Les communautés microbiennes sont des acteurs majeurs du fonctionnement biologique du sol à travers notamment leur implication dans les transformations des cycles biogéochimiques (C, N, P…). Dans les agro-écosystèmes, la diversité de ces communautés est régulièrement modifiée par des perturbations liées aux pratiques agricoles et la question des conséquences de ces modifications pour le maintien du fonctionnement biologique et des fonctionnalités des systèmes agricoles est aujourd’hui centrale. Si le rôle de la diversité biologique pour le fonctionnement des écosystèmes a été bien étudié chez les macro-organismes, et notamment les plantes ; la relation biodiversité/activité est encore très mal connue pour les microorganismes du sol. Pourtant, dans la mouvance agroécologique actuelle, cette connaissance est nécessaire pour définir de nouvelles pratiques culturales intégrant une gestion de la diversité microbienne pour une utilisation durable des agrosystèmes. Dans ce travail, l’objectif général était de tester l’importance de la diversité pour la stabilité (résistance/résilience) et l’activité des communautés microbiennes (bactéries et champignons) impliquées dans les transformations de la matière organique dans le sol, une fonction déterminante pour la fertilité des sols, la qualité de l’environnement et les changements globaux. D’un point de vue expérimental, nos questions ont été abordées par le couplage d’expérimentations au laboratoire avec des échantillonnages réalisés au terrain. Dans un premier travail basé sur une manipulation de la diversité au laboratoire, nous avons montré que la stabilité de la structure et de l’activité des communautés en réponse à différentes perturbations est positivement liée à la diversité microbienne (i.e. nombre d’espèces). Ce lien a ensuite été validé par une expérimentation basée sur un échantillonnage de terrain qui nous a permis de démontrer (i) que la diversité microbiennes peut être modulée (augmentée ou diminuée) en fonction de l’intensité d’usage des sols, et (ii) que la minéralisation de la matière organique est plus intense dans les sols présentant les plus hauts niveaux de diversité. Enfin, dans le cadre d’une expérimentation réalisée au terrain (SOERE-ACBB, Lusignan), nous avons montré que la réponse des communautés de bactéries et de champignons à un apport de résidus de blé, en termes de successions de populations et d’activité de minéralisation de la matière organique, dépend de l’historique cultural du sol. Ces travaux apportent de nouvelles connaissances sur l’importance de la diversité microbienne (richesse, composition) pour la stabilité et l’activité des communautés impliquées dans les transformations de la matière organique dans le sol. Ils montrent également que la modulation de la diversité des communautés microbiennes du sol par les pratiques agricoles, présentes ou passées, peut affecter significativement le turnover de la MOS. / Soil microbial communities act as important agents of the biological soil functioning, particularly through their involvements in the transformations of biogeochemical cycles (C, N, P…). In agro-ecosystems, the diversity of these communities is affected by perturbations associated to agricultural practices, and the significance of these modifications in terms of preservation of biological functioning and sustainability of agricultural systems has emerged as a central issue in the environmental sciences. Whereas the role of biodiversity has been well studied for macroorganisms, in particular for plants; the biodiversity/activity relationship is still largely unknown for soil microorganisms. However, in the current agro-ecological movement, this knowledge is needed to define new agricultural practices including a best management of microbial diversity for the sustainable use of agro-ecosystems. In this context, the objective of this Phd was to test the significance of microbial diversity for the stability (resistance/resilience) and the activity of microbial community (bacteria and fungi) involved in the turnover of soil organic matter, a major function for soil fertility, environment quality and global changes. From an experimental point of view, these issues were addressed by coupling laboratory with field experiments. In a first work, by manipulating microbial diversity in laboratory condition, we have shown that the stability of both microbial genetic structure and activity in response to different perturbations is positively linked to microbial diversity (i.e. number of species). This link was then validated by a sampling based on a field experiment that allowed us to demonstrate that (i) the soil microbial diversity can be modulated (increased or decreased) depending the intensity of land use management, and (ii) the mineralization of organic matter is more intense in the soil with the highest level of diversity. Finally, thanks to an experiment carried out in the field (SOERE-ACBB, Lusignan), we showed that the response of bacterial and fungal communities to wheat residues supply in terms of successions of microbial populations and activities of organic matter mineralization depends on the soil management history. These works provide new insights into the significance of microbial diversity (richness, composition) for the stability and the activity of communities involved in the soil organic matter turnover. They also suggest that the modulation of the diversity of soil microbial communities by agricultural practices, past or present, can significantly affect the turnover of soil organic matter.

