Animal manures are land applied in agronomic systems to supply essential crop nutrients and decrease dependency on chemical fertilizers. Liquid manures are traditionally surface broadcast to fields and sometimes incorporated to reduce odor and nutrient losses; however, incorporation is incompatible with no-till agriculture. Subsurface manure injection is a no-till compatible alternative application method which addresses these concerns, but likely changes the dynamics of nutrient cycling. Comparison of the two application methods has yielded mixed results and warrants further research. Therefore, the objectives of this research were to contrast the surface broadcast and subsurface injection of dairy slurry on nitrogen and carbon cycling, crop yield, and biologic responses to proxy soil health. In a forced air-flow laboratory incubation, manure injection reduced ammonia volatilization by 87% and 98% in a sandy loam and clay loam soil, respectively. The increased ammoniacal nitrogen recovery resulted in increases of soil nitrate of 13% for the sandy loam and 26% for the clay loam after 40 days of incubation. Microbial measurements were inconclusive in the laboratory. In 7 site-years of field study, soil nitrate was greater in 7 of 25 measurements under manure injection and 30% higher under injection on average during the corn pre side-dress nitrate test (PSNT) time. Soil nitrate sampling methods were assessed for fields injected with manure; a standard random sampling method had a coefficient of variation (C.V.) of 28% and was as equally repeatable as utilizing an equi-spaced distribution of cores taken across an injection band, C.V. of 30%. Both biological responses, carbon mineralization (C-min) and substrate induced respiration (SIR), were not different between application methods; both were highly variable and C-min was especially intensive logistically. Corn yield showed no consistent response to application method, but probably was not nitrogen limited. In 2 years of field study conducted on a university research farm injection resulted in greater 0-15cm soil nitrate levels than surface broadcast 1 week after application and persisted for 9 additional weeks. In injected plots, nitrate was concentrated in the injection band; nitrate movement was significant only 10cm lateral to the injection band but overall distribution fit well to a second degree polynomial, especially 2 and 4 weeks after application, R2>0.80. Evidence of leaching was observed in one year after receiving considerable rainfall in weeks 1 and 2 after application. When corn grain yield was averaged year over year, injection was 26% greater than the no- manure control, and 15% greater than surface application. Both biological metrics, C-min and microbial biomass, were stratified by depth; C-min was concentrated within the manure band leading to greater mineralization under injected applications. Microbial biomass was significantly higher under injection at the 15-30cm depth. Overall biological response to manure application method was inconclusive, however manure injection is superior to surface application in terms of nitrogen recovery. / Doctor of Philosophy / Animal manures supply nutrients essential to crop growth (notably nitrogen and phosphorous); liquid manures (pigs and dairy cattle) are commonly applied by spraying them on soils before tillage. Where no-tillage is used as a conservation measure subsurface injection can be used as an alternative to leaving manure on the soil surface. The purpose of this research was to assess nutrient cycling, crop yield, and soil health impacts of surface applied and injected dairy manure applications. Manure injection greatly reduces a nitrogen loss pathway, and as a result supplies more plant available nitrogen to the crop. Methods of soil sampling fields using injection were compared and a recommended sampling method was defined. Transport of a form of nitrogen vulnerable to movement in the ground was found to only travel 10cm away from where manure was injected. Transport of this form of nitrogen below the injection area was observed after abundant rainfall. Crop yields were sometimes higher under injection however, yields are also determined by factors other than nitrogen. Soil health was not repeatably improved under one application method, but microbial activity was greater at shallower soil depths.

