Soil respiration represents the second largest CO 2 flux from terrestrial ecosystems to the atmos... more Soil respiration represents the second largest CO 2 flux from terrestrial ecosystems to the atmosphere, and a small rise could significantly contribute to further increase in atmospheric CO 2 . Unfortunately, the extent of this effect cannot be quantified reliably, and the outcomes of experiments designed to study soil respiration remain notoriously unpredictable. In this context, the mathematical simulations described in this article suggest that assumptions of linearity and presumed irrelevance of micro-scale heterogeneity, commonly made in quantitative models of microbial growth in subsurface environments and used in carbon stock models, do not appear warranted. Results indicate that microbial growth is non-linear and, at given average nutrient concentrations, strongly dependent on the microscale distribution of both nutrients and microbes. These observations have far-reaching consequences, in terms of both experiments and theory. They indicate that traditional, macroscopic soil measurements are inadequate to predict microbial responses, in particular to rising temperature conditions, and that an explicit account is required of microscale heterogeneity. Furthermore, models should evolve beyond traditional, but overly simplistic, assumptions of linearity of microbial responses to bulk nutrient concentrations. The development of a new generation of models along these lines, and in particular incorporating upscaled information about microscale processes, will undoubtedly be challenging, but appears to be key to understanding the extent to which soil carbon mineralization could further accelerate climate change.
Plasmid (mainly as the supercoiled form) and linear chromosomal DNA were compared in terms of the... more Plasmid (mainly as the supercoiled form) and linear chromosomal DNA were compared in terms of their mechanisms and degree of adsorption on three clay minerals, kaolinite, montmorillonite, and illite. Based on adsorption isotherms on Ca-clays, adsorption was complete for both plasmid and linear DNA at low concentrations of DNA. Amounts of DNA adsorbed on illite in water were at least 2-fold greater than the amounts adsorbed on kaolinite and montmorillonite, regardless of whether excess divalent Ca (5 mM) was present in the solution. Increasing the concentration of DNA (>25 µg mL -1 ) increased the adsorption of linear DNA, whereas the adsorption of plasmid DNA molecules decreased, probably as the result of selfaggregation in solution. Titration of acidic groups of DNA showed a narrow range of strong acidity for the plasmid form, whereas the pH of linear chromosomal acidic groups ranged from very low to neutral or slightly alkaline pKa values. The amount of acidic groups per gram of DNA was higher in linear DNA (13.4 cmol g -1 ) than in supercoiled plasmid DNA (1.8 cmol g -1 ). Direct observations of plasmid DNA adsorbed on clay minerals by low temperature scanning electron microscopy (LTSEM) indicated that these molecules could act as bridges between clay domains by the ends of the supercoiled molecule. The location and strength of the acidic groups of DNA determine the interaction between clay and DNA. Supercoiled plasmid DNA interacts by a low number of strongly acidic groups, presumably located at the maximum of bending of the double strand where a high charge density exists. Linear chromosomal molecules appear to attach on the clay surface and edges, as demonstrated by previous observations et al. FEMS Microbiol Letters 1992, 97, 31), through acidic groups distributed along the DNA molecules. Such differences in interactions between clay and DNA should influence the accessibility to nucleases and persistence of DNA in soil environments.
ABSTRACT In vegetative filter strips used to intercept pesticides present in run-off, particulate... more ABSTRACT In vegetative filter strips used to intercept pesticides present in run-off, particulate organic matter derived from the vegetation plays an important function in pesticide sorption processes, because it accumulates at the soil surface and quickly responds to changes in land use. Two herbicides with contrasted properties: isoproturon, moderately hydrophobic (log Kow= 2.5), diflufenican, strongly hydrophobic (log Kow= 4.9), and isopropylaniline, a metabolite of isoproturon, were used to characterize the sorption and desorption properties of POM originating from soils under three different land uses: a cropped plot under conventional wheat/maize rotation, an adjacent 10-year-old grassed strip and a nearby 80-year-old oak/chestnut forest. Chemical structural composition information obtained from solid-state 13C CPMAS NMR and estimation of hydrophobicity from contact angle measurements were used to explain the different sorption capacities of POM according to their size and origins. Sorption of isoproturon and diflufenican increased with hydrophobicity of POM, which was greater in the forest soil. Aromaticity of POM was positively correlated to sorption coefficients (Koc). Desorption of the more hydrophobic compounds, diflufenican and isopropylaniline was weak for all POM fractions, regardless of their origin and size. On the other hand, desorption of isoproturon depended on land use and POM characteristics. The sorption capacities of POM were not only controlled by their chemical composition, but also by their size, due to a greater number of sorptive sites related to a greater surface area with decreasing particle-size.
