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Biochar is useful for soil organic carbon (SOC) sequestration. However, the effects of biochar aging and addition rates on SOC stabilization are unclear. A field experiment with four biochar application rates (0% (control), 1% (LB), 2% (MB), and 4% (HB) of dry fluvo-aquic soil) was conducted. Soil samples were sampled after 8, 12, and 24 months of its application to clarify the question. In general, SOC gradually increased with the biochar application rate. SOC with HB was higher than that in other treatments, while the ratio of microbial biomass carbon (MBC)/SOC and readily oxidizable carbon (ROC)/SOC with HB was lower than that in other treatments (p < 0.05), indicating a positive effect of HB for C stabilization over time. The effects of biochar on the soil carbon pool management index (CPMI) changed from negative to positive after 8 and 24 months of biochar application. The activities of β-D-glucosidase (βG), cellobiohydrolase (CBH), and β-N-acetylglucosaminidase (NAG) under HB were higher than with other treatments after 12 and 24 months of biochar application (p < 0.05) and negatively correlated with the ratio of MBC/SOC and ROC/SOC over time. The CPMI was positively related with βG and CBH activities after 8 and 24 months of biochar application, respectively (p < 0.05). HB increased the relative abundance of oligotrophs, including Acidobacteria, Actinobacteria, and Chloroflexi, but decreased the relative abundance of copiotrophs, including γ-Proteobacteria and Bacteroidetes over time (p < 0.05). The ratio of dissolved organic carbon (DOC)/SOC was positively correlated with the bacterial oligotroph/copiotroph ratio and significantly affected the oligotrophic and copiotrophic bacterial communities, especially after 8 and 12 months of biochar application (p < 0.05). These findings reinforce that increasing the biochar application rate and time enhances SOC stabilization by decreasing the proportions of labile organic carbon and making oligotrophic/copiotrophic communities and enzyme activities more conducive to C sequestration.

— Cropping system is an effective agricultural practice which play crucial role in soil carbon stabilization, soil health and fertility as well as in sequestering atmospheric CO2 in soil for long period of time. With these considerations in mind, a research was conducted in the Prayagraj district of eastern Uttar Pradesh to evaluate how major agricultural systems affect soil carbon stock. The major cropping system includes Wheat-Wheat, Mustard- Mustard, Rice-Wheat and Rice-Mustard Soil samples were collected from eight tehsil of Prayagraj district randomly from depth 0-15 cm and 15-30 cm depth. The findings show that soil organic carbon store in rice-wheat cropping systems is higher than in other cropping systems.

Goethite, hematite, ferrihydrite, and other iron oxides bind through various sorption reactions with humic substances (HS) in soils creating nano-, micro-, and macro-aggregates with a specific nature and stability. Long residence times of soil organic matter (SOM) have been attributed to iron-humic substance (Fe-HS) complexes due to physical protection and chemical stabilization at the organic-mineral interface. Humic acids (HA) and fulvic acids (FA) contain many acidic functional groups that interact with Fe oxides through different mechanisms. Due to the numerous interactions between mineral Fe and natural SOM, much research has led into a better identification and definition of HS. In this review, we first focus on the surface colloidal properties of Fe oxides and their reactivity toward HS. These minerals can be efficiently identified by usual techniques, such as XRD, FTIR spectroscopy, XAS, Mössbauer, diffuse reflectance spectroscopies (DRS), HRTEM, ATM, NanoSIMS. Second, we present the recent state of art regarding the adsorption/precipitation of HS onto iron mineral surfaces and their effects on binding metalloid and trace elements. Finally, we consider future research directions based on recent scientific literature, with particular focus on the ability of Fe nano-particles to increase Fe bioavailability, improve carbon sequestration, reduce greenhouse gas emissions, and decrease the impact of persistent organic and inorganic pollutants. The methodology in this field has rapidly developed over the last decade. However, new procedures to estimate the nature of Fe-HA bonds will be important contributions in clarifying the role of natural iron oxides in soil for carbon stabilization.