This work aimed to contribute to the understanding of tree - grass inter-cropping interactions so that the productivity and sustainability of extensive livestock husbandry can be increased. The work was carried out in the context of a small farm in Oaxaca, Mexico, where increases in productivity are limited by shortage of capital and where the tree component would be used as green manure. It is difficult to investigate the effectiveness of such a system by only using conventional field trials. I constructed a mathematical model to simulate how the main components of the system function under conditions that would not be evaluated in the field. Issues such as how many trees to plant and what tree species combine with grass cattle and environment, can be answered with the model. The particular features of the model are: 1) It describes an agro-ecosystem where trees perform several biological functions like nitrogen capture for use in the silvopastoral system, 2) It links grass and trees with the animal and 3) Nutrient availability depends mainly on soil organic matter decomposition and mineralisation rather than on external inputs. The present research consisted of 1) constructing the model prototype using data from the literature, 2) conducting field experiments to investigate the actual performance of the silvopastoral system, 3) perform laboratory research and greenhouse experiments complementarily to the field experiments and 4) elaborate on the carbon and nitrogen balance of the silvopastoral experiment, by combining research results and the mathematical model. The field experiment consisted of an array of 13 plots with one of the tree species Gliricidia sepium, Leucaena leucocephala, Delonix regia and Lysiloma auritum in a gradient of plant densities within a Brachiaria decumbens paddock. Results showed that the presence of trees in pastures is potentially useful for retaining nitrogen and carbon that would be lost in the grass mono-crop. Trees did not incorporate nitrogen through biological fixation, perhaps because the lack of adequate nodulation and they did not established their rooting systems to a depth beyond the grass roots (> 1.20m) so as to recover leached nutrients. However, trees produced mulch that was rich in nitrogen (3.8%) and whose decomposition rate ensures a slow release to prevent leaching. At the plant density used, the tree population caused no harm to grass as to production and nutritive value. Further increments in tree density in order to improve the potential for nitrogen capture should be evaluated in terms of the reduction of grass production. Several biological attributes of the species were determined, in some cases for the first time: biomass productivity, specific leaf area, nutritive value, phenolic content, root biomass, grass root longevity, root vertical distribution, etc. Such characterisation is useful for the understanding of the system inter-cropping and specially for the parameterisation of the silvopastoral model. Even though the mixtures proved able to survive for the span of the experiment, the sustainability of tree - grass inter-cropping as to the stabilisation of soil fertility requires longer monitoring. Other limiting factors such as phosphorus availability and the management of grazing systems have to be incorporated for an adequate evaluation of the silvopastoral system.

Cheatgrass is an invasive weed that has come to dominate large areas of the western United States. Once an ecosystem has been converted to a cheatgrass monoculture, it is extremely difficult to restore native vegetation. Cheatgrass negatively impacts wildlife and increases wildfire frequency and intensity. Understanding how cheatgrass so effectively invades western ecosystems is essential to turning the tide of invasion. One possible key to cheatgrass’ success is alteration of soil nutrient cycling. The goal of this study is to explore how nitrogen (N) may accumulate in cheatgrass soils via redistribution of N within soil N pools. To accomplish this we investigated soil N cycling in soils underneath cheatgrass and crested wheatgrass. We used a 15N isotope tracer to determine the contribution of root exudates to soil N pools. During the 1-week 15N tracer experiment, cheatgrass roots exuded more than twice as much N (0.11 mg N kg-1 soil d-1) as crested wheatgrass roots (0.05 mg N kg-1 soil d-1). We propose that exudation of high N content root exudates leads to the changes in soil N pool size and transformation rates commonly observed in soils under cheatgrass. This research uses a simple and relatively inexpensive isotope tracer to shed light on mechanisms by which invasive plants may alter soil processes. By understanding these mechanisms we may be able to develop strategies for better managing cheatgrass invasion.