Magister Scientiae (Biodiversity and Conservation Biology) / This study examined the magnitude of soil CO2 efflux in an arid South African ecosystem, the flux responses as well as those of key limiting nutrients to soil temperature increases and moisture reductions consistent with a future climate change scenario, and compared measured soil respiration rates with those predicted with empirically and theoretically-based soil respiration models. Measurements of soil respiration rate, temperature, moisture, N and P contents were conducted monthly over a 12-month period in natural environments and those artificially manipulated with replicated open-top warming chambers (average 4.1oC increase) and precipitation exclusion chambers (average 30.1% decrease in rainfall, 26.2% decrease in fog and dewfall) distributed in five different soil-vegetation units.Measured soil respiration rates were over 3-fold less than those reported for temperate and tropical forest ecosystems with 61.5% of the total soil CO2 efflux contributed by root respiration (derived from the differences between moderately vegetated and sparsely vegetated areas) in moderately vegetated soils. Massive increases (up to 15 times) in soil CO2 efflux occurred during wet phases, but even these large CO2 pulses were only comparable in magnitude with soil CO2 effluxes reported for temperate semi-arid grasslands. There was considerable intra-annual and inter-site variability in the magnitude and direction of soil respiration and N and P responses to elevated temperatures and reduced precipitation levels with poor correspondence evident between soil CO2 efflux and soil organic matter content. Soil CO2 effluxes declined in response to precipitation exclusion by 7.1% over all sites and increased in response to warming by 42.1% over all sites. The large increase in response to warming was assisted by a 7.5% enhancement in soil moisture content due to precipitation interception by the chamber walls and its channelling to the soil surface.Relatively smaller respiration increases in response to warming occurred in moderately vegetated soils, these attributed to soil thermal insulation by the plant canopy cover. Soil P and N contents increased in response to warming by 11.3% and 13.3% respectively over all sites, with soil P declining in response to precipitation exclusion by 5.8% over all sites and soil N increasing in response to precipitation exclusion over all sites by 5.8%. Standard least squares regressions quantified the relationships between soil respiration rate and measured soil physical and chemical properties, and their interactions for each of the 5 soil-vegetation units. These relationships were incorporated in an empiricallybased soil respiration (EMR) model which was compared with a theoretically based generalized soil respiration model (GRESP). GRESP model functions included measured Q10 coefficients at soil moisture contents above field capacity, these assumed reduced by half for dry conditions, and maximum retentive and field capacities of soils. EMR modelled soil respiration rates displayed slightly better correspondence with measured soil respiration rates than GRESP modelled soil respiration rates. This apparent from the higher regression coefficients and lower sums of squared residuals, with EMR model residuals also more closely approximating normal distributions. However, despite the EMR model’s slight superiority, it was concluded that more precise laboratory-based measurements of soil retentive and field capacities and their Q10 coefficients at different soil moisture contents could improve the GRESP model’s accuracy thereby providing a more convenient and uncomplicated means of predicting respiration responses to current and future climates over a wide range of arid soil types

This diploma thesis is focused on changes of stability in organic matter by extraction in different agents. Two soils of a different type and isolated humic acids were used for stability determination. Extraction agents were selected usually used for soil metal extraction, and a changes in the structure of the organic matter was observed. Samples were characterized using FTIR analysis and elemental analysis, the extracts were measured by UV-VIS spectroscopy, absorption ratios E2/E3, E4/E6 were discovered, dynamic light scattering were determined for particle size distributions. Next part of the thesis was the assessment of the change of thermal stability using thermogravimetric analysis and differential scanning calorimetry, degradation temperatures of the extracted samples were discovered and compared with the original samples.