Soil organic matter stabilisation by the mineral phase can take place through adsorption and aggr... more Soil organic matter stabilisation by the mineral phase can take place through adsorption and aggregation. For subsoil horizons, most studies addressed stabilisation by interaction with minerals. In this study, we distinguished both processes, i.e. organic carbon (OC) adsorption onto clay-sized particles and OC occlusion in silt-size aggregates. The objective was to evaluate their relative importance for organic matter storage and
The distribution of organic matter in soil aggregates was investigated by fractionating aggregate... more The distribution of organic matter in soil aggregates was investigated by fractionating aggregates and measuring carbon contents. The distribution of recently incorporated organic carbon was analyzed using 13C natural abundance. The soils of the experiment, which previously only had C3 vegetation, were cropped to maize, a C4 plant, for 6 or 23 years.
A resistant soil organic residue, 'humin', has been analysed by solid-state 13 C-NMR and pyrolysi... more A resistant soil organic residue, 'humin', has been analysed by solid-state 13 C-NMR and pyrolysis followed by molecular and 13 C determination of the pyrolysate alkane-alkene fraction. The results show the occurrence of highly aliphatic, straight-chain biopolymer material as a substantial part of soil organic matter. They confirm the hypothesis by which a part of soil organic matter can be formed by selective preservation of resistant highly aliphatic microbial polymers. This pathway comes in addition to the longstanding neogenesis hypothesis involving recondensation of small polar molecules such as amino acids, carbohydrates and phenols.
Although many "in vitro" experiments demonstrated that Fe and Al oxides stabilise organ... more Although many "in vitro" experiments demonstrated that Fe and Al oxides stabilise organic carbon (OC) in soil, very few field studies validated that result, and none of them took into account the influence of scale and that of depth on the relationships linking OC to metal oxides in soil. In the present study, we characterised the latter as a function
ABSTRACT Liming is a common agricultural practice worldwide, used for increasing productivity in ... more ABSTRACT Liming is a common agricultural practice worldwide, used for increasing productivity in acid agricultural soils. Liming reduces Al saturation and toxicity and/or increases pH up to values where the availability of nutrients is higher. The effect of this practice on soil properties has been extensively studied, with focus of most studies upon pH, exchangeable cations and productivity. In turn, the effects of liming on soil organic C (SOC) stocks still remain poorly known. The net effect on SOC can be the result of several factors: first, liming increases the soil biological activity, thus favoring the mineralization of organic matter, which should result in CO2 losses and a decrease of the SOC stocks. Second, liming ameliorates soil structure, increasing the stability of clay assemblages and clay-organic matter bonds, which should bring an increase in SOC physical and physicochemical protection. Finally, as liming ameliorates soil conditions to plant growth, plant productivity increases and also the return of C inputs to soil, thus potentially increasing SOC concentrations. The net effect of these processes is not well understood yet. Still, some overall trends can be deduced from data currently available in the literature. Liming does modify SOC stocks, increasing them in most cases, what seems to be caused by higher C inputs to limed soils due to increased productivity. Reductions in SOC have also been reported, probably in connexion with increased mineralization, whereas the role of improved soil structure remains unclear. Overall, these insights are deduced from published data which are still scarce, so we encourage the scientific community to synthesize unpublished SOC data from existing in situ experiments, in order to enlarge the span of experimental conditions and gain knowledge about the role of such a widespread agricultural practice on SOC stocks.
A B S T R A C T In this paper, we attempt to analyse the respective influences of land-use and cl... more A B S T R A C T In this paper, we attempt to analyse the respective influences of land-use and climate changes on the global and regional balances of soil organic carbon (SOC) stocks. Two time periods are analysed: the historical period 1901-2000 and the period 2000-2100. The historical period is analysed using a synthesis of published data as well as new global and regional model simulations, and the future is analysed using models only. Historical land cover changes have resulted globally in SOC release into the atmosphere. This human induced SOC decrease was nearly balanced by the net SOC increase due to higher CO 2 and rainfall. Mechanization of agriculture after the 1950s has accelerated SOC losses in croplands, whereas development of carbon-sequestering practices over the past decades may have limited SOC loss from arable soils. In some regions (Europe, China and USA), croplands are currently estimated to be either a small C sink or a small source, but not a large source of CO 2 to the atmosphere.