The improvement of carbon (C) accumulation in soils has been one of the main purposes of the conservation systems in agricultural production. This study aimed to assess the long-term effect of conventional tillage (CT) and no-tillage (NT) combined with winter cover crops, black oat and oilseed radish, and fallow on C accumulation and stabilization in a very clayey Oxisol in Southern Brazil. Soil samples were collected in the 0-0.05, 0.05-0.10 and 0.10-0.20 m layers of a 26-year-old experiment. Distribution of size-class aggregates, C stock in aggregates, total C stock, and C stocks in the physical fractions, free particulate organic matter (free-POM), occluded particulate organic matter (occluded-POM) and mineral-associated organic matter (min-OM) were assessed. NT had a higher percentage of macroaggregates and C stock in this size-class, and also higher C stock in bulk soil, free-POM and occluded-POM fractions than CT in 0-0.05 m (Tukey’s test p < 0.05), due to higher input of biomass and minimum soil mobilization in NT. Oat and radish had higher C stock in macroaggregates than fallow in 0.05-0.10 m (Tukey’s test p < 0.05). Radish had the highest C stock in the free-POM (0-0.05 m). Fallow decreased the stabilization of macroaggregates and C accumulation in free-POM, due to the lower C input from aboveground biomass over the years. In conclusion, NT after 26 years improved C accumulation and stabilization, mainly in the superficial layer and in POM fractions, and winter cover crops favored the formation and stability of macroaggregates.

Agricultural activities generate CO2, CH4, and N2O, affecting the global climate and the sustainability of agricultural production systems. This topic is essential in those areas where agriculture has caused soil decarbonization. The soil can regenerate by implementing sustainable soil management (SSM), and this regeneration is finite. Therefore, it is necessary to determine the maximum carbon (C) storage capacity to establish the most SSM for soil recarbonization. This research analyzes the C storage capacity in soils with rainfed olive groves and traditional tillage in the largest olive-oil-producing area in the world (Jaén, Andalusia, Spain). The results show that these soils had low soil organic C (SOC) content, ranging from 5.16 g kg−1 (topsoil) to 1.60 g kg−1 (subsoil) and low SOC stock (SOC-S) (43.12 Mg ha−1; 0–120 cm depth). In addition, the SOC fractionation showed that the highest SOC concentrations were in the particulate organic C form. The SOC-S linked to the fine mineral fraction (

Increasing the potential of soil to store carbon (C) is an acknowledged and emphasized strategy for capturing atmospheric CO2. Well-recognized approaches for soil C accretion include reducing soil disturbance, increasing plant biomass inputs, and enhancing plant diversity. Yet experimental evidence often fails to support anticipated C gains, suggesting that our integrated understanding of soil C accretion remains insufficient. Here we use a unique combination of X-ray micro-tomography and micro-scale enzyme mapping to demonstrate for the first time that plant-stimulated soil pore formation appears to be a major, hitherto unrecognized, determinant of whether new C inputs are stored or lost to the atmosphere. Unlike monocultures, diverse plant communities favor the development of 30–150 µm pores. Such pores are the micro-environments associated with higher enzyme activities, and greater abundance of such pores translates into a greater spatial footprint that microorganisms make on the soil and consequently soil C storage capacity., The processes driving soil carbon accretion remain to be poorly understood. Here the authors combined X-ray micro-tomography and zymography to demonstrate that plant-stimulated soil pore formation is a major, hitherto unrecognized, determinant of whether new C inputs are stored or lost to the atmosphere.

In the Arctic zone, where up to 1024 × 1013 kg of organic matter is stored in permafrost-affected soils, soil organic matter consists of about 50% humic substances. Based on the analysis of the molecular composition of humic acids, we assessed the processes of accumulation of the key structural fragments, their transformations and the stabilization rates of carbon pools in soils in general. The landscape of the Lena River delta is the largest storage of stabilized organic matter in the Arctic. There is active accumulation and deposition of a significant amount of soil organic carbon from terrestrial ecosystems in a permafrost state. Under ongoing climate change, carbon emission fluxes into the atmosphere are estimated to be higher than the sequestration and storing of carbon compounds. Thus, investigation of soil organic matter stabilization mechanisms and rates is quite an urgent topic regarding polar soils. For study of molecular elemental composition, humic acids were separated from the soils of the Lena River delta. Key structural fragments of humic matter were identified and quantified by CP/MAS 13C NMR spectroscopy: carboxyl (–COOR), carbonyl (–C=O), CH3–, CH2–, CH-aliphatic, –C-OR alcohols, esters and carbohydrates, and the phenolic (Ar-OH), quinone (Ar = O) and aromatic (Ar–) groups as benchmark Cryosols of the Lena delta river terrestrial ecosystem. Under the conditions of thermodynamic evolutionary selection, during the change between the dry and wet seasons, up to 41% of aromatic and carboxyl fragments accumulated in humic acids. Data obtained showed that three main groups of carbon played the most important role in soil organic matter stabilization, namely C, H-alkyls ((CH2)n/CH/C and CH3), aromatic compounds (C-C/C-H, C-O) and an OCH group (OCH/OCq). The variations of these carbon species’ content in separated humics, with special reference to soil–permafrost organic profiles’ recalcitrance in the current environment, is discussed.

Evidence-based decisions governing sustainable agricultural land management practices require a