This research combines observation, experimentation, and modeling to evaluate the influence of lakes on dissolved organic matter (DOM) quantity, quality and export in subalpine watersheds of the Sawtooth Mountain Lake District, central Idaho. First, I conducted an empirical study of the hydrologic and biogeochemical controls on DOM dynamics in stream-lake fluvial networks. I hypothesized that lakes would decrease temporal variability (i.e., buffer) and alter the characteristics of DOM from inflow to outflow. I tested these hypotheses by evaluating DOM temporal patterns and measuring annual export in seven-paired lake inflows and outflows. I then evaluated how ultraviolet (UV) exposure affected DOM characteristics during snowmelt and baseflow, and how UV alters baseflow DOM bioavailability and nutrient limitation. Given that increased water residence time increases UV exposure, I hypothesized that lake outflow DOM would be more photorecalcitrant than DOM from lake inflows. I further hypothesized that UV exposure would increase DOM quality, heterotrophic processing, and nutrient demand. Results indicate that lakes can buffer stream temporal variability by acting as a DOM sink during snowmelt and a DOM source during baseflow. Lake outflow DOM photodegradation was similar to lake inflows during snowmelt (p=0.66). Conversely, outflow DOM was 2X more photorecalcitrant than inflow DOM during baseflow (ANOVA, p=0.03) and was strongly related to water residence time (WRT). During baseflow, light exposure increased inflow and outflow DOM bioavailability (p=0.059 and 0.024, respectively) and nutrient limitation (p=0.03 and 0.09, respectively). Combined, these results indicate that WRT in subalpine lakes strongly influences DOM temporal variability and DOM degradation and processing. Thus, lakes can provide temporal stability of DOM and potentially increase both carbon and nutrient uptake by heterotrophs in lake outflows. I then evaluated how global changes could alter hydrologic and nutrient dynamics in a subalpine lake. Model results indicate that the magnitude and timing of snowmelt runoff can have a substantial effect on water and nutrient exports. In phosphorus (P)-limited lakes, increases in inorganic N concentrations within and exported from lakes are likely to occur with increased temperatures and lake WRT. Increases in atmospheric N deposition will further enhance inorganic N exports in P-limited subalpine lakes.

In the Intermountain West, aspen (Populus tremuloides) has declined mainly due to a combination of successional processes, fire suppression and long-term use of ungulates which has led to replacement by conifers, sagebrush or other shrub communities. Conifer encroachment is believed to cause critical changes in the ecosystem properties. In order to understand the impacts of conifer encroachment on soil properties such as soil organic carbon (SOC) storage, soil morphology, and soil chemical properties, and the implications of such changes, it is very important to assess the soil properties under the two vegetation types. The objectives of this study were to i) quantify SOC stocks and their variability in pure aspen forests; ii) evaluate the role of various biotic and abiotic site parameters as drivers of this SOC; iii) evaluate the effect of conifer encroachment on SOC storage, soil morphology, soil microclimate and soil chemical properties. The study was conducted in three catchments in Northern Utah in two phases: i) a transect study with 33 sampling points in a pure aspen community; ii) a paired plot study based on comparing six plots in to aspen and nearby conifer plots as representatives of end-member communities. Soils under aspen were mainly Mollisols, whereas the soils associated with conifers were classified as Alfisols, Inceptisols and Entisols. Even under pure aspen there was a significant SOC variability among sampling points and aspects, and SOC was negatively correlated with soil moisture index and average tree diameter and positively correlated with vegetation density. The paired plot comparison showed that SOC in the mineral soil (0-60 cm) was significantly higher under aspen, while O horizon thickness and C content was higher under conifers. The total SOC (O layer + mineral soil) was not significantly different among the vegetation types, suggesting an upward redistribution of SOC in conifer soils. The soil moisture in summer was also higher under aspen compared to conifers. Other chemical properties were not affected by vegetation types. Our study indicates that i) no differences in SOC can be detected in surface soil horizons (<20 >cm); ii) SOC is highly variable and greatly influenced by soil moisture and forest characteristics; iii) conifer encroachment is likely to alter soil microclimatic and SOC amount and distribution.

