Note: / The soil-solution partitioning of dissolved organic carbon (DOC) withinmineral soil horizons is primarily controlled by processes of adsorption and desorption. These abiotic processes largely occur within a short equilibration time of seconds to minutes, which generally occur faster than microbial processes. To characterise the adsorption of DOC to mineral soils, I used the Langmuir adsorption isotherm, which holds several advantages to the commonly used linear initial mass (IM) isotherm. One advantage to using the Langmuir isotherm is anestimation of the maximum DOC adsorption capacity (Qmax). The Qmax estimates the number of remaining DOC binding sites available on the mineral soil particle surfaces. I modified the traditional Langmuir isotherm in order to estimate the DOC desorption potential of native soil organic matter (SOC).[...] / Le partitionnement entre les solutions de sols du carbone organiquedissous (COD) dans les horizons des sols minéraux est essentiellement contrôle par les processus d'adsorption et de désorption. Ces processus abiotiques se déroulent normalement dans un bref temps d'équilibration variant de quelques secondes a quelques minutes, ce qui est en général plus rapide que les processus microbiens. Pour caractériser Fadsorption de COD aux sols minéraux, nous avons utilise l'isotherme d'adsorption de Langmuir. Cette isotherme présente plusieurs avantages par rapport a Fisotherme de masses initiales (IM) linéaires couramment utilisée, en particulier F estimation de la capacité d'adsorption maximale du COD (Qmax). Le Qmax estime le nombre de sites de liaison de COD restants a la surface du sol minéral. Nous avons aussi modifie Fisotherme de Langmuir traditionnelle afin d'évaluer le potentiel de désorption de COD de la matière organique du sol indigène (MOS).[...]

Dissolved organic matter (DOM) is operationally defined as soluble/colloidal material passing through a 0.45mum filter paper. The importance of DOM in soils relies on its role in soil formation and weathering processes, plant and microbial assimilation and soil and water acidification. However, the scientific community studying DOM still disagrees on whether fresh or humified material is the major source of DOM within the forest floor. One of the factors that could influence the overall importance of DOM production by organic horizons is its potential for biodegradability. In addition, the interaction occurring between the nutrients (i.e. nitrogen (N) and phosphorus (P)) and carbon (C) substrate is believed to be of major importance. / To acquire more knowledge on the production and biodegradation of dissolved C, N and P during decomposition of organic matter (OM), I performed laboratory incubations to evaluate rates of production and transformation, the influence of the degree of OM decomposition and stand type on these rates, and the stoichiometric relationships of the different quotients during the incubations. First, I performed a 30-day incubation of coniferous and deciduous OM from 10 Canadian forest floors representing various degrees of OM decomposition and subsequently measured the amount of: dissolved organic carbon (DOC), total dissolved nitrogen (TDN), nitrate (NO3-N), ammonium (NH4-N), dissolved organic nitrogen (DON), total dissolved phosphorus (TOP) and carbon dioxide (CO2-C). I performed water extractions with the same set of samples to evaluate the biodegradability of DOC and DON and the transformations of TDN, NO3-N and NH4-N. / Fresh material produces more DOM than humified material; material in the midpoint of decomposition (F horizon) produced the largest amount of DIN. Coniferous and deciduous samples did not display different rates of DOM production, most likely because of the overshadowing effect of OM degree of decomposition. I found strong links between the organic matter and dissolved phase C and N content and C:N quotient. The biodegradation, measured as DOC disappearance and mineralization of CO2-C, showed a discrepancy, reflecting the importance of increasing microbial biomass at the beginning of the incubation in response to priming effect. The sharp decrease of TDN and DON observed in the first few days of the incubation, in addition to increasing amount of dissolved inorganic N as waste products during decomposition of DON, supports this hypothesis. A better understanding of the dynamics of dissolved C, N and P in soil is essential to further understand their role in global elemental cycles, including climate change, forest management and pollution.

