The fate and behaviour of hydrophobic organic compounds (e.g. pesticides) in soils are largely controlled by sorption processes. Recent findings suggest that the chemical properties of soil organic carbon (OC) significantly control the extent of sorption of such compounds in soil systems. However, currently there is no practical tool to integrate the effects of OC chemistry into sorption predictions. Therefore, the K [subscript]oc model, which relies on the soil OC content (foc), is used for predicting soil sorption coefficients (K[subscript]d) of pesticides. The K[subscript]oc model can be expressed as K[subscript]d = K[subscript]oc × foc, where K[subscript]oc is the OC-normalized sorption coefficient for the compound. Hence, there is a need for a prediction tool that can effectively capture the role of both the chemical structural variation of OC as well as foc in the prediction approach. Infrared (IR) spectroscopy offers a potential alternative to the K[subscript]oc approach because IR spectra contain information on the amount and nature of both organic and mineral soil components. The potential of mid-infrared (MIR) spectroscopy for predicting K[subscript]d values of a moderately hydrophobic pesticide, diuron, was investigated. A calibration set of 101 surface soils from South Australia was characterized for reference sorption data (K[subscript]d and K[subscript]oc) and foc as well as IR spectra. Partial least squares (PLS) regression was employed to harness the apparent complexity of IR spectra by reducing the dimensionality of the data. The MIR-PLS model was developed and validated by dividing the initial data set into corresponding calibration and validation sets. The developed model showed promising performance in predicting K[subscript]d values for diuron and proved to be a more efficacious than the K[subscript]oc model. The significant statistical superiority of the MIR-PLS model over the K[subscript]oc model was caused by some calcareous soils which were outliers for the K[subscript]oc model. Apart from these samples, the performance of the two compared models was essentially similar. The existence of carbonate peaks in the MIR-PLS loadings of the MIR based model suggested that carbonate minerals may interfere or affect the sorption. This requires further investigation. Some other concurrent studies suggested excellent quality of prediction of soil properties by NIR spectroscopy when applied to homogenous samples. Next, therefore, the performance of visible near-infrared (VNIR) and MIR spectroscopy was thoroughly compared for predicting both foc and diuron K[subscript]d values in soils. Some eleven calcareous soils were added to the initial calibration set for an attempt to further investigate the effect of carbonate minerals on sorption. MIR spectroscopy was clearly a more accurate predictor of foc and K[subscript]d in soils than VNIR spectroscopy. Close inspection of spectra showed that MIR spectra contain more relevant and straightforward information regarding the chemistry of OC and minerals than VNIR and thus useful in modelling sorption and OC content. Moreover, MIR spectroscopy provided a better (though still not great) estimation of sorption in calcareous soils than either VNIR spectroscopy or the K[subscript]oc model. Separate research is recommended to fully explore the unusual sorption behaviour of diuron in calcareous soils. In the last experiment, two dimensional (2D) nuclear magnetic resonance/infrared heterospectral correlation analyses revealed that MIR spectra contain specific and clear signals related to most of the major NMR-derived carbon types whereas NIR spectra contain only a few broad and overlapped peaks weakly associated with aliphatic carbons. 2D heterospectral correlation analysis facilitated accurate band assignments in the MIR and NIR spectra to the NMR-derived carbon types in isolated SOM. In conclusion, the greatest advantage of the MIR-PLS model is the direct estimation of Kd based on integrated properties of organic and mineral components. In addition, MIR spectroscopy is being used increasingly in predicting various soil properties including foc, and therefore, its simultaneous use for K[subscript]d estimation is a resource-effective and attractive practice. Moreover, it has the advantage of being fast and inexpensive with a high repeatability, and unlike the K[subscript]oc approach, MIR-PLS shows a better potential for extrapolating applications in data-poor regions. Where available, MIR spectroscopy is highly recommended over NIR spectroscopy. 2D correlation spectroscopy showed promising potential for providing rich insight and clarification into the thorough study of soil IR spectra. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1415416 / Thesis (Ph.D.) - University of Adelaide, School of Earth and Environmental Sciences, 2009