Little is known about spatial variability of microbial activity, particularly at microscales. Thi... more Little is known about spatial variability of microbial activity, particularly at microscales. This is especially true for the fate and degradation of pesticides. The objective was to sample soil from micro to field scales and apply geostatistics on the potential mineralization of a widely used herbicide 2,4-dichlorophenoxy acetic acid (2,4-D; C 8 H 6 Cl 2 O 3 ). Soil cores were sampled in the plow layer of a cultivated soil with a systematic sampling procedure. In a first experiment 2,4-D mineralization was measured on 39 crushed cores and we analyzed variability of mineralization at the field scale, from decameter to meter and from meter to decimeter scale. In a second experiment, 432 soil cubes (about 216 mm 3 ) were used to study the variability of mineralization at the ''microhabitat'' scale from meter to millimeter. The spatial dependence of 2,4-D mineralization was first quantified by computing an empirical variogram function. Spatial independence was then tested by comparing the empirical variogram function to its individual confidence bounds at 95% level obtained under independence assumption by a Monte-Carlo Method.
Microbial communities exist and are active in a complex 3-D physical framework which can cause a ... more Microbial communities exist and are active in a complex 3-D physical framework which can cause a variety of micro-environments to develop that are more or less suitable for microbial growth, activity and survival. If there is a significant microbial biogeography at the pore scale in soil, then the relationship between microbial diversity and ecosystem function is likely to be affected by micro-environmental variations at the pore scale. In this laboratory study we show that there is a significant pore-scale microbial biogeography by labelling microbial communities in different pore size classes of undisturbed soil cores with 13 C-labelled fructose (a soluble, labile substrate). This was achieved by adding the substrate solution to the samples at different matric potentials (À100 kPa, À3.15 kPa and À1 kPa; placing the substrate in pores with maximum diameter of 0.97, 9.7 and 97 mm, respectively) and incubating the samples for two weeks. The mineralisation of soil organic carbon and fructose was measured as CO 2 and 13 CeCO 2 , respectively, in the jar headspace throughout the incubation. At the end of incubation we analysed the total microbial community structure using PLFA. The structure of microbial communities in different pore size classes was measured by PLFA stable isotope probing. Total PLFA profiles suggested that there was little effect of the incubation conditions on microbial community structure. However, labelled PLFA profiles showed that microbial community structure differed significantly among pore size classes, the differences being due primarily to variations in the abundance of mono-unsaturated lipids (Gram-biomarkers) and of the fungal biomarker (C18:2(9,12)). This is the first evidence for a significant microbial biogeography at the pore scale in undisturbed soil cores.
The distribution and composition of carbohydrates were investigated in slaking-resistant aggregat... more The distribution and composition of carbohydrates were investigated in slaking-resistant aggregates and in particle size fractions of two silty soils from the Paris Basin, France. Three hydrolysis procedures, involving hot water, dilute acid and concentrated acid were used and hydrolysates were analysed on a HPLC system to further assess the origin of the carbohydrates. Results showed the predominantly plant origin of the particulate organic matter (>50 mm) and the microbial origin of the clay + silt fraction (<50 mm) carbohydrates. Total organic C content and carbohydrate content, both increased with aggregate size. The proportion of plant-derived carbohydrates also increased with aggregate size, which supported previous results on the distribution of particulate organic matter in aggregates. The clay + silt fraction (<50 mm), located within stable aggregates >50 mm, was enriched in carbohydrates produced by microorganisms. This is consistent with the hypothesis that aggregate stability is mediated partly by the extracellular polysaccharides of microorganisms developing on plant debris occluded within the aggregates. #
polysaccharide secretion (EPS) is a salient feature of many soil bacteria. This study investigate... more polysaccharide secretion (EPS) is a salient feature of many soil bacteria. This study investigates whether an EPS shell modulates the diffusion rate of nutrients in the immediate microenvironment of soil microorganisms. The diffusion of glucose through pure microbial polysaccharides xanthan and dextran, and through kaolinite and EPS-amended kaolinite was measured at several water potentials using a steady state method. A specific device was developed to simultaneously monitor water potential (Y) using polyethylene glycol (PEG) solutions, and measure diffusion rates. The diffusion rate of glucose increased with increasing I and volumetric water content in both polysaccharides. At all Y values studied, glucose diffused faster in EPS or in EPS-amended clay than in pure clay, due to the higher volumetric water content of EPS and their water-saturated porosity. Water retention and diffusion characteristics in EPS may thus help soil bacteria to maintain physiological functions at low water potential.
