and V20 were also evaluated against a control in Merlot and Cabernet Franc. Compound dormant buds were sampled monthly and primary bud survival was assessed. In the early part of the acclimation process, V, V20, V40, and V + V20 treatments showed significant improvements in bud cold hardiness Chardonnay and Syrah, as well as in Cabernet Franc. Bud cold hardiness was unaffected across all treatments in Merlot early on all the way past mid-winter. While no significant difference was detected after October between treatments and bud survivability compared to the control in all varieties, a few treatments showed positive significant differences from month to month. Yield and basic fruit components were not affected. However, anthocyanin accumulation was significantly greatest in the V20 group for Cabernet Franc. Foliar applications of abscisic acid show potential as future cold hardiness methods and should be evaluated further over several growing seasons for potential prolonged increases in bud cold hardiness.
Open Access
Interplay between selenium hyperaccumulator plants and their microbiome
(Colorado State University. Libraries, 2016) Cochran, Alyssa T., author; Pilon-Smits, Elizabeth, advisor; Leach, Jan, committee member; von Fischer, Joseph, committee member; Stromberger, Mary, committee member
The plant microbiome includes all microorganisms that occur on the plant root (rhizosphere) and shoot (phyllosphere) or inside plants (endosphere). Many of these microbes benefit their host by promoting growth, helping acquire nutrients or by alleviating biotic or abiotic stress. In addition to its intellectual merit, better knowledge of plant-microbiome interactions is important for agriculture and medicine. Microbiome studies are gaining popularity in multiple research areas, particularly due to advances in next generation sequencing, which has advantages over cultivable methods by revealing the complete microbial community. Still relatively little is known about the microbiomes of plants with extreme properties, including plants that hyperaccumulate (HA) toxic elements such as selenium (Se). Selenium HAs may contain up to 1.5% of their dry weight in Se, which can cause toxicity to herbivores and pathogens as well as neighboring plants. Many advances are yet to be made with regard to the interaction of Se and the plant microbiome: does plant Se affect microbial diversity and composition, and do plant-associated microbes affect plant Se accumulation? The first chapter of this thesis will discuss aspects of the plant microbiome as well as the discoveries to date with regard to plant-associated microbes and Se, mostly explored through culture-dependent methods. Selenium HA appear to harbor equally diverse endophytic microbial communities as non-hyperaccumulators. Thus, plant Se does not impair associations with microbes. A variety of microbes have been isolated from plants or soil in seleniferous areas, including some bacteria and fungi with extreme Se tolerance. Inoculation of plants with individual strains or consortia of microbes was able to promote plant growth, Se uptake and/or Se volatilization. Thus, microbes may facilitate their host’s fitness in seleniferous areas. Exploiting and optimizing plant-microbe associations may facilitate applications like phytoremediation (bio-based environmental cleanup) or biofortification (nutritionally fortified crops). Plant-derived microbial isolates may also be applicable without their plant host, e.g. for cleanup of wastewaters. Culture-dependent studies have dominated the plant-microbe interactions research in regards to hyperaccumulators thus far, painting an elaborate but incomplete picture. In the second chapter of this thesis, we use a mix of culture based and culture-independent methods to investigate the bacterial rhizobiome of selenium Se HAs. Using 16S rRNA Illumina sequencing, we show that the rhizobiomes of Se HAs are significantly different from non-accumulators from the same naturally seleniferous site, with a higher occurrence of Pedobacter and Deviosa surrounding HAs. In addition, we found that HAs harbor a higher species richness when compared to non-accumulators on the same seleniferous site. Thus, hyperaccumulation does not appear to negatively affect rhizobiome diversity, and may select for certain bacterial taxa in the rhizobiome. The bacterial isolates, independent from site or host plant species were in general extremely resistant to toxic concentrations of Se (up to 200mM selenate or selenite) and could reduce selenite to elemental Se. Thus, microbial Se resistance may be widespread and not be under selection by Se HAs. In future studies it will be interesting to further investigate the mechanisms by which Se HA species similarly shape their rhizobiome; this is perhaps due to Se-related root exudates. Future studies may also focus on elucidating the effects of microbes on plant Se accumulation and tolerance.