Research Doctorate - Doctor of Philosophy (PhD) / Amidst growing concerns about global warming, efforts to reduce atmospheric CO2 concentrations (i.e. C sequestration) have received widespread attention. One approach to C sequestration is to increase the amount of C stored in terrestrial ecosystems, through improved land management. Terrestrial ecosystems represent a critical element of the C interchange system, however a lack of understanding of the C cycle at regional and sub-regional scales means that they represent a source of primary uncertainty in the overall C budget. This thesis aims to address this deficiency by developing an understanding of catchment-scale processes critical for accurate quantification of C in the landscape. An investigation into the spatial and temporal dynamics of soil organic carbon (SOC) was conducted for a 150ha temperate grassland catchment in the Upper Hunter Valley, New South Wales, Australia. The major factors controlling the movement, storage, and loss of SOC were investigated, including climate, vegetation cover, soil redistribution processes, topography, land use, and soil type. This study falls into four broad areas. In the first part of this study the spatio-temporal dynamics of soil moisture and temperature at the catchment scale are assessed for a range of soil depths. Data recorded from a network of monitoring sites located throughout the study catchment was compared with independently derived soil moisture and temperature data sets. The data indicates that soil moisture and temperature in surface soil layers were highly dynamic, in their response to rainfall and incoming solar radiation, respectively. Deeper soil layers however were less dynamic, with longer lag times observed with increasing soil depth, as topography, soil type, and landscape position were the dominant controlling factors. Climate related variables are important factors affecting plant growth and net primary productivity. The second part of the study quantified spatial and temporal vegetation patterns using both field-based measurements of above-ground biomass and remotely sensed vegetation indices from the MODIS and Landsat TM 5 platforms. A strong and statistically significant relationship was found between climate variables and MODIS derived NDVI, leading to the development of a predictive vegetation cover model using ground-based soil moisture, soil temperature, and sunshine hours data. The ability of remotely sensed data to capture vegetation spatial patterns was found to be limited, while it was found to be a good predictor of temporal above-ground biomass trends, enabling net primary productivity to be quantified over the three-year study period. In the third part of the thesis soil redistribution patterns and erosion rates were quantified using the caesium-137 method and empirical and physically-based modelling approaches. The impact of soil redistribution processes on SOC distribution was investigated, and the amount of erosion derived SOC loss quantified. A significant proportion of SOC stored within the catchment was found below a soil depth of 0.30m, which is the depth of sampling set out in the IPCC and Australian Greenhouse Office guidelines for carbon accounting. Soil depth was identified as a key factor controlling the spatial distribution of SOC, which is in turn determined by position in the landscape (i.e. topography). The fourth and final part of the study describes how data on erosion derived SOC loss were used in conjunction with net primary productivity estimates, to establish a SOC balance. This involved mapping the spatial distribution of SOC using a high resolution digital elevation model of the catchment, in conjunction with soil depth measurements, and quantifying the total SOC store of the catchment. It was observed that temporal changes in SOC were minimal over the limited three-year study period, however, the continuity of catchment management practices over the previous decades suggest that steady-state conditions have perhaps been reached. The study concludes that the key to increasing the amount of SOC and enhancing carbon sequestration in the soil, is to increase the amount of SOC stored at depth within the soil profile, where factors such as soil moisture and temperature, which control decomposition rates, are less dynamic in space and time, and where SOC concentrations will be less vulnerable to changes occurring at the surface in response to global warming and climate change.