The submitted diploma thesis aims to extend the use of thermogravimetry for the analysis of organic C and total N contents in soils. The advantages of thermogravimetry in comparison to conventional analytical methods are that it is a versatile technique that provides fast analysis, does not require sample pretreatment and chemicals– and can be used for the analysis of various soil types. The research work performed so far showed correlations between thermogravimetric data and some soil properties. In the past, intact soils exposed to 76% relative humidity (RH) were analyzed by thermogravimetry for these purposes. However, this humidity is problematic to achieve and maintain for most thermogravimeters. Recent work has shown that correlations can be observed in agricultural land exposed to lower RH. Therefore, it can be assumed that a correlation between TG data exists in soils exposed to any RH. TG could then be used to analyze soil properties under any known RH conditions. The aim of this work was to verify this hypothesis and try to incorporate knowledge of RH into the relationships between TG and soil properties. For this reason, intact soils exposed to the relative humidity of 30, 55, and 76% were analyzed in this work. It was demonstrated the dependence between organic C content and mass loss between 320 and 330°C as well as total nitrogen content and mass loss between 410 and 420°C independently of relative humidity. Based on that knowledge, we have derived equations enabling determination of the content of organic C and total N for RH ranging 43% - 76% using mass losses and knowledge of RH. Nevertheless, due to the low number of tested RH, the equations are still only preliminary and need to be improved by analyzing soil samples at a larger number of RH.

The United Nations organization for food and agriculture argues that humanity hastaken the soils of the world for granted. Due to chemical pollution, erosion,salinization, compaction, and acidification, 33 percent of the soils are moderately orhighly degraded. If humanity loses more productive soils, there is a risk that foodinsecurity and poverty would increase as well as the diminishing of severalecosystem services.This study focuses on the anthropogenic external factors that affect the agriculturalfields since the conventional and organic farmers use different methods relating tofertilizers, manure, and pesticides. A comparison of soil organic matter between soilsamples from organic and conventional farms in Sweden was carried out, throughsoil sampling and then analyzed with the combustion method. The study alsoevaluates the effect of erosion on the fields in the sampling region.According to this study’s results, there is no significant difference between theamount of soil organic matter in conventional or organic farming soil in southeastSweden, and there has been no erosion on the fields in the past seven years since theSOM content had neither increased nor decreased. This knowledge can be of furtheruse in soil science studies.

Indiana University-Purdue University Indianapolis (IUPUI) / Up-to-date and accurate information on soil properties is important for precision farming and environmental management. The spatial information of soil properties allows adjustments of fertilizer applications to be made based on knowledge of local field conditions, thereby maximizing agricultural productivity and minimizing the risk of environmental pollution. While conventional soil sampling procedures are labor-intensive, time-consuming and expensive, remote sensing techniques provide a rapid and efficient tool for mapping soil properties. This study aimed at examining the capacity of hyperspectral reflectance data for mapping soil organic matter (SOM), total nitrogen (N) and total phosphorus (P). Soil samples collected from Eagle Creek Watershed, Cicero Creek Watershed, and Fall Creek Watershed were analyzed for organic matter content, total N and total P; their corresponding spectral reflectance was measured in the laboratory before and after oven drying and in the field using Analytical Spectral Devices spectrometer. Hyperion images for each of the watersheds were acquired, calibrated and corrected and Hyperion image spectra for individual sampled sites were extracted. These hyperspectral reflectance data were related to SOM, total N and total P concentration through partial least squares (PLS) regressions. The samples were split into two datasets: one for calibration, and the other for validation. High PLS performance was observed during the calibration for SOM and total N regardless of the type of the reflectance spectra, and for total P with Hyperion image spectra. The validation of PLS models was carried out with each type of reflectance to assess their predictive power. For laboratory reflectance spectra, PLS models of SOM and total N resulted in higher R2 values and lower RMSEP with oven-dried than those with field-moist soils. The results demonstrate that soil moisture degrades the performance of PLS in estimating soil constituents with spectral reflectance. For in-situ field spectra, PLS estimated SOM with an R2 of 0.74, N with an R2 of 0.79, and P with an R2 of 0.60. For Hyperion image spectra, PLS predictive models yielded an R2 of 0.74 between measured and predicted SOM, an R2 of 0.72 between measured and predicted total N, and an R2 of 0.67 between measured and predicted total P. These results reveal slightly decreased model performance when shifting from laboratory-measured spectra to satellite image spectra. Regardless of the spectral data, the models for estimating SOM and total N consistently outperformed those for estimating total P. These results also indicate that PLS is an effective tool for remotely estimating SOM, total N and P in agricultural soils, but more research is needed to improve the predictive power of the model when applied to satellite hyperspectral imagery.