Open Access
Irrigation effects on growth and visual quality of three ornamental grass species
(Colorado State University. Libraries, 2014) Rozum, Jane, author; Klett, Jim, advisor; Koski, Tony, committee member; Stromberger, Mary, committee member
Ornamental grasses have become ubiquitous in the landscape and are popular with consumers and industry professionals because of their favorable low-input cultural characteristics. These characteristics include low water and nutrient requirements, decreased maintenance, fast growth and few disease and insect problems. A study conducted at Colorado State University (Fort Collins, CO) examined the effects of four irrigation levels (0, 25, 50 and 100% of potential evapotranspiration (ET)) on growth and visual quality of three species of ornamental grasses (Panicum virgatum 'Rotstrahlbusch' (Rotstrahlbusch Switchgrass), Schizachyrium scoparium 'Blaze' (Blaze Little Bluestem) and Calamagrostis brachytricha (Korean Feather Reed grass)). Averaged across species, maximum plant height and width was observed at the 25% irrigation level. We found that plant dry weight increased as irrigation level increased from 0 to 50% of ET, but there was a decrease in total plant dry weight at 100% of ET. This indicates that watering these species of ornamental grasses at 100% ET may be detrimental to growth and plant quality. The greatest drought stress, as measured by leaf water potential, was found with the mesic species C. brachytricha. Averaged across species, leaf water potential was most negative (greatest drought stress) at 0% of ET and the least amount of stress was observed at 50 and 100% of ET. At the conclusion of the study, visual ratings of plant form, floral impact and landscape impact were highest at the 25% of ET irrigation level. Our research with these three species in Colorado suggests that irrigation at 25% of ET produces the healthiest plants, with greater height, width, dry weight and visual impact in the landscape. This agrees with anecdotal observations that ornamental grasses will perform better in a landscape with limited irrigation and other inputs.
Open Access
Microbial responses to plant functional types and historical resources additions in the shortgrass steppe
(Colorado State University. Libraries, 2009) Bontti, Eliana E., author; Burke, Ingrid C., advisor; Lauenroth, William K., committee member; Stromberger, Mary, committee member; von Fischer, Joseph, committee member
Nutrient addition in rangelands is an appealing way to increase plant biomass and quality, but little is known about the long-term effects of these additions on soil microbial activity and nutrient cycling. In addition, microbial activity may be affected by plant functional types (PFT) through influence on the levels of inorganic nitrogen (N) and labile carbon in the rhizosphere. This is particularly important in the shortgrass steppe (SGS), where plants with the C3 or C4 photosynthetic pathway differ in phenology, which affects the timing of maximum N uptake and root exudate production. To understand the effect of PFT (C3 and C4 species) and historical nutrient additions on temporal patterns of N partitioning between microbes and plants, I estimated seasonal trends in plant biomass and N content, microbial N) and soil N availability. In addition, I evaluated monthly emissions of the greenhouse gases C02 and N20, discriminating between fungal and bacterial production through incubations of soils under the influence of different PFTs and historical N additions. Last, I tested the effect of biosolid application on C02 and N20 emissions from fungi and bacteria in SGS soils. Seasonal trends in plant and microbial N concentration indicated that the two were synchronous during most of the plant growing season and both strongly influenced by precipitation. Plant functional type did not explain differences in microbial N and available soil N, but historical N amendments increased plant N content, decreased microbial N, and had no detectable effect on soil available N. Fungi showed higher emissions of C02 and N20 compared to bacteria in the SGS, whereas there was no difference in emissions between the two groups in the historically N amended plots. There were no effects of PFT on bacterial and fungal emissions of C02 and N20 but high historical N fertilization resulted in increased C02 and N20 emissions from bacteria. Fungal emissions of C02 were higher than bacterial emissions in SGS sites compared to biosolid amended sites, but I detected no differences between microbial groups in N20 emissions. C02 and N20 emissions were higher in biosolid treated sites than non-treated SGS sites even 20 years after amendments ceased. Biosolid treated sites dominated by forbs showed higher C02 emissions compared to sites dominated by C3 grasses, while C3-dominated sites with high available inorganic N had higher N20 emissions than C4-dominated sites. In summary, historical N additions had long lasting effects on SGS by increasing plant biomass and N. Given that N additions to ecosystems are increasing worldwide, it may be important to evaluate the impacts of these changes in processes on ecosystems services that grasslands provide. My results suggest that high levels of nutrient additions have unintended consequences such us increased C02 and N20 emissions, and in particular carbon additions through biosolids increase fungal activity, which is also conducive to N20 production. These additions have a profound impact, since the elevated greenhouse gas emissions and changes in microbial communities last at least 20 years after the amendment was carried out.