The fate and behaviour of hydrophobic organic compounds (e.g. pesticides) in soils are largely controlled by sorption processes. Recent findings suggest that the chemical properties of soil organic carbon (OC) significantly control the extent of sorption of such compounds in soil systems. However, currently there is no practical tool to integrate the effects of OC chemistry into sorption predictions. Therefore, the K [subscript]oc model, which relies on the soil OC content (foc), is used for predicting soil sorption coefficients (K[subscript]d) of pesticides. The K[subscript]oc model can be expressed as K[subscript]d = K[subscript]oc × foc, where K[subscript]oc is the OC-normalized sorption coefficient for the compound. Hence, there is a need for a prediction tool that can effectively capture the role of both the chemical structural variation of OC as well as foc in the prediction approach. Infrared (IR) spectroscopy offers a potential alternative to the K[subscript]oc approach because IR spectra contain information on the amount and nature of both organic and mineral soil components. The potential of mid-infrared (MIR) spectroscopy for predicting K[subscript]d values of a moderately hydrophobic pesticide, diuron, was investigated. A calibration set of 101 surface soils from South Australia was characterized for reference sorption data (K[subscript]d and K[subscript]oc) and foc as well as IR spectra. Partial least squares (PLS) regression was employed to harness the apparent complexity of IR spectra by reducing the dimensionality of the data. The MIR-PLS model was developed and validated by dividing the initial data set into corresponding calibration and validation sets. The developed model showed promising performance in predicting K[subscript]d values for diuron and proved to be a more efficacious than the K[subscript]oc model. The significant statistical superiority of the MIR-PLS model over the K[subscript]oc model was caused by some calcareous soils which were outliers for the K[subscript]oc model. Apart from these samples, the performance of the two compared models was essentially similar. The existence of carbonate peaks in the MIR-PLS loadings of the MIR based model suggested that carbonate minerals may interfere or affect the sorption. This requires further investigation. Some other concurrent studies suggested excellent quality of prediction of soil properties by NIR spectroscopy when applied to homogenous samples. Next, therefore, the performance of visible near-infrared (VNIR) and MIR spectroscopy was thoroughly compared for predicting both foc and diuron K[subscript]d values in soils. Some eleven calcareous soils were added to the initial calibration set for an attempt to further investigate the effect of carbonate minerals on sorption. MIR spectroscopy was clearly a more accurate predictor of foc and K[subscript]d in soils than VNIR spectroscopy. Close inspection of spectra showed that MIR spectra contain more relevant and straightforward information regarding the chemistry of OC and minerals than VNIR and thus useful in modelling sorption and OC content. Moreover, MIR spectroscopy provided a better (though still not great) estimation of sorption in calcareous soils than either VNIR spectroscopy or the K[subscript]oc model. Separate research is recommended to fully explore the unusual sorption behaviour of diuron in calcareous soils. In the last experiment, two dimensional (2D) nuclear magnetic resonance/infrared heterospectral correlation analyses revealed that MIR spectra contain specific and clear signals related to most of the major NMR-derived carbon types whereas NIR spectra contain only a few broad and overlapped peaks weakly associated with aliphatic carbons. 2D heterospectral correlation analysis facilitated accurate band assignments in the MIR and NIR spectra to the NMR-derived carbon types in isolated SOM. In conclusion, the greatest advantage of the MIR-PLS model is the direct estimation of Kd based on integrated properties of organic and mineral components. In addition, MIR spectroscopy is being used increasingly in predicting various soil properties including foc, and therefore, its simultaneous use for K[subscript]d estimation is a resource-effective and attractive practice. Moreover, it has the advantage of being fast and inexpensive with a high repeatability, and unlike the K[subscript]oc approach, MIR-PLS shows a better potential for extrapolating applications in data-poor regions. Where available, MIR spectroscopy is highly recommended over NIR spectroscopy. 2D correlation spectroscopy showed promising potential for providing rich insight and clarification into the thorough study of soil IR spectra. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1415416 / Thesis (Ph.D.) - University of Adelaide, School of Earth and Environmental Sciences, 2009