Les sols sont la principale composantede l’écosystème terrestre et le plus grand réservoir de carbone organique sur Terre, étant très réactifs aux perturbations humaines et aux changements climatiques. Malgré leur importance dans les réservoirs de carbone, la dynamique du carbone des sols est une source importante d'incertitudes pour les prévisions climatiques futures. Le but de la thèse était d'explorer différents aspects d’études du carbone des sols (mesures expérimentales, modélisation et évaluation de bases de données) à différentes échelles spatiales (de l'échelle d'un profil à l'échelle globale). Nous avons souligné que l'estimation des stocks globaux de carbone du sol est encore assez incertaine.Par conséquent le rôle du carbone des sols dans la dynamique du climat devient l'une des principales incertitudes dans les modèles du système terrestre utilisés pour prédire les changements climatiques futurs. La deuxième partie de la thèse porte sur la présentation d'une nouvelle version du modèle IPSL-Land Surface appelé ORCHIDEE-SOM, intégrant la dynamique du 14C dans le sol. Plusieurs tests effectués supposent que les améliorations du modèle devraient se focaliser davantage sur une paramétrisation dépendante de la profondeur,principalement pour la diffusion, afin d'améliorer la représentation du cycle global du carbone dans les modèles de surface terrestre, contribuant ainsi à contraindre les prédictions futures du réchauffement climatique. / Soils are the major components ofthe terrestrial ecosystems and the largest organiccarbon reservoir on Earth, being very reactive tohuman disturbance and climate change. Despiteits importance within the carbon reservoirs, soilcarbon dynamics is an important source ofuncertainties for future climate predictions. Theaim of the thesis was to explore different aspectsof soil carbon studies (Experimentalmeasurements, modeling, and databaseevaluation) at different spatial scales (from thescale of a profile to the global scale). Wehighlighted that the estimation of the global soilcarbon stocks is still quite uncertain.Consequently, the role of soil carbon in theclimate dynamics becomes one of the majoruncertainties in the Earth system models (ESMs)used to predict future climate change. Thesecond part of thesis deals with the presentationof a new version of the IPSL-Land SurfaceModel called ORCHIDEE-SOM, incorporatingthe 14C dynamics in the soil. Several tests doneassume that model improvements should focusmore on a depth dependent parameterization,mainly for the diffusion, in order to improve therepresentation of the global carbon cycle inLand Surface Models, thus helping to constrainthe predictions of the future soil organic carbonresponse to global warming.

Les systèmes agroforestiers stockent du carbone dans la biomasse des arbres. Cependant leur intérêt ne se limite pas à ce carbone stocké sous forme de bois. En effet, les arbres produisent de grandes quantités de litières, et apportent également du carbone dans les horizons profonds du sol par la mortalité et l’exsudation racinaire. Or, les sols agricoles, ayant de très faibles teneurs en matière organique, ont un potentiel de stockage en carbone bien plus important que les sols forestiers. A ce jour, il n’existe pratiquement pas de travaux permettant d’avoir une estimation de l’impact des arbres agroforestiers sur le carbone du sol. La plupart des études sont en effet menées sur le stockage de carbone dans la biomasse aérienne des arbres. Une étude a ainsi estimé qu’en climat tempéré et pour des densités comprises entre 50 et 100 arbres/ha, le stockage de carbone serait compris entre 1.5 et 4 tC/ha/an, ce qui est très important comparé au potentiel de stockage d’autres systèmes de culture. On se propose donc dans ce travail de contribuer significativement à la connaissance sur les possibilités de stockage de C dans les sols en agroforesterie. Tout d’abord, nous quantifierons les stocks de C dans les parcelles agroforestières et les comparerons aux témoins agricoles. Nous étudierons également l’hétérogénéité spatiale de ces stocks, sous la ligne d’arbres ou sous la culture intercalaire, et ce à différentes profondeurs. Dans un deuxième temps, nous étudierons les entrées de carbone au sol, notamment via la mortalité racinaire des arbres. Puis, nous étudierons les processus liés à la stabilisation de ce carbone dans les horizons profonds du sol. Enfin, nous chercherons à savoir si l’apport de carbone frais dans les horizons du sol ne pourrait pas entraîner une minéralisation d’une partie du carbone stable du sol, phénomène connu sous le nom du priming effect, et qui pourrait jouer un rôle non négligeable dans le bilan de carbone de ces systèmes. La modélisation sera utilisée afin d’estimer le stockage de carbone sur le long terme. L’étude sera menée dans un contexte de système de culture méditerranéen, sur un site expérimental d’exception. L’analyse mécaniste fournira le cadre conceptuel pour la compréhension de la dynamique du C dans d’autres systèmes agroforestiers à l’avenir. / Agroforestry is a land use type where trees are associated with crops and/or animals within the same field. This agroecosystem could help mitigating climate change, and also contribute to its adaptation. The goal of this thesis was to evaluate the potential of soil organic carbon storage under agroforestry systems. This study was performped at the oldest experimental site in France, a trial supervised by INRA since 1995, but also at farmers' fields. Soil organic carbon stocks were compared between agroforestry and agricultural plots, down to 2 m soil depth. All organic inputs to the soil were quantified (tree roots, leaf litter, crop roots and residues). The stability of additionnal stored carbon was caracterised with soil organic matter fractionation, and soil incubations. A model of soil organic carbon dynamic was described in order to better undrestand this dynamic in agroforestry, especially in deep soil layers. This study revealed the interest and the potential of agroforestry systems in increasing soil organic carbon stocks, with accumulation rates of 0.09 to 0.46 t C ha -1 yr -1. It also reveals the role of tree rows in this storage, and the importance of carbon inputs from root mortality. However, it raises concerns about the stability of this storage.