Open Access
Molecular ecology of Listeria spp., Salmonella, Escherichia coli O157:H7, and non-O157 Shiga toxin producing E. coli in northern Colorado wilderness areas
(Colorado State University. Libraries, 2011) Ahlstrom, Christina, author; Nightingale, Kendra Kerr, 1977-, advisor; Goodridge, Lawrence, committee member; Stromberger, Mary, committee member
Foodborne disease is a substantial concern in the United States and receives a great deal of attention from the government, industry, and the media. Government initiatives have alleviated some of the burden; however, without improved knowledge of the molecular epidemiology of the pathogens in a variety of environments, a comprehensive understanding of foodborne disease will remain out of reach. Listeria monocytogenes, Salmonella, Escherichia coli O157:H7 and non-O157 Shiga toxin producing E. coli (STEC) play a prominent role in the incidence of bacterial foodborne illness in the United States. Molecular subtyping methods are used extensively in foodborne disease surveillance, yet there is a knowledge gap regarding the presence, transmission and molecular ecology of these pathogens in non-food associated environments. We collected foodborne pathogen isolates from pristine wilderness environments to obtain subtyping data that may aid in the interpretation of clinical and food isolates particularly during outbreak investigations. Furthermore, the identification of subtypes present in different environments but not commonly linked to human disease may provide key information regarding the evolution of virulence in these organisms. To achieve these goals, five wilderness locations in Colorado were selected to represent pristine locations and three areas (approximately 100m2) within each location were designated; each area was sampled once during the spring, summer, and fall seasons in 2009 and 2010. A total of 450 soil, 450 water, 90 drag swab (surface soil) and 276 fecal samples were collected. Five soil samples and five water samples from each area were composited and all samples were microbiologically analyzed to detect Listeria spp. (i.e., L. monocytogenes and other Listeria spp.), Salmonella, E. coli O157:H7, and non-O157 STEC. After non-selective pre-enrichment, samples were divided and microbiologically analyzed to detect each target organism using modified versions of the United States Food and Drug Administration Bacteriological Analytical Manual. Up to four presumptive colonies for each target organism from each sample were confirmed by PCR to detect gene fragments specific to each respective organism. Overall, three samples tested positive for L. monocytogenes, including two fecal samples and one water sample. Nineteen samples contained Listeria spp. other than L. monocytogenes, 14 of which were determined to be Listeria welshimeri by sigB sequencing. The remaining five Listeria spp. were presumptively identified as Listeria rocourtiae sp. nov. by 16s rDNA sequencing; however, these isolates demonstrated notably different biochemical properties than L. rocourtiae. Salmonella was found in two samples, including one water and one fecal sample, and five non-O157 STEC were found in one fecal, one sediment, and three water samples. E. coli O157:H7 was not detected in the natural environments in Northern Colorado surveyed in this study. A molecular serotyping PCR assay revealed two L. monocytogenes isolates belonging to the 1/2 b serogroup and two isolates belonging to the 1/2a serogroup, with a single fecal sample containing two different L. monocytogenes serogroups (1/2a and 1/2b). Pulsed field gel electrophoresis typing results indicated three unique DNA fingerprints among the L. monocytogenes isolates. Two unique strains were isolated from a single fecal sample; with one strain isolated from two different fecal samples collected from the same area. All five non-O157 STEC- and both Salmonella -positive samples had unique PFGE fingerprints. All subtyping data has been deposited in PathogenTracker (www.pathogentracker.com), a publicly available WWW database. In conclusion, results from this study demonstrate a rare presence of foodborne pathogens in pristine environments and the utility of molecular subtyping from distinct environments. Further characterization of the foodborne pathogen isolates obtained from non-food associated environments in this study will expand our knowledge on the molecular ecology of foodborne pathogens in nature.