An important portion of the large northern permafrost soil organic carbon (SOC) pool might be released into the atmosphere as greenhouse gases following permafrost thawing and subsequent SOC decomposition under future warming conditions, resulting in a warming amplification known as the permafrost carbon feedback. Improved knowledge about the amount, composition and distribution of the permafrost SOC pool is essential when assessing the potential magnitude and timing of the permafrost carbon feedback. This study investigates and compares the SOC storage, composition and distribution in two contrasting permafrost environments: a lowland tundra area in NE Siberia (Tiksi study site), and an alpine area in the Russian Altai Mountains (Aktru Valley study site). Soil pedons were sampled down to 1 m depth and analyzed for key soil properties, i.e., DBD, water content, coarse fraction content, %OC, %IC, C/N ratios and δ¹⁵N values. These soil properties are upscaled by vertical subdivisions based on land cover classes. The role of geomorphology in the accumulation and distribution of SOC in the alpine study site is tested by using a landform and a combined land cover-land form upscaling approach. The estimated mean SOC storage in the upper meter of soils in the alpine site is 3.5 ± 0.8 kg C m¯² compared to 21.4 ± 3.2 kg C m¯² in the lowland tundra site (95% confidence intervals). The inclusion of geomorphology in the upscaling in some cases allows identification of SOC hotspots and areas with very low SOC storage within former land cover classes, therefore improving the landscape SOC storage distribution in the area. The much lower SOC stocks in the alpine site of Aktru Valley can be largely explained by the presence of extensive unvegetated areas in high altitudes (60%), the occurrence active layers deeper than the active soil formation, the enhanced SOM decomposition due to coarse grained, well-drained non-frozen soils, and the negligible occurrence of peatlands and buried organics. Instead, the lowland tundra site in NE Siberia presents important amounts of relatively undecomposed SOM in the permafrost layer. Thus, under future climate warming, alpine permafrost environments such as Aktru Valley may become a net C sink due to an upward shift of vegetation zones and an increase in plant productivity and soil development. Contrarily, lowland tundra areas such as Tiksi may become important C sources since the small increase in C uptake by photosynthetic plants will be outweighed by the thawing and subsequent decomposition of the much larger permafrost SOC pool.

FundaÃÃo Cearense de Apoio ao Desenvolvimento Cientifico e TecnolÃgico / Soil degradation can occur because of inadequate management, which may result in soil unproductive and, in certain situations, lead to the desertification process. One of the causes of degradation is overgrazing, causing loss of biodiversity of plant strata by the pressure of ramoneio and soil compaction by excessive trampling of animals, bringing implications to the ground in the physical, chemical and biological properties, thereby undermining the sustainability of the agroecosystem. An alternative to recover soil degradation is the practice of fallow, because that is easy to perform and inexpensive to restore soil properties that provides input of organic matter, important soil conditioner. With that aimed to prove the hypothesis that the time of 14 year fallow improves the physical and chemical soil in degraded areas in the process of desertification. The areas selected for the study are located in the municipality of IrauÃuba in the state of CearÃ, Brazil. Soil samples were taken in five areas of overgrazing, in five areas of fallow and in a forest area, is serving as the reference area. Physical and chemical analysis of the soil as soil density, total porosity, flocculation, pH, electrical conductivity, cation exchange capacity (CTC), total organic carbon, carbon management index and physical fractions of organic carbon, were done to assess the