To assess the in¯uence of the pore space structure and organic matter on denitri®cation, a compar... more To assess the in¯uence of the pore space structure and organic matter on denitri®cation, a comparative study was performed on clods in a soil under cropped and pasture managements. For each management, the potential denitri®cation rate was estimated. Denitri®cation under oxic conditions was also measured on 100 clods, which were saturated with KNO 3 solution (4 g l À 1 ). Size and density fractions of the soil were separated, and the C and N contents of their particulate organic matter were determined. Clod porosities were measured and the distributions of distances of any point within the clod to the nearest air-®lled pore were estimated on 20 thin sections for each soil management. Potential denitri®cation rates were similar (105 Â 10 À 11 and 98 Â 10 À 11 mol N 2 O kg À 1 dry soil s À 1 for pasture and cropped soil, respectively). The mean denitri®cation rate under oxic conditions was only equal to 0.14% of the potential denitri®cation rate for pasture, whereas it was about 2.1% for cropped soil. The total and soluble organic C content was signi®cantly higher in pasture than in cropped soil clods. The quantity or the quality of organic matter fractions did not explain the dierence in denitri®cation activities. Even if macroporosity represented a small fraction of the total porosity in both soils, the dierences in macropore distribution induced by soil management practices led to signi®cantly dierent maximal distances between any clod point and the nearest air-®lled pore (8 and 14 mm for pasture and cropped soils, respectively). Consequently, we demonstrated that the pore space structure appears to be the major factor explaining the dierence in mean denitri®cation rates between pasture and cropped soil clods, while the distribution of particulate organic matter is suspected to be involved in the dierences in denitrifying activity distribution between the clods of the two soils. #
Tillage has been reported to reduce organic matter concentrations and increase organic matter tur... more Tillage has been reported to reduce organic matter concentrations and increase organic matter turnover rates to a variable extent. The change of soil climate and the incorporation of aboveground C inputs within the soil lead to no unique effect on biodegradation rates, because of their strong interaction with the regional climate and the soil physical properties. The periodical perturbation of soil structure by tools and the subsequent drying±rewetting cycles may be the major factor increasing organic matter decomposition rates by exposing the organic matter that is physically protected in microaggregates to biodegradation. This paper reviews the assessed effects of tillage on organic matter, the scale, extent and mechanisms of physical protection of organic matter in soils. #
Land use changes result in significant decreases in soil organic matter stocks due to enhanced mi... more Land use changes result in significant decreases in soil organic matter stocks due to enhanced mineralization attributed to increased tillage, and due to decreased organic matter inputs. The current paradigm of soil organic matter dynamics suggests that decreasing organic matter stocks are also associated with shifts in organic matter quality to more resistant fractions as the more labile pools are decomposed. The objectives of the current study were to characterize changes in clay-associated organic matter quality in a soil C depletion sequence in terms of thermal properties, and to thermally characterize peroxideresistant organic matter. Clay-sized fractions were isolated from a sequence of soils ranging from native forest to long-term bare fallow, and analyzed using thermogravimetry (TG) and differential scanning calorimetry (DSC) before and after hydrogen peroxide treatment. Differential scanning calorimetry traces had exothermic maxima near 285 and 333 8C, consistent with analyses of chemically extracted organic matter and humic substances reported in the literature. Peak fitting analyses showed that these maxima consisted of several hidden peaks, but their interpretation is problematic. The qualitative shifts in the thermal properties of clay-associated organic matter with differing land use observed in the DSC traces were quantified using cultivation and fallow treatment to forest ratios. Thermogravimetric mass loss ratios were greater in the thermally labile (180-310 8C) exothermic region than in the more thermally resistant (310-450 8C) exothermic region. Similarly, ratios of peak heights and areas for the fitted DSC peaks were higher for the 324 8C peak, compared to peaks at 257 and 284 8C. The higher ratios indicate that the more thermally resistant organic matter has been retained and the more thermally labile organic matter is lost. The observed shift in the distribution of organic matter from thermally labile to thermally resistant fractions from forest to long-term bare fallow clay samples is consistent with the current paradigm of decomposition consisting of a shift to more biologically resistant fractions with increasing time under cultivation and decreasing organic matter inputs. These results suggest that the thermal properties of clay-associated organic matter are related to their biological decomposability. Hydrogen peroxide treatment of clay samples removed approximately 87% of the initial organic C in all samples, and peroxide-resistant organic matter was found to be more thermally stable than the whole. However, thermal analyses of the peroxide-resistant fraction did not show any changes in thermal properties with decreases 0016-7061/$ -see front matter D Geoderma 129 www.elsevier.com/locate/geoderma in total C content. Rather than isolating a biologically resistant pool of organic matter, peroxide-resistant organic matter may isolate an inert pool of soil organic matter with thermal properties consistent with the presence of black carbon, which remains unchanged by changes in land use. D
Soil respiration represents the second largest CO 2 flux from terrestrial ecosystems to the atmos... more Soil respiration represents the second largest CO 2 flux from terrestrial ecosystems to the atmosphere, and a small rise could significantly contribute to further increase in atmospheric CO 2 . Unfortunately, the extent of this effect cannot be quantified reliably, and the outcomes of experiments designed to study soil respiration remain notoriously unpredictable. In this context, the mathematical simulations described in this article suggest that assumptions of linearity and presumed irrelevance of micro-scale heterogeneity, commonly made in quantitative models of microbial growth in subsurface environments and used in carbon stock models, do not appear warranted. Results indicate that microbial growth is non-linear and, at given average nutrient concentrations, strongly dependent on the microscale distribution of both nutrients and microbes. These observations have far-reaching consequences, in terms of both experiments and theory. They indicate that traditional, macroscopic soil measurements are inadequate to predict microbial responses, in particular to rising temperature conditions, and that an explicit account is required of microscale heterogeneity. Furthermore, models should evolve beyond traditional, but overly simplistic, assumptions of linearity of microbial responses to bulk nutrient concentrations. The development of a new generation of models along these lines, and in particular incorporating upscaled information about microscale processes, will undoubtedly be challenging, but appears to be key to understanding the extent to which soil carbon mineralization could further accelerate climate change.