Nitrous oxide (N₂O) is a powerful greenhouse gas, and its atmospheric concentration is increasing dramatically. N₂O is produced through the microbially-mediated processes of nitrification and denitrification. Since these processes have different substrates and isotopic enrichment factors, stable isotope analysis (δ¹⁵N and δ¹⁸O) of N₂O can be used to study the production of this important greenhouse gas. Although production in rivers accounts for a significant portion of the global N₂O budget, the isotopic composition of N₂O from this source is poorly characterized. Most of the previous work using stable isotopes of N₂O has been conducted in terrestrial or oceanic environments, and only one published study has measured δ¹⁵N and δ¹⁸O of N₂O produced in a riverine environment. The purpose of this research project was to use stable isotope analysis to characterize the processes responsible for N₂O production in the Grand River, Ontario, Canada, and to determine the spatial and temporal variability of the isotopic composition of the N₂O flux. To meet the study objectives, an offline “purge and trap” method was developed to collect and purify dissolved N₂O for stable isotope analysis. Using this method, δ¹⁵N and δ¹⁸O analysis of dissolved N₂O is possible for samples with concentrations as low as 6 nmol N₂O/L. Due to the isotopic effects of gas exchange and the back flux of tropospheric N₂O, there is a complex relationship between the δ¹⁵N and the δ¹⁸O of source, dissolved, and emitted N₂O in aquatic environments. A simple box model (SIDNO – Stable Isotopes of Dissolved Nitrous Oxide) was developed to properly interpret isotopic data for dissolved N₂O. Using this model, it was determined that the isotopic composition of emitted N₂O is much more representative of N₂O production in aquatic environments than the isotopic composition of dissolved N₂O. If the concentration, δ¹⁵N and δ¹⁸O of dissolved N₂O are measured, the magnitude and isotopic composition of the N₂O flux can be calculated. Sampling downstream of the major wastewater treatment plants (WWTPs) on the Grand River indicates that nitrification and denitrification in the river are strongly tied to diel changes in dissolved oxygen (DO) concentration. During the day, when DO concentrations are high, nitrification or nitrifier-denitrification is the dominant N₂O production pathway, with sediment denitrification also contributing to N₂O production. At night, when DO concentrations are low, denitrification in the sediments and at the sediment / water interface is the dominant production pathway. Using the SIDNO model, N₂O produced during the day was found to have a δ¹⁵N of -22‰ and a δ¹⁸O of 43‰. N₂O produced at night had a δ¹⁵N of -30‰ and a δ¹⁸O of 30‰. The isotopic composition of N₂O emitted from the Grand River is dominated by night-time production downstream of the Waterloo and Kitchener WWTPs during the summer. The flux and time weighted annual average isotopic composition of N₂O emitted from the Grand River is -18.5‰ and 32.7‰ for δ¹⁵N and δ¹⁸O respectively. These values are significantly more depleted than the only other published data for riverine N₂O production. If the Grand River is representative of global riverine N₂O production, these results will have significant implications for the global isotopic budget for atmospheric N₂O.

Nitrous oxide (N₂O) is a powerful greenhouse gas, and its atmospheric concentration is increasing dramatically. N₂O is produced through the microbially-mediated processes of nitrification and denitrification. Since these processes have different substrates and isotopic enrichment factors, stable isotope analysis (δ¹⁵N and δ¹⁸O) of N₂O can be used to study the production of this important greenhouse gas. Although production in rivers accounts for a significant portion of the global N₂O budget, the isotopic composition of N₂O from this source is poorly characterized. Most of the previous work using stable isotopes of N₂O has been conducted in terrestrial or oceanic environments, and only one published study has measured δ¹⁵N and δ¹⁸O of N₂O produced in a riverine environment. The purpose of this research project was to use stable isotope analysis to characterize the processes responsible for N₂O production in the Grand River, Ontario, Canada, and to determine the spatial and temporal variability of the isotopic composition of the N₂O flux. To meet the study objectives, an offline “purge and trap” method was developed to collect and purify dissolved N₂O for stable isotope analysis. Using this method, δ¹⁵N and δ¹⁸O analysis of dissolved N₂O is possible for samples with concentrations as low as 6 nmol N₂O/L. Due to the isotopic effects of gas exchange and the back flux of tropospheric N₂O, there is a complex relationship between the δ¹⁵N and the δ¹⁸O of source, dissolved, and emitted N₂O in aquatic environments. A simple box model (SIDNO – Stable Isotopes of Dissolved Nitrous Oxide) was developed to properly interpret isotopic data for dissolved N₂O. Using this model, it was determined that the isotopic composition of emitted N₂O is much more representative of N₂O production in aquatic environments than the isotopic composition of dissolved N₂O. If the concentration, δ¹⁵N and δ¹⁸O of dissolved N₂O are measured, the magnitude and isotopic composition of the N₂O flux can be calculated. Sampling downstream of the major wastewater treatment plants (WWTPs) on the Grand River indicates that nitrification and denitrification in the river are strongly tied to diel changes in dissolved oxygen (DO) concentration. During the day, when DO concentrations are high, nitrification or nitrifier-denitrification is the dominant N₂O production pathway, with sediment denitrification also contributing to N₂O production. At night, when DO concentrations are low, denitrification in the sediments and at the sediment / water interface is the dominant production pathway. Using the SIDNO model, N₂O produced during the day was found to have a δ¹⁵N of -22‰ and a δ¹⁸O of 43‰. N₂O produced at night had a δ¹⁵N of -30‰ and a δ¹⁸O of 30‰. The isotopic composition of N₂O emitted from the Grand River is dominated by night-time production downstream of the Waterloo and Kitchener WWTPs during the summer. The flux and time weighted annual average isotopic composition of N₂O emitted from the Grand River is -18.5‰ and 32.7‰ for δ¹⁵N and δ¹⁸O respectively. These values are significantly more depleted than the only other published data for riverine N₂O production. If the Grand River is representative of global riverine N₂O production, these results will have significant implications for the global isotopic budget for atmospheric N₂O.