Indiana University-Purdue University Indianapolis (IUPUI) / This study investigated nutrient export during three spring storm events in two different land use watersheds (agricultural and mixed land use) in a glacial till landscape of the Midwestern, USA. The objectives of the study were: (1) to determine how land use affects water, nitrate, soluble reactive phosphorus (SRP) and dissolved organic carbon (DOC) delivery (timing, amount) to streams during spring storms in two central Indiana watersheds with contrasting land use; and (2) to determine nitrate, SRP and DOC flow pathways to streams during spring storms. High frequency stream sampling of nutrients and cations, coupled with hydrograph separations using δ18O, was used to identify water flow pathways and event and pre-event water contributions to the streams. Data indicate land use and storm characteristics play a role in the export of water and nutrients. In the agricultural watershed (Watershed A), the storm hydrograph is dominated by pre-event water, whereas the mixed land use watershed (Watershed M) storm hydrograph is more event water dominated. Watershed A also contains higher nutrient concentrations, especially nitrate. High bulk precipitation and greater maximum intensity export more nitrate, SRP, and DOC to the streams. Results also indicate nitrate, DOC, and SRP concentrations display distinct temporal patterns during spring storm events. DOC concentration increased with stormflow and peaked on the rising limb/with maximum discharge regardless of land use or storm event. In Watershed A, SRP concentration followed a similar pattern to DOC during small storms; therefore they are believed to be exported together with flushing of saturated near-surface soil waters via macropores/overland flow. However, SRP likely has multiple flowpaths, one dominated over another depending on the storm. Nitrate concentrations matched Ca2+, Mg2+, and Na+ trajectories and decreased with stormflow, suggesting a tile drain/subsurface flowpath. Nitrate and SRP peak concentrations are delayed relative to DOC in Watershed M. The wet retention ponds in the headwaters are believed to delay the stormflow response, and therefore, the delivery of nutrients to the stream.

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[pt] Os copolímeros de poliacrilamida desempenham um papel importante na recuperação avançada de petróleo, cuja utilização visa o aumento da produção de óleo. A avaliação do polímero na água produzida é, portanto, necessária para monitorar a operação do campo petrolífero e para questões ambientais. Entretanto, tal avaliação é desafiadora devido à complexidade da matriz e à possível degradação da macromolécula durante o processo. Entre os diferentes métodos para determinação de polímero, o carbono orgânico total (TOC, do inglês total organic carbon) é vantajoso em comparação com outros métodos reportados na literatura devido ao seu baixo custo, fácil operação e curto tempo de análise. Devido a isso, um método de análise por TOC foi desenvolvido para a determinação do teor de poliacrilamida, e a influência da salinidade, massa molar e dois tipos de degradação química (hidrólise e por Fe2+/O2) na quantificação do polímero por este método foram exploradas neste trabalho. Medidas reológicas, análises de ressonância magnética nuclear (RMN) e de espectroscopia de infravermelho (ATR-FTIR, do inglês Fourier-transform infrared with attenuated total reflectance) também foram realizadas com o intuito de avaliar a degradação. A quantificação do polímero na presença de diferentes teores de sal não foi afetada. De modo similar, as diferentes massas molares dos polímeros não afetaram os resultados das análises por TOC. O grau de hidrólise dos polímeros foi confirmado por 13C RMN e ATR-FTIR, enquanto medidas reológicas confirmaram a degradação por Fe2+/O2. Notavelmente, ambos os mecanismos de degradação investigados não impactaram a determinação de poliacrilamida. Para todos os fatores avaliados, os erros não ultrapassaram 8 por cento. Assim, os resultados obtidos destacam o método TOC como adequado e confiável para a determinação de poliacrilamida com boa exatidão, conferindo-lhe um potencial de aplicação para análises de amostras reais. / [en] Polyacrylamide copolymers play an important role in enhanced oil recovery (EOR), whose widespread use aims to improve oil production. Assessment of polymer in the produced water is thus required for monitoring the oilfield operation and for environmental concerns. However, this task is challenging due to the matrix complexity and the possible degradation of the macromolecule during the process. Among the different methods for polymer determination, total organic carbon (TOC) is advantageous in comparison to others reported in the literature because of its low cost, easy operation and short analysis time. Due to this, a TOC method was developed for the determination of polyacrylamide content, and the influence of salinity, molar mass, and two types of chemical degradation (by hydrolysis and Fe2+/O2) on the polymer quantification through this method were explored in this work. Rheological measurements, NMR and ATR-FTIR analyses were also carried out to evaluate the extent of degradation. The polymer quantification was not affected in the presence of different salt contents. Similarly, analyses of samples with different molar masses did not affect TOC results. The hydrolysis degree of the polymers was confirmed by 13C NMR and ATR-FTIR, while rheological measurements confirmed degradation by Fe2+/O2. Noticeably, both degradation mechanisms investigated did not impair polyacrylamide determination. For all factors evaluated, the errors were never greater than 8 percent. The results obtained highlight the TOC as a suitable and reliable method for the determination of polyacrylamide with good accuracy, showing its potential application in the analysis of real samples.