Plasmid (mainly as the supercoiled form) and linear chromosomal DNA were compared in terms of the... more Plasmid (mainly as the supercoiled form) and linear chromosomal DNA were compared in terms of their mechanisms and degree of adsorption on three clay minerals, kaolinite, montmorillonite, and illite. Based on adsorption isotherms on Ca-clays, adsorption was complete for both plasmid and linear DNA at low concentrations of DNA. Amounts of DNA adsorbed on illite in water were at least 2-fold greater than the amounts adsorbed on kaolinite and montmorillonite, regardless of whether excess divalent Ca (5 mM) was present in the solution. Increasing the concentration of DNA (>25 µg mL -1 ) increased the adsorption of linear DNA, whereas the adsorption of plasmid DNA molecules decreased, probably as the result of selfaggregation in solution. Titration of acidic groups of DNA showed a narrow range of strong acidity for the plasmid form, whereas the pH of linear chromosomal acidic groups ranged from very low to neutral or slightly alkaline pKa values. The amount of acidic groups per gram of DNA was higher in linear DNA (13.4 cmol g -1 ) than in supercoiled plasmid DNA (1.8 cmol g -1 ). Direct observations of plasmid DNA adsorbed on clay minerals by low temperature scanning electron microscopy (LTSEM) indicated that these molecules could act as bridges between clay domains by the ends of the supercoiled molecule. The location and strength of the acidic groups of DNA determine the interaction between clay and DNA. Supercoiled plasmid DNA interacts by a low number of strongly acidic groups, presumably located at the maximum of bending of the double strand where a high charge density exists. Linear chromosomal molecules appear to attach on the clay surface and edges, as demonstrated by previous observations et al. FEMS Microbiol Letters 1992, 97, 31), through acidic groups distributed along the DNA molecules. Such differences in interactions between clay and DNA should influence the accessibility to nucleases and persistence of DNA in soil environments.
ABSTRACT In vegetative filter strips used to intercept pesticides present in run-off, particulate... more ABSTRACT In vegetative filter strips used to intercept pesticides present in run-off, particulate organic matter derived from the vegetation plays an important function in pesticide sorption processes, because it accumulates at the soil surface and quickly responds to changes in land use. Two herbicides with contrasted properties: isoproturon, moderately hydrophobic (log Kow= 2.5), diflufenican, strongly hydrophobic (log Kow= 4.9), and isopropylaniline, a metabolite of isoproturon, were used to characterize the sorption and desorption properties of POM originating from soils under three different land uses: a cropped plot under conventional wheat/maize rotation, an adjacent 10-year-old grassed strip and a nearby 80-year-old oak/chestnut forest. Chemical structural composition information obtained from solid-state 13C CPMAS NMR and estimation of hydrophobicity from contact angle measurements were used to explain the different sorption capacities of POM according to their size and origins. Sorption of isoproturon and diflufenican increased with hydrophobicity of POM, which was greater in the forest soil. Aromaticity of POM was positively correlated to sorption coefficients (Koc). Desorption of the more hydrophobic compounds, diflufenican and isopropylaniline was weak for all POM fractions, regardless of their origin and size. On the other hand, desorption of isoproturon depended on land use and POM characteristics. The sorption capacities of POM were not only controlled by their chemical composition, but also by their size, due to a greater number of sorptive sites related to a greater surface area with decreasing particle-size.