<p><p>Invasive species are rapidly expanding in riparian wetlands while concurrently anthropogenic causes are increasing nitrogen (N) into these ecosystems. <italic>Microstegium vimineum (Microstegium) </italic> is a particularly abundant invasive grass in the Southeast United States. To evaluate impacts of <italic>Microstegium</italic> on both plant diversity and N cycling in a riparian floodplain, paired plots of <italic>Microstegium</italic> hand-weeded and unweeded were established for three years. Plots without <italic>Microstegium</italic> increased from 4 to 15 species m<super>-2</super> and 90% of the newly establishing species were native. The <italic>Microstegium</italic> community accumulated approximately half the annual N in biomass of the diverse community, 5.04 versus 9.36 g-N m<super>-2</super> year<super>-1</super>, respectively (p=0.05). Decomposition and release of N from <italic>Microstegium</italic> detritus was much less than in the diverse community, 1.19 versus 5.24 g-N m<super>-2</super> year<super>-1</super>. Rates of soil N mineralization estimated by in-situ incubations were relatively similar in all plots. While <italic>Microstegium</italic> invasion appears to greatly diminish within-ecosystem circulation of N through the under-story plants, it might increase ecosystem N losses through enhanced denitrification (due to lower redox potentials under Microstegium plots). Microstegium removal ceased in the fourth growing season and formerly weeded plots increased to 59% (&plusmn; 11% SE) Microstegium cover and species richness decreased to <8 species m<super>-2</super>. </p></p><p><p>To learn how <italic>Microstegium</italic> responds to increased N, we conducted a greenhouse competition experiment between <italic>Microstegium</italic> and four native plants across an N gradient. There was a unique competition outcome in each species combination, yet <italic>Microstegium</italic> was most dominant in the high levels of N. </p></p><p><p>Last, we disturbed a floodplain similar to wetland restoration disturbance and tracked available N. We also established a native community of plants with and without <italic>Microstegium</italic> in three levels of N. Disturbance to the floodplain dramatically increased inorganic N, especially in the form of NO₃ which was five times higher in the disturbed floodplain than the undisturbed floodplain. N levels remained elevated for over a year. <italic>Microstegium</italic> was N responsive, but did not show negative effects to the planted vegetation until the second year. Ironically, restoration activities are increasing available N, and favoring invasive species which in turn detracts from restoration success.</p> / Dissertation

In the summer of 2010, fieldwork was conducted in the Robertson Valley, Canadian Rocky Mountains, Alberta to assess the magnitude and controls of microbial nitrification in proglacial till and in supraglacial, subglacial, and proglacial streams. Seasonal precipitation and glacial and proglacial runoff was sampled for hydrochemical and stable isotope analyses (δ18O and δ15N of nitrate [NO3-]). Lower Ca:Mg ratios, higher mean Σmajor ions, and an increased importance of reactions with slower dissolution kinetics in subglacial streams and proglacial seeps indicated waters here experienced longer rock-water contact time than in dilute supraglacial streams. Additionally, waters emanating from longer residence time flowpaths acquired substantial NO3- from nitrification reactions. Using δ18O-NO3- in a simple end-member mixing model, the fraction of NO3- derived from microbial nitrification was estimated to be 44 to 56% in the two subglacial streams, and greater than 80% in proglacial seeps. These results show that atmospherically-derived nitrogen (N) in this glacial valley undergoes substantial biological cycling prior to export in surface runoff. Water flowing from the east subglacial stream (RE) received a larger portion of its melt from a sediment-rich, slow drainage system and had a higher proportion of nitrified NO3- compared to the west subglacial stream (RW), where runoff was similar in composition to supraglacial runoff, indicating that the nature of subglacial flowpaths is an important factor in determining the amount of microbially-cycled nutrients that are exported from a glacier. Sixteen 34-day in situ soil incubations revealed that net mineralization and net nitrification occurred at all four sampling sites in the glacier forefield along a 1.6 km chronosequence; however, there was no significant difference among these rates with time since deglaciation or temperature. Instead, net mineralization and net nitrification rates were significantly correlated (p < 0.05, n = 16) with measured physical and chemical soil variables, including total organic carbon, total N, bulk density, pH, and clay content, suggesting that substrate availability is a larger control on N-cycling processes than time since deglaciation. High variability in inorganic soil N pools and N-cycling rates indicates that there are likely hot spots of biogeochemical activity within glacial till. / Thesis (Master, Geography) -- Queen's University, 2012-04-26 14:47:17.29

Thesis (Ph. D.)--Earth and Atmospheric Sciences, Georgia Institute of Technology, 2007. / Martial Taillefert, Committee Member ; Jay Brandes, Committee Member ; Markus Huettel, Committee Member ; Philip Froelich, Committee Member ; Ellery Ingall, Committee Member ; Richard A. Jahnke, Committee Chair.

Human activities such as fossil fuel combustion and deforestation have increased atmospheric concentrations of carbon dioxide, reactive nitrogen, and other greenhouse gases. As a result, Earth's surface has warmed by 0.85 °C since the pre-industrial era and will continue to warm. Many northern latitude temperate forest ecosystems mitigate the effects of both elevated carbon dioxide and atmospheric nitrogen deposition through retention of carbon and nitrogen in plants and soils. However, the continued ability of these ecosystems to store carbon and nitrogen will be altered with continued climate change. Warmer winters will lead to reduced depth and duration of snowpack, which insulates soils from cold winter air. Climate change over the next century will therefore affect soil temperatures in northern temperate forests in opposing directions across seasons, with warmer soils in the growing season and colder, more variable soil temperatures in winter. Warmer growing seasons generally increase ecosystem uptake and storage of carbon and nitrogen, whereas a smaller snowpack and colder soils in winter reduce rates of ecosystem nutrient cycling and plant growth. My dissertation aims to understand how climate change in the growing season and winter interact to affect function and nitrogen cycling in northern hardwood forest ecosystems. I accomplished this goal through formal literature review and two climate change manipulation experiments at Hubbard Brook Experimental Forest, NH. I found that although 67% of climate change experiments were conducted in seasonally snow covered ecosystems, only 14% take into account the effects of distinct climate changes in winter. By simulating climate change across seasons, I demonstrated that changes in nitrogen cycling caused by increased soil freezing in winter are not offset by warming in the growing season. Moreover, shifts in plant function due to winter climate change are mediated through a combination of changes in snow depth, soil temperature, and plant-herbivore interactions that differentially affect above- and belowground plant components. These results would not be evident from examining climate change in either the growing season or winter alone and demonstrate the need for considering seasonally distinct climate change to determine how nitrogen and carbon cycling will change in the future.