In marine ecosystems around the world, iron (Fe) is a well-studied and important trace element which has been found to stimulate phytoplankton and cyanobacteria growth, as well as limit nitrogen fixation and being essential for Legionella pneumophila which is known as the main cause of Legionnaires’ disease. Fe also contributes to the brownification of freshwater and has recently been documented to increase in concentration in rivers in Sweden. The aim of this study was to investigate the link between Fe and humic substances (HS) in rivers and a coastal area in the northern Baltic Sea. In late winter, surface samples from rivers and the coast were analyzed for Fe, HS, dissolved organic carbon (DOC), pH, salinity, and temperature. Overall, the results showed a positive correlation between Fe and HS, indicating a terrestrial input of Fe. Other variables affecting Fe and HS including pH, salinity, and temperature were analyzed using principal component analysis (PCA), which showed spatial variations between river and coastal samples. Fe is known to be transported in water as two main carrier phases, Fe-OM complexes and Fe-oxyhydroxides. Which carrier phase is the most contributing depends on multiple factors, including catchment characteristics, seasonal variations, and water chemistry. Understanding the link between Fe and HS in land-sea transitional areas is important since terrestrial Fe is identified as a source of bioavailable Fe in marine waters such as the Baltic Sea.

The ecological significance of coarse woody debris (CWD) is usually highlighted in forests where CWD constitutes much of an ecosystem's carbon (C) source and stores. However, a unique addition of CWD is occurring in semi-deserts for which there is no ecological analog. To stem catastrophic wildfires and create firebreaks, whole Juniperus osteosperma (Torr.) and Pinus edulis (Engelm.) trees are being mechanically shredded into CWD fragments and deposited on soils previously exposed to decades of tree-induced changes that encourage "tree islands of fertility." To investigate consequences of CWD on C and nitrogen (N) cycling, we evaluated microbial metabolic activity and N transformation rates in Juniperus and Pinus surface and subsurface soils that were either shredded or left untreated. We sampled three categories of tree cover on over 40 tree cover encroachment sites. Tree cover categories (LOW = 0-15%, MID ≥ 15-45%, HIGH ≥ 45%) were used to indicate tree island development at time of treatment. In conjunction with our microbial measurements, we evaluated the frequency of three exotic grasses, and thirty-five native perennial grasses to identify links between belowground and aboveground processes. The addition of CWD increased microbial biomass by almost two-fold and increased microbial efficiency, measured as the microbial quotient, at LOW Juniperus cover. C mineralization was enhanced by CWD only in Pinus soils at the edge of tree canopies. The addition of CWD had little impact on microbial activity in subsurface soils. CWD enhanced the availability of dissolved organic C (DOC) and phosphorus (P) but tended to decrease the overall quality of labile DOC, measured as the ratio of soil microbial biomass to DOC. This suggested that the increase in DOC alone or other environmental factors novel to CWD additions lead to the increase in biomass and efficiency. P concentrations were consistently higher following CWD additions for all encroachment levels. The CWD additions decreased N mineralization and nitrification in Juniperus and Pinus soils at LOW and MID tree cover but only in surface soils, suggesting that less inorganic N was available to establishing or residual plants. The frequency of native perennial grasses, especially Elymus elymoides (Raf.), was at least 65% higher under CWD additions for all categories of tree cover, while the frequencies of exotic annual and perennial grasses were not impacted by CWD. The frequency of all perennial grasses ranged from 10-27%. Our results suggest that CWD enhanced microbial activity even when the quality of C substrates declined requiring microbes to immobilize more N. The reduction in inorganic N may promote the establishment and growth of native perennial grasses. Ultimately, the addition of CWD improved soil conditions for microbes in tree islands of fertility.