<p>Soil organic carbon (OC) is one of the most important terrestrial carbon pools and plays a major role in climate regulation, water quality, provisional services, and numerous other ecosystem functions. The conversion of natural vegetation and the supporting soil to intensively managed agricultural systems put soil at risk for loss due to erosion and enhanced microbial degradation with loss rates increased by orders of magnitude above the pre-managed system. The process has negatively impacted agricultural productivity on hillslopes by diminishing soil health, as well as the quality of stream water and coastal aquatic environments, and it is an important but as of yet poorly quantified factor in the region’s terrestrial C budgets. There have been substantial debates on the role of erosional and depositional processes on the landscape as a control on exchange of C between the land surface and the atmosphere. A central aspect of the debate stems from the limited data regarding the fate of soil erosion-induced transport of OC through stages of detachment and splash, transport and redistribution, deposition and burial. The overarching purpose of this thesis is to evaluate how dynamic patterns of soil OC erosion due to intensive agricultural management influences soil aggregate strength, the chemical nature of mobilized organic particles, and connectivity and sourcing between hillslope and streams. Using both simulated and natural, short-term, event-based erosive rainfall processes, with a multiproxy geochemical approach, we attempt to develop a comprehensive understanding of how upland watershed mechanistic controls soil movement and associated chemical alterations to the material exported through dissected segments from hillslope to the fluvial network. </p> <p>Our results demonstrate that erosive processes on hillslope connects between terrestrial sources to receiving potential deposition settings, actively ‘filter’ soil aggregates and particles and associated OC at each erosional stage (i.e., detachment and transport downhill/downstream), with distinct geochemistry in low relief and poorly drained agricultural systems, like the CCW. Complex interactions among tillage intensity, tillage practice-induced, oriented surface roughness, and storm-induced hydrological connectivity, that potentially impact the fate of these transported OC upon decomposition, deposition and burial, and have important implications for predicting landscape level heterogeneity in surface and buried soil chemistry upon mobilization and burial, as well as the dynamics of sourcing and transformation of material exported to inland water systems.</p>

The ancient Maya of the Classic period (1700-1050 B.P.) relied on maize agriculture to support their populations. The agricultural systems they employed to produce that maize varied in form and degree of intensity, with more productive forms of agriculture needed in the Late Classic period (1350-1050 B.P.) to sustain the peaking population. It is likely that the ancient systems of production agriculture contributed to environmental degradation that in turn contributed to the pressures that culminated in the collapse of the civilization. In this study, stable carbon isotope ratios contained in the soil organic matter were used to further investigate ancient maize cultivation in the Petexbatan region of Guatemala. Maize, a plant that uses the C4 photosynthetic pathway, leaves a different carbon (13C) isotopic signal in the soil than the C3 plants of the native forest vegetation. Soil profiles were collected from various landscape features around the Classic Maya site of Aguateca: bajos (or wetlands), control locations (areas not conducive to agriculture), defensible locations (areas within or near defensive walls), rehoyadas (natural karst depressions), and upland locations (level soils from across the rest of the landscape). The samples were tested for various chemical and physical properties, and the bulk soil organic matter and humin fractions were analyzed on a mass spectrometer to determine δ13C values. Graphs of the isotopic values were examined and the bulk and humin δ13C enrichment values for the landscape feature categories were compared statistically using ANOVA methods. We determined that the bulk and humin δ13C values of the Bajo and Rehoyada categories showed significantly greater enrichment than the Control, Defensible, and Upland locations. This enrichment was likely the result of sustained periods of maize cultivation, especially in the Rehoyadas. Surface soil bulk and humin δ13C enrichment values do not show that the Bajo or Rehoyada categories are influenced by modern C4 vegetation, nor do the enrichment values of the Control, Defensible or Upland soils. In other words, C3 vegetation dominates each of the landscape features today under natural conditions and probably would have done so anciently absent human interference. (Savanna soils are an exception, but none were identified in the study area.) When the thinner midslope (Backslope) Rehoyada profiles were compared to Control, Defensive, and Upland samples of similar depth, the Backslope Rehoyada profiles had significantly greater bulk δ13C enrichment values that may have resulted from maize cultivation. There was also a significant difference among the surface soil bulk and humin δ13C values of the Backslope Rehoyada, Control, Defensive, and Upland categories, though the reasons are less clear. Both the Bajos and Rehoyadas would have been valuable agricultural resources for maize production for the ancient Maya.

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