Dans les prairies subalpines, l’abandon de certaines pratiques agricoles (fertilisation, fauche ou pâturage) ou à l’inverse, leurs intensifications entrainent une altération de la diversité fonctionnelle et spécifique des végétaux, de l’activité microbienne du sol ainsi que de la disponibilité de l’azote du sol et des processus de transformation de l’azote. L’hypothèse émise concerne les changements dans la distribution de la dominance des traits de plantes qui agiraient comme un déterminant important sur la productivité des plantes, la diversité fonctionnelle des micro-organismes du sol et sur les mécanismes des cycles de l’azote (N) et du carbone (C). Les hypothèses testées sont que : (i) l’augmentation de la dominance des traits liés aux stratégies conservatives et exploitatives promeut les espèces bactériennes K et r sélectionnées ; et (ii) ces interactions plantes-micro-organismes établiront les rythmes des cycles de l’N et du C et donc les services écologiques associés. L’approche originale de ce projet consiste en une étude de quatre espèces que l’on retrouve sur les prairies typique des bassins versant agricoles subalpins: deux espèces de monocotylédone et deux espèces dicotylédones, avec une espèce à stratégie conservative et une espèce à stratégie exploitative pour chacun des groupes. Ces plantes seront cultivées avec un sol pauvre en nutriment qui subira ou non un traitement de fertilisation ; ce qui permettra un contrôle de la disponibilité en N du sol. En parallèle, l’étude d’un mésocosme contenant un sol inoculé sera effectuée avec pour objectif de préciser le rôle des micro-organismes dans les mécanismes de rétroaction entre le sol et les plantes. Dans le cadre du projet (VITAL, EU Biodiversa) dans laquelle entre cette étude, ces mêmes espèces seront cultivées dans les Alpes autrichiennes et dans les prairies du Royaume-Unis. In subalpine grasslands, the neglect of some agricultural practices (fertilization, mowing or pasture) or on the contrary, their intensification involve a deterioration of the functional and specific diversity of plant, soil microbial diversity as well as soil nitrogen availability and nitrogen transformation processes. Our overarching hypothesis is that changes in the dominance distribution of plant traits will act as an important determinant of plant productivity, microbial functional diversity and carbon and nitrogen cycling. The hypothesis tested are that: (i) increased dominance of traits linked to conservative or exploitative strategies promotes K and r-selected microbial species; and (ii) these plant-microbial linkages will determine carbon and nitrogen cycling rates, and hence the associated ecosystem services. The original approach of this project consists studying four species from the typical grasslands of subalpine agricultural mountainside basins: two grasses species and two forbs species, with conservative strategiy species and exploitative strategy species for each group. These plants will be cultivated with low nutrient soil which will undergo or not fertilization treatement to control soil nitrogen availability. In a parallel study, mesocoms containing an inoculated soil will be carried out to clarify the role of microbe in plant-soil feedback processes. Thanks to the EU framework of the project (VITAL, EU Biodiversa) in which enters this study, these same species will be cultivated in the Austrian Alps and the United Kingdom grasslands. / Plant communities are strongly linked with soil microbial communities through symbiotic or resource competitive interactions. Plant functional traits have been often used to study these relationships and have highlighted, for example, plant litter or root exudate effects on soil microbial communities. However, few studies have taken into account both leaf and root functional traits to elucidate plant-microbe relationships. My thesis work has focused on functional approach of aboveground and belowground plant traits and microbial functional parameters of nitrogen (N) cycle. My main objective was to understand subtle mechanisms implied in plant-microbes relationships and their impacts on ecosystem functioning. These researches have greatly benefited from experiments based on a gradient of nested scales ranging from individual to ecosystem, with an intermediary step using artificial plant communities in controlled conditions. Methodologies varied from isotope labeling of nitrogen fluxes to enzymatic activities and abundances of key microbial genes of N cycling. I have shown close relationships between plant functional traits and microbial functional parameters. I found that root functional traits were the plant functional traits to be principally implied in relationships with microbial communities related to N cycling, particularly those related to denitrifying activities. I have also shown that ecosystem functioning is the result of the influence of plant and microbial interactions, and that the influence of one or the other was modulated by soil nutrient availability. Finally, my results suggest that responses of plant species to nutrient availability, in turn influence microbial communities related to N cycling to favor either recycling or retention of nitrogen nutrients.