Open Access
Nitrate removal from groundwater using a reactive stream stabilization structure
(Colorado State University. Libraries, 2010) Mitchell, Christina M., author; Carlson, Kenneth, advisor; Watson, Chester, committee member; Stromberger, Mary, committee member
Riparian zones that remove nitrate (NO3-) from groundwater play a significant role in protecting and improving the quality of receiving surface waters. Denitrification, the microbial conversion of NO3 to gaseous forms of nitrogen (N ) is an important removal mechanism in these systems. For this process to occur there must be a supply of organic carbon (C ). High levels of organic C may be found in the subsurface of relatively undisturbed riparian zones. However, in areas where streambank erosion has resulted in the loss of riparian vegetation (C source) and organic-rich sediments, the amount of C available for denitrification is likely to be low. Vegetation may become established in these areas soon after the banks are stabilized using standard structural and/or bioengineering techniques. However, it will take time for organic C to accumulate in the soil. Thus, significant NO3 removal via denitrification will not be immediately observed following the completion of bank stabilization work. This study examined the potential for improving existing streambank stabilization designs to accelerate and maximize groundwater NO3 removal benefits. A simple, cost-effective structure, called the reactive stream stabilization (R S 2) structure, was designed for the purpose of this study. The RS2 structure combines a permeable reactive barrier composed of solid-phase organic C (sawdust) with a common bank stabilization technique (longitudinal peaked stone toe protection). A small field-scale RS2 structure and a control (no organic C amendment) were constructed along a stream in July 2003. The two systems were monitored from August to December 2003 and from M ay to September 2004. During the initial monitoring period, N O 3 removal in the reactive barrier averaged 93% (7.27 mg N L-1 along the upslope edge, versus 0.48 mg N L"' along the downslope edge). In comparison, NO3 removal in the control averaged 30% (12.3 mg N L-1 along the upslope edge, versus 8.65 mg N L-1 along the downslope edge). It was not possible to measure NO3 removal in the control the following spring and summer because the artificially generated plume of NO3 was not intercepted by the monitoring wells in the system. The plume was, however, intercepted by the wells located in the reactive barrier. Nitrate loss in the reactive barrier was high and averaged 97% (17.9 mg N L-1 along the upslope edge, versus 0.51 mg N L-1 along the downslope edge) during this period. The results of this study suggest that RS2 structures can enhance groundwater NO3 removal along streams. Additional field testing needs to be completed to verify these results, but it appears that the RS2 structure could be an effective tool for reducing NO3 loading to waterways.
Open Access
Nitrogen mineralization from biofertlizer Azolla mexicana compared to traditional organic fertilizers
(Colorado State University. Libraries, 2018) Jama, Aisha, author; Davis, Jessica G., advisor; Stromberger, Mary, committee member; Uchanski, Mark, committee member
Organic agriculture has become an essential approach to meeting the growing global food production demand and long-term soil sustainability, as well as addressing environmental problems connected with the use of synthetic agrochemicals. As a result, biofertilizers (biological fertilizers) have become promising resources to meet the growing demands for healthy and safe food production. Biofertlizers supply nutrients and take advantage of microorganisms that contribute to sustainable practices. One such biofertilizer is the aquatic pteridophyte Azolla mexicana which can be found in both tropical and temperate climates. Azolla can multiply rapidly, ensuring year-round biomass, and has also been found to have fast and high rate of N fixation. Azolla strains have been successfully exploited as effective biofertilizers in Asia but strains native to the Great Plains have not. There is no literature that assesses N mineralization (Nmin) rates of Azolla mexicana compared to other organic fertilizers in Colorado soils. A laboratory soil incubation was conducted to determine the rates of Nmin, N availability and total C and N of Azolla mexicana applied to soils compared to commonly-used organic fertilizers. Then, a greenhouse study was conducted to assess the organic fertilizer and urea treatment responses on kale growth and yield, leaf and petiole N percentage, total N uptake and percentage N recovery. In this study, we hypothesized that Azolla biofertilizer application will enhance soil inorganic nitrogen (soil ammonium-N and nitrate-N) concentrations and that soil amended with Azolla will also enhance vegetable plant growth parameters (plant height, leaf and petiole N percentage, total N uptake and percentage N recovery). In the incubation study, soil NH4+-N for all treatments tended to increase until day 56 where they all peaked then subsequently decreased until the end of the incubation period. Compost treatment recorded higher initial soil NH4+-N while Azolla + Watanabe treatment recorded higher soil NH4+-N concentration towards the end of the study. The soil NO-3-N concentrations in all treatments increased throughout the 140-day study. The Azolla + Watanabe treatment showed highest average soil NO-3-N concentration at day 140 while the Control treatment had the lowest soil NO-3-N concentration throughout the experiment. The decline in soil NH4+-N concentration formed during ammonification was followed by an increase in soil NO-3-N concentration because of nitrification. In the greenhouse study, Azolla + Watanabe treatments had taller kale, significantly higher leaf fresh weight as well as significantly higher leaf dry weight. Both Azolla + Watanabe and Urea treatments recorded significantly higher yields compared to the other treatments. The Azolla + Watanabe and both Cyano treatments recorded significantly higher root dry weights compared other treatments. Control treatment had significantly higher root to shoot ratio. There were no significant differences in leaf N (%) among Azolla + Watanabe, Azolla, Cyano and Cyano + Moringa treatments. Azolla + Watanabe treatment also had significantly higher total N uptake among the organic fertilizers but was not significantly different from Azolla. Urea treatment recorded significantly higher N recovery and showed a similar pattern as the total N uptake whereby Azolla + Watanabe had significantly higher N recovery.