changes the physical and chemical attributes of these areas. The management of fallow showed improvements in physical and chemical soil when assessed after 14 years of withdrawal of animals, but when compared to the work done in the same area in 2009, with 7 years of fallow, evidence that the areas are still in the process of degradation. The organic carbon content was higher in the areas of fallow and the fraction with the highest proportion was the organic carbon associated to minerals. / A degradaÃÃo do solo pode ocorrer em funÃÃo do manejo inadequado, o qual pode acarretar na improdutividade do solo e, em certas situaÃÃes, levar ao processo de desertificaÃÃo. Uma das causas da degradaÃÃo à o sobrepastejo, ocasionando perda da biodiversidade do estrato vegetal pela pressÃo do ramoneio e compactaÃÃo do solo pelo pisoteio excessivo dos animais, trazendo implicaÃÃes ao solo nas propriedades fÃsicas, quÃmicas e biolÃgicas, comprometendo assim a sustentabilidade do agrossistema. Uma alternativa para recuperar a degradaÃÃo do solo à a prÃtica do pousio, pois essa à de fÃcil execuÃÃo e de baixo custo para restaurar as propriedades do solo que proporciona aporte de matÃria orgÃnica, importante condicionador do solo. Com isso, objetivou-se comprovar a hipÃtese que o tempo de pousio de 14 anos melhora os atributos fÃsicos e quÃmicos do solo em Ãreas degradadas em processo de desertificaÃÃo. As Ãreas selecionadas para o estudo estÃo localizadas no municÃpio de IrauÃuba, no Estado do CearÃ, Brasil. Foram realizadas coletas de solo em cinco Ãreas de sobrepastejo, em cinco Ãreas de pousio e em uma Ãrea de mata, esta servindo como Ãrea de referÃncia. AnÃlises fÃsicas e quÃmicas do solo como: densidade do solo, porosidade total, grau de floculaÃÃo, pH, condutividade elÃtrica, capacidade de troca de cÃtions (CTC), carbono orgÃnico total, Ãndice de manejo de carbono e fracionamento fÃsico do carbono orgÃnico, foram feitas a fim de avaliar as modificaÃÃes nos atributos fÃsicos e quÃmicos destas Ãreas. O manejo de pousio apresentou melhorias em atributos fÃsicos e quÃmicos do solo quando avaliado apÃs 14 anos de retirada dos animais, mas quando comparada ao trabalho realizado na mesma Ãrea em 2009, com 7 anos de pousio, evidÃncia que as Ãreas ainda estÃo em processo de degradaÃÃo. O teor de carbono orgÃnico foi maior nas Ãreas de pousio e a fraÃÃo com maior proporÃÃo foi o carbono orgÃnico associado aos minerais.

No description available.

Recent studies have shown that permafrost soils in the northern circumpolar region store almost twice as much carbon as the atmosphere. Since soil organic carbon (SOC) pools have large regional and landscape-level variability, detailed SOC inventories from across the northern permafrost region are needed to assess potential remobilization of SOC with permafrost degradation and to quantify the permafrost carbon-climate feedback on global warming. This thesis provides high-resolution data on SOC storage in five study areas located in undersampled regions of the continuous permafrost zone (Zackenberg in NE Greenland; Shalaurovo and Cherskiy in NE Siberia; Ary-Mas and Logata in Taymyr Peninsula). The emphasis throughout the five different study areas is put on SOC partitioning within the landscape and soil horizon levels as well as on soil forming processes under periglacial conditions. Our results indicate large differences in mean SOC 0–100 cm storage among study areas, ranging from 4.8 to 30.0 kg C m-2, highlighting the need to consider numerous factors as topography, geomorphology, land cover, soil texture, soil moisture, etc. in the assessment of landscape-level and regional SOC stock estimates. In the high arctic mountainous area of Zackenberg, the mean SOC storage is low due to the high proportion of bare grounds. The geomorphology based upscaling resulted in a c. 40% lower estimate compared to a land cover based upscaling (4.8 vs 8.3 kg C m-2, respectively). A landform approach provides a better tool for identifying hotspots of SOC burial in the landscape, which in this area corresponds to alluvial fan deposits in the foothills of the mountains. SOC burial by cryoturbation was much more limited and largely restricted to soils in the lower central valley. In the lowland permafrost study areas of Russia the mean SOC 0–100 cm storage ranged from 14.8 to 30.0 kg C m-2. Cryoturbation is the main burial process of SOC, storing on average c. 30% of the total landscape SOC 0–100 cm in deeper C-enriched pockets in all study areas. In Taymyr Peninsula, the mean SOC storage between the Ary-Mas and Logata study areas differed by c. 40% (14.8 vs 20.8 kg C m-2, respectively). We ascribe this mainly to the finer soil texture in the latter study area. Grain size analyses show that cryoturbation is most prominent in silt loam soils with high coarse silt to very fine sand fractions. However, in profiles and samples not affected by C-enrichment, C concentrations and densities were higher in silt loam soils with higher clay to medium silt fractions. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript.</p>