Soil organic matter stabilisation by the mineral phase can take place through adsorption and aggr... more Soil organic matter stabilisation by the mineral phase can take place through adsorption and aggregation. For subsoil horizons, most studies addressed stabilisation by interaction with minerals. In this study, we distinguished both processes, i.e. organic carbon (OC) adsorption onto clay-sized particles and OC occlusion in silt-size aggregates. The objective was to evaluate their relative importance for organic matter storage and
The distribution of organic matter in soil aggregates was investigated by fractionating aggregate... more The distribution of organic matter in soil aggregates was investigated by fractionating aggregates and measuring carbon contents. The distribution of recently incorporated organic carbon was analyzed using 13C natural abundance. The soils of the experiment, which previously only had C3 vegetation, were cropped to maize, a C4 plant, for 6 or 23 years.
A resistant soil organic residue, 'humin', has been analysed by solid-state 13 C-NMR and pyrolysi... more A resistant soil organic residue, 'humin', has been analysed by solid-state 13 C-NMR and pyrolysis followed by molecular and 13 C determination of the pyrolysate alkane-alkene fraction. The results show the occurrence of highly aliphatic, straight-chain biopolymer material as a substantial part of soil organic matter. They confirm the hypothesis by which a part of soil organic matter can be formed by selective preservation of resistant highly aliphatic microbial polymers. This pathway comes in addition to the longstanding neogenesis hypothesis involving recondensation of small polar molecules such as amino acids, carbohydrates and phenols.
Although many "in vitro" experiments demonstrated that Fe and Al oxides stabilise organ... more Although many "in vitro" experiments demonstrated that Fe and Al oxides stabilise organic carbon (OC) in soil, very few field studies validated that result, and none of them took into account the influence of scale and that of depth on the relationships linking OC to metal oxides in soil. In the present study, we characterised the latter as a function
ABSTRACT Liming is a common agricultural practice worldwide, used for increasing productivity in ... more ABSTRACT Liming is a common agricultural practice worldwide, used for increasing productivity in acid agricultural soils. Liming reduces Al saturation and toxicity and/or increases pH up to values where the availability of nutrients is higher. The effect of this practice on soil properties has been extensively studied, with focus of most studies upon pH, exchangeable cations and productivity. In turn, the effects of liming on soil organic C (SOC) stocks still remain poorly known. The net effect on SOC can be the result of several factors: first, liming increases the soil biological activity, thus favoring the mineralization of organic matter, which should result in CO2 losses and a decrease of the SOC stocks. Second, liming ameliorates soil structure, increasing the stability of clay assemblages and clay-organic matter bonds, which should bring an increase in SOC physical and physicochemical protection. Finally, as liming ameliorates soil conditions to plant growth, plant productivity increases and also the return of C inputs to soil, thus potentially increasing SOC concentrations. The net effect of these processes is not well understood yet. Still, some overall trends can be deduced from data currently available in the literature. Liming does modify SOC stocks, increasing them in most cases, what seems to be caused by higher C inputs to limed soils due to increased productivity. Reductions in SOC have also been reported, probably in connexion with increased mineralization, whereas the role of improved soil structure remains unclear. Overall, these insights are deduced from published data which are still scarce, so we encourage the scientific community to synthesize unpublished SOC data from existing in situ experiments, in order to enlarge the span of experimental conditions and gain knowledge about the role of such a widespread agricultural practice on SOC stocks.
A B S T R A C T In this paper, we attempt to analyse the respective influences of land-use and cl... more A B S T R A C T In this paper, we attempt to analyse the respective influences of land-use and climate changes on the global and regional balances of soil organic carbon (SOC) stocks. Two time periods are analysed: the historical period 1901-2000 and the period 2000-2100. The historical period is analysed using a synthesis of published data as well as new global and regional model simulations, and the future is analysed using models only. Historical land cover changes have resulted globally in SOC release into the atmosphere. This human induced SOC decrease was nearly balanced by the net SOC increase due to higher CO 2 and rainfall. Mechanization of agriculture after the 1950s has accelerated SOC losses in croplands, whereas development of carbon-sequestering practices over the past decades may have limited SOC loss from arable soils. In some regions (Europe, China and USA), croplands are currently estimated to be either a small C sink or a small source, but not a large source of CO 2 to the atmosphere.