The soil-solution partitioning of dissolved organic carbon (DOC) within mineral soil horizons is primarily controlled by processes of adsorption and desorption. These abiotic processes largely occur within a short equilibration time of seconds to minutes, which generally occur faster than microbial processes. To characterise the adsorption of DOC to mineral soils, I used the Langmuir adsorption isotherm, which holds several advantages to the commonly used linear initial mass (IM) isotherm. One advantage to using the Langmuir isotherm is an estimation of the maximum DOC adsorption capacity (Qmax). The Qmax estimates the number of remaining DOC binding sites available on the mineral soil particle surfaces. I modified the traditional Langmuir isotherm in order to estimate the DOC desorption potential of native soil organic matter (SOC). / Sorption characteristics were derived for a broad range of52 mineral soils collected from 17 soil profiles spanning across Canada from British Columbia to Quebec. Mineral horizons with the greatest Qmax included the Fe-enriched B horizons of acidic Podzols and Volcanic soils, followed by B horizons not enriched in Fe, followed by A and C horizons. Podzol B horizons were distinct from all other horizons due to significantly higher desorption potential. Soil properties predicting the adsorption characteristics of DOC also predicted the adsorption characteristics of dissolved organic nitrogen (DON). Adsorption of DOC and DON was tightly coupled (R 2 = 0.86), however the ratio of DOC:DON in the final equilibrium solution lowered for 48 out of 52 minerals horizons. These results suggest that DON may be slightly more mobile than DOC. / A short-term (32 day) incubation was perform to establish the fate of indigenous soil C, relative to newly adsorbed soil C to four mineral soils with different adsorption characteristics. Soil columns were leached periodically and sampled for DOC and CO2 production. Two Fe-enriched mineral horizons with high adsorption capacity released low amounts of old SOC, yet released almost all of the newly adsorbed SOC. In contrast, two B horizons without Fe-enrichment released greater amounts of old SOC, and retained a greater fraction of the newly adsorbed SOC than the Fe-enriched horizons. These results identify a contrast between the fate of indigenous and newly adsorbed SOC on mineral soils with differing Qmax. / The final component of this study examined changes to the molecular structure of DOC after equilibration with mineral soils. Multiple techniques were used to assess changes in the molecular composition of DOC, including the analysis of aromatic content by specific UV absorbance (SUVA) and fluorescence spectroscopy, analysis of molecular weight distribution (MWD) with high performance size exclusion chromatography (HPSEC) and functional group analysis with Fourier transform infra-red spectroscopy (FTIR). The solution phase DOC generally showed a reduced aromatic content, along with the removal of organic compounds with carboxyl groups. The MWD of DOC was reduced after equilibration to mineral soils, and the reduction in average molecular weight was related to the Qmax of mineral soils. / The various components of this thesis have contributed to the overall understanding of controls on the adsorption of DOC and DON species to mineral soils of the Canadian temperate and boreal forest.

Soil properties and their spatial variability was measured to provide a strong database to assess the modification in soil properties associated with future changes in land use. Surface (0--10 cm) soil samples were collected from a 9 ha, 46-year-old pasture being converted to a native tree plantation and a neighboring control pasture near Sardinilla, Panama. A small-scale nested grid of surface soil samples was replicated in the future plantation and a primary forest in the region to evaluate the spatial variability of soil properties. Seven 1 m profiles were sampled in the future plantation and litter samples were collected at the forest and the future plantation. / Assuming the three sites were identical before the conversion to pasture, the difference in surface soil organic carbon (SOC) was 0.75 kg m-2 or equivalent to a loss of 0.017 kg m-2 yr -1 since the original land-use change. The control pasture is higher in bulk density but lower in percent SOC than the future plantation, which is critical to future comparisons. / The pasture soils showed signs of soil compaction and of the homogenization of soil properties. For the 7 profiles in the future plantation, total SOC mass ranged from 13.45 to 23.80 kg m-2, and stable isotopes revealed that 82% of the SOC in the top 10 cm is derived from the pasture vegetation, down to 23% at 1 m depth. / Spatially, the full scale of spatial variability was not determined from the nested grids alone. In addition, the 15 x 15 m grid in the future plantation generally over-estimated the semivariance at the scales below 3 m. The nested grids assessed the minimum measurable semivariance below 5 m. / The precision of surface estimations from point observations can be improved by adding a 5 x 5 m grid to any large-scale sampling scheme.

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Dissolved organic matter (DOM) is operationally defined as soluble/colloidal material passing through a 0.45~m filter paper. The importance of DOM in soils relies on its role in soil formation and weathering processes, plant and microbial assimilation and soil and water acidification. However, the scientific community studying DOM still disagrees on whether fresh or humified material is the major source of DOM within the forest floor.[...] / La matière organique dissoute (DOM) est composée de particules dissoutes et colloïdales passant au travers un filtre de 0.45 Ilm. L'importance de DOM dans les sols est liée à son rôle dans la pédogenèse, les processus d'altération des minéraux, l'assimilation par les plantes et microbes, ainsi que l'acidification des plans d'eau et des sols.[...]

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