Open Access
Nutrient limitation of microbial decomposition in Arctic tussock tundra soil
(Colorado State University. Libraries, 2013) Melle, Caroline, author; Wallenstein, Matthew, advisor; von Fischer, Joseph, committee member; Stromberger, Mary, committee member; Steltzer, Heidi, committee member
Cold, wet conditions limit microbial activity in many parts of the Arctic tundra, resulting in slow decomposition of soil organic matter, low nitrogen (N) mineralization rates and the accumulation of massive amounts of soil organic carbon (SOC). Climate change is currently reducing these physical environmental constraints, allowing for Arctic SOC to become vulnerable to decomposition. However, historically low decomposition rates due to climatic inhibition have resulted in soils with extremely poor nutrient availability in the active soil layer for much of the year further inhibiting ecosystem productivity and limiting microbial decomposition. N limitation of both primary productivity and microbial activity, in addition to extremely low soil N availability throughout much of the active season, make many Arctic tundra ecosystems among the most N limited in the world. Changing climatic conditions can potentially allow for increased annual N mineralization resulting in greater soil N availability. Enduring increases in soil N availability would alter microbial driven biogeochemical cycles with cascading long-term effects on Arctic tundra ecosystems. Despite previous experimental findings of N limitation of microbial decomposition in Arctic tundra, seasonal variability in soil N availability in conjunction with the influences of other soil factors indicate that N may not be the primary control of microbial activity in these soils during the entirety of the Arctic active season. The tight coupling of biogeochemical cycles suggests that labile carbon (C) may be co-limiting for portions of the active season when there is greater soil N available. Furthermore, most observations of N stimulation of microbial activities have originated from relatively few research sites due to the inaccessibility of much of the Arctic, but N limitation of decomposition may be site dependent and vary across small geographic areas. Questions of inter-annual and intersite variability of soil microbial activities within a singular Arctic soil type have never previously been directly addressed. I conducted laboratory soil incubations to examine intra-seasonal and annual variability of soil microbial N limitation, the potential for co-limitation of labile C and N, and the extent of intersite variability in microbial N limitation across two comparable moist acidic tundra (MAT) sites within close proximity and of similar topography, climate and vegetation. I found, contrary to previous studies and my hypotheses, that soil microbial biomass growth, C mineralization, and extracellular enzyme activities were not consistently stimulated by N additions, but rather found that N was primarily immobilized in microbial biomass. Stimulation of C mineralization by N addition was short-lived and variable across the course of a single active season. Additionally, there was significant variation in microbial responses to nutrient amendments and temperature across the two consecutive study years; differences in temperature sensitivities of C mineralization and conflicting effects of N amendment on enzyme activities were seen between study years. Intersite variability was also significant; despite the close physical proximity and similar topography, climate, and vegetation of the sample sites investigated, they differed markedly in their responses to N additions as well indications of labile C co-limitation. The uniquely uniform properties of MAT tussock soils may lead to the presumption of homogeneity of soil microbial activities. However, I found that the significance of microbial N limitation and occurrence of co-limitation by labile C were dependent on the soil sampling site even though soil properties were consistent across sites. These findings of extensive variability and labile C co-limitation within some MAT tussock soils elucidate some of the current knowledge gaps in Arctic microbial ecology and suggest that the current paradigm of Arctic N limitation as one of the primary active season controls on ecosystem activity needs to be expanded and further refined to better predict the fate of the large amounts of C currently sequestered in Arctic tundra soils.