Little is known about spatial variability of microbial activity, particularly at microscales. Thi... more Little is known about spatial variability of microbial activity, particularly at microscales. This is especially true for the fate and degradation of pesticides. The objective was to sample soil from micro to field scales and apply geostatistics on the potential mineralization of a widely used herbicide 2,4-dichlorophenoxy acetic acid (2,4-D; C 8 H 6 Cl 2 O 3 ). Soil cores were sampled in the plow layer of a cultivated soil with a systematic sampling procedure. In a first experiment 2,4-D mineralization was measured on 39 crushed cores and we analyzed variability of mineralization at the field scale, from decameter to meter and from meter to decimeter scale. In a second experiment, 432 soil cubes (about 216 mm 3 ) were used to study the variability of mineralization at the ''microhabitat'' scale from meter to millimeter. The spatial dependence of 2,4-D mineralization was first quantified by computing an empirical variogram function. Spatial independence was then tested by comparing the empirical variogram function to its individual confidence bounds at 95% level obtained under independence assumption by a Monte-Carlo Method.
Microbial communities exist and are active in a complex 3-D physical framework which can cause a ... more Microbial communities exist and are active in a complex 3-D physical framework which can cause a variety of micro-environments to develop that are more or less suitable for microbial growth, activity and survival. If there is a significant microbial biogeography at the pore scale in soil, then the relationship between microbial diversity and ecosystem function is likely to be affected by micro-environmental variations at the pore scale. In this laboratory study we show that there is a significant pore-scale microbial biogeography by labelling microbial communities in different pore size classes of undisturbed soil cores with 13 C-labelled fructose (a soluble, labile substrate). This was achieved by adding the substrate solution to the samples at different matric potentials (À100 kPa, À3.15 kPa and À1 kPa; placing the substrate in pores with maximum diameter of 0.97, 9.7 and 97 mm, respectively) and incubating the samples for two weeks. The mineralisation of soil organic carbon and fructose was measured as CO 2 and 13 CeCO 2 , respectively, in the jar headspace throughout the incubation. At the end of incubation we analysed the total microbial community structure using PLFA. The structure of microbial communities in different pore size classes was measured by PLFA stable isotope probing. Total PLFA profiles suggested that there was little effect of the incubation conditions on microbial community structure. However, labelled PLFA profiles showed that microbial community structure differed significantly among pore size classes, the differences being due primarily to variations in the abundance of mono-unsaturated lipids (Gram-biomarkers) and of the fungal biomarker (C18:2(9,12)). This is the first evidence for a significant microbial biogeography at the pore scale in undisturbed soil cores.
The distribution and composition of carbohydrates were investigated in slaking-resistant aggregat... more The distribution and composition of carbohydrates were investigated in slaking-resistant aggregates and in particle size fractions of two silty soils from the Paris Basin, France. Three hydrolysis procedures, involving hot water, dilute acid and concentrated acid were used and hydrolysates were analysed on a HPLC system to further assess the origin of the carbohydrates. Results showed the predominantly plant origin of the particulate organic matter (>50 mm) and the microbial origin of the clay + silt fraction (<50 mm) carbohydrates. Total organic C content and carbohydrate content, both increased with aggregate size. The proportion of plant-derived carbohydrates also increased with aggregate size, which supported previous results on the distribution of particulate organic matter in aggregates. The clay + silt fraction (<50 mm), located within stable aggregates >50 mm, was enriched in carbohydrates produced by microorganisms. This is consistent with the hypothesis that aggregate stability is mediated partly by the extracellular polysaccharides of microorganisms developing on plant debris occluded within the aggregates. #
polysaccharide secretion (EPS) is a salient feature of many soil bacteria. This study investigate... more polysaccharide secretion (EPS) is a salient feature of many soil bacteria. This study investigates whether an EPS shell modulates the diffusion rate of nutrients in the immediate microenvironment of soil microorganisms. The diffusion of glucose through pure microbial polysaccharides xanthan and dextran, and through kaolinite and EPS-amended kaolinite was measured at several water potentials using a steady state method. A specific device was developed to simultaneously monitor water potential (Y) using polyethylene glycol (PEG) solutions, and measure diffusion rates. The diffusion rate of glucose increased with increasing I and volumetric water content in both polysaccharides. At all Y values studied, glucose diffused faster in EPS or in EPS-amended clay than in pure clay, due to the higher volumetric water content of EPS and their water-saturated porosity. Water retention and diffusion characteristics in EPS may thus help soil bacteria to maintain physiological functions at low water potential.