Although, the potential of management induced changes of soil organic matter, soil hydraulic properties (SHPs) and soil physical quality has been studied particularly in relation to tillage, few studies have evaluated combined effect of tillage, crop residue retention and cropping sequence, which are essential components of conservation agriculture (CA), on stratification and storage of soil organic matter, its effect on near-saturated soil hydraulic properties and soil physical quality in intensive cereal based irrigated cropping systems. Hence, the present study critically analyses the effects of CA on organic matter and hydro-physical properties of soil in a long-term CA field trial in NWIGP, India, which is one of the most fragile agro-ecosystems in the world. The objectives were (I) to investigate the stratification of soil organic carbon (SOC), total nitrogen (TN), C/N ratio and evaluate SR as an indicator of storage of SOC and TN and soil quality for different CA practices, (II) to assess the long-term effect of CA practices and short-term effect of crops on near-saturated soil hydraulic conductivity and water transmission properties, and (III) to assess the effect of CA practices on soil physical quality using capacitive and dynamic indicators. There were four treatments: (1) conventionally tilled rice-wheat cropping system (CT-RW), (2) reduced till CA-based rice-wheat-mungbean system (RT-RWMB), (3) no-till CA-based rice-wheat-mungbean system (NT-RWMB), and (4) no-till CA-based maize-wheat-mungbean system (NT-MWMB). To achieve these objectives, soil bulk density, SOC and TN were measured in an increment of 5 cm up to 30 cm soil depth. Furthermore, the effects of CA were also evaluated in terms of soil hydro-physical properties. Soil physical properties such as bulk density and soil aggregate distribution were evaluated in two cropping seasons along with near saturated hydraulic properties. Steady state infiltration rates were obtained at four pressure heads by hood infiltrometer consecutively over two cropping seasons, i.e. during harvest season of rice/maize (October 2017) and maximum crop growth stage of wheat (February 2018). Data were analysed in terms of soil hydraulic conductivity, k(h), flow weighted mean pore radius (r0), hydraulically active porosity (ε) and threshold pore radius (rbp), a new pore measure indicative of macropore stability derived by substituting soil’s bubble pressure in the capillary equation. Finally, the effects of CA on soil physical quality in terms of both capacitive and dynamic indicators, derived from soil moisture retention curve and field measured hydraulic conductivity, respectively, were assessed and related with crop yield to infer which indicator better represented the soil physical quality and its effect on crop yield under irrigated intensive cereal based cropping systems. Results showed that CA had profound impacts on distribution of SOC and TN in the soil profile. Significantly higher proportion of both SOC and TN were observed in the top soil in the CA-based treatments as compared with conventional intensive tillage-based treatment. The mean stratification ratio of both SOC and TN were found > 2 in CA-based treatments whereas the same was < 2 in intensive tillage-based treatment. Storage of SOC and TN in the 0-30 cm were found higher in CA-based treatments as compared with the intensive tillage-based treatment. These results on vertical distribution and storage of SOC and TN indicated a relatively better soil carbon sequestration and soil quality in CA-based treatment. The higher concentrations and storage of soil organic matter in CA-based treatments were, however, not translated into significantly (p < 0.05) lower bulk density due to probable compaction effect of no-tillage and harvest machinery