To assess the in¯uence of the pore space structure and organic matter on denitri®cation, a compar... more To assess the in¯uence of the pore space structure and organic matter on denitri®cation, a comparative study was performed on clods in a soil under cropped and pasture managements. For each management, the potential denitri®cation rate was estimated. Denitri®cation under oxic conditions was also measured on 100 clods, which were saturated with KNO 3 solution (4 g l À 1 ). Size and density fractions of the soil were separated, and the C and N contents of their particulate organic matter were determined. Clod porosities were measured and the distributions of distances of any point within the clod to the nearest air-®lled pore were estimated on 20 thin sections for each soil management. Potential denitri®cation rates were similar (105 Â 10 À 11 and 98 Â 10 À 11 mol N 2 O kg À 1 dry soil s À 1 for pasture and cropped soil, respectively). The mean denitri®cation rate under oxic conditions was only equal to 0.14% of the potential denitri®cation rate for pasture, whereas it was about 2.1% for cropped soil. The total and soluble organic C content was signi®cantly higher in pasture than in cropped soil clods. The quantity or the quality of organic matter fractions did not explain the dierence in denitri®cation activities. Even if macroporosity represented a small fraction of the total porosity in both soils, the dierences in macropore distribution induced by soil management practices led to signi®cantly dierent maximal distances between any clod point and the nearest air-®lled pore (8 and 14 mm for pasture and cropped soils, respectively). Consequently, we demonstrated that the pore space structure appears to be the major factor explaining the dierence in mean denitri®cation rates between pasture and cropped soil clods, while the distribution of particulate organic matter is suspected to be involved in the dierences in denitrifying activity distribution between the clods of the two soils. #
Tillage has been reported to reduce organic matter concentrations and increase organic matter tur... more Tillage has been reported to reduce organic matter concentrations and increase organic matter turnover rates to a variable extent. The change of soil climate and the incorporation of aboveground C inputs within the soil lead to no unique effect on biodegradation rates, because of their strong interaction with the regional climate and the soil physical properties. The periodical perturbation of soil structure by tools and the subsequent drying±rewetting cycles may be the major factor increasing organic matter decomposition rates by exposing the organic matter that is physically protected in microaggregates to biodegradation. This paper reviews the assessed effects of tillage on organic matter, the scale, extent and mechanisms of physical protection of organic matter in soils. #
Land use changes result in significant decreases in soil organic matter stocks due to enhanced mi... more Land use changes result in significant decreases in soil organic matter stocks due to enhanced mineralization attributed to increased tillage, and due to decreased organic matter inputs. The current paradigm of soil organic matter dynamics suggests that decreasing organic matter stocks are also associated with shifts in organic matter quality to more resistant fractions as the more labile pools are decomposed. The objectives of the current study were to characterize changes in clay-associated organic matter quality in a soil C depletion sequence in terms of thermal properties, and to thermally characterize peroxideresistant organic matter. Clay-sized fractions were isolated from a sequence of soils ranging from native forest to long-term bare fallow, and analyzed using thermogravimetry (TG) and differential scanning calorimetry (DSC) before and after hydrogen peroxide treatment. Differential scanning calorimetry traces had exothermic maxima near 285 and 333 8C, consistent with analyses of chemically extracted organic matter and humic substances reported in the literature. Peak fitting analyses showed that these maxima consisted of several hidden peaks, but their interpretation is problematic. The qualitative shifts in the thermal properties of clay-associated organic matter with differing land use observed in the DSC traces were quantified using cultivation and fallow treatment to forest ratios. Thermogravimetric mass loss ratios were greater in the thermally labile (180-310 8C) exothermic region than in the more thermally resistant (310-450 8C) exothermic region. Similarly, ratios of peak heights and areas for the fitted DSC peaks were higher for the 324 8C peak, compared to peaks at 257 and 284 8C. The higher ratios indicate that the more thermally resistant organic matter has been retained and the more thermally labile organic matter is lost. The observed shift in the distribution of organic matter from thermally labile to thermally resistant fractions from forest to long-term bare fallow clay samples is consistent with the current paradigm of decomposition consisting of a shift to more biologically resistant fractions with increasing time under cultivation and decreasing organic matter inputs. These results suggest that the thermal properties of clay-associated organic matter are related to their biological decomposability. Hydrogen peroxide treatment of clay samples removed approximately 87% of the initial organic C in all samples, and peroxide-resistant organic matter was found to be more thermally stable than the whole. However, thermal analyses of the peroxide-resistant fraction did not show any changes in thermal properties with decreases 0016-7061/$ -see front matter D Geoderma 129 www.elsevier.com/locate/geoderma in total C content. Rather than isolating a biologically resistant pool of organic matter, peroxide-resistant organic matter may isolate an inert pool of soil organic matter with thermal properties consistent with the presence of black carbon, which remains unchanged by changes in land use. D
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Papers by Claire Chenu