and hydraulic pressure exerted by the flooded irrigation water. However, the increased soil organic matter in the top soil in CA-based treatments improved the soil aggregation significantly which helped in enhancing soil structural quality. Improvement in soil structure was reflected in relatively higher near saturated hydraulic conductivity in CA-based treatments. Irrespective of crop seasons, higher k(h) was observed under CA due to formation of macropores with better continuity, greater size and numbers as compared with conventional intensive tillage treatment. Moreover, higher r0 values were observed for a given k(h) for CA treatments suggesting that interaggregate pores are the dominant pathways of infiltration in CA. A relatively smaller temporal variation of rbp was indicative of a more stable macropore system established by rice-based CA as compared with maize-based CA. CA also enhanced hydraulically active macropores as compared with intensive tillage based conventional agriculture. Results also indicated that crops play an important role in relative distribution of the hydraulically active macropores in the root zone. The impact of CA on soil organic matter stratification and soil hydraulic properties were found to be expressed in terms of changes in soil physical quality. Soil moisture retention curves and pore size distributions under different treatments suggested higher soil water storage in structural pores in CA as compared with intensive tillage-based conventional agriculture. The impact of CA on soil physical quality and consequent effect on crop yield was found to be more expressed through dynamic indicators such as hydraulically active porosity rather than capacitive indicators derived from soil moisture retention curve. Overall, this study reveals that conservation agriculture has great potentials to reverse the intensive tillage induced degradation of soil resources in Indo-Gangetic Plains of India by improving the soil hydro-physical properties and soil physical quality.:Table of Contents Declaration i Declaration of Conformity ii Acknowledgements iii Table of Contents v List of Figures vii List of Tables xi List of Symbols, Abbreviations and Acronyms xiv Abstract xvii 1 Introduction and Background 1 1.1 General Overview 1 1.2 Statement of the Research Problem 5 1.3 Objectives 6 1.4 Research Flow and Chapter Description 7 2 Materials and Methods 9 2.1 Study Area Description 9 2.1.1 Study site 9 2.1.2 Climate 9 2.1.3 Soil 10 2.1.4 Treatments 10 2.1.5 Field Campaigns and Measurement/Analysis 14 2.2 Methods and Theoretical Considerations 14 2.2.1 Soil Sampling and Analysis 14 2.2.1.1 Calculation of Stratification Ratio 15 2.2.1.2 Calculation of SOC and TN Storage 15 2.2.1.3 Aggregate Size Distribution 16 2.2.2 Infiltration Measurements 16 2.2.3 Soil Moisture Retention Experiments 17 2.2.4 Derivation of Hydraulic Properties from Steady State Infiltration Rates 18 2.2.4.1 Near-Saturated Hydraulic Conductivity 18 2.2.4.2 Flow Weighted Mean Pore Radius 20 2.2.4.3 Equivalent Threshold pore Radius 21 2.2.4.4 Hydraulically Active Porosity 21 2.2.5 Determiation of Soil Moisture Charachtristics and Pore Size Distribution 22 2.2.6 Derivation of Soil Physical Quality Indicators 23 2.3 Statistics 25 3 Results and Discussion 26 3.1 Stratification and Storage of Soil Organic Matter 26 3.1.1 Bulk Density 26 3.1.2 Concenrations of SOC 27 3.1.3 Concentrations of TN 28 3.1.4 C/N Ratio 29 3.1.5 Stratification Ratio of SOC, TN and C/N Ratio 30 3.1.6 Storage of SOC and TN 33 3.1.7 Discussion 34 3.1.8 Summary of Results 39 3.2 Soil Hydro-Physical Properties 40 3.2.1 Soil Physical Properties 40 3.2.2 Near-Saturated Hydraulic Conductivity 43 3.2.3 Soil Pore Characteristics-Conductivity Relationship 47 3.2.4 Hydrailically active Porosity 51 3.2.5 Summary of Results 54 3.3 Soil Physical Quality (SPQ) 56 3.3.1 Soil Moisture Retention Curve (SMRC) 56 3.3.2 Soil Pore Size Distribution (SPSD) 58 3.3.3 Capacitive Indicators 59 3.3.4 Dynamic Indicators 60 3.3.5 Relationship between capacitive indicators of SPQ with dynamic indicators of SPQ and long-term crop yield 60 3.3.6 Relationship between dynamic indicator of SPQ (hydraulically active porosity) and Long-term Crop Yield 62 3.3.7 Summary of Results 64 4 Synthesis and Conclusions 65 5 Implications and Outlook 69 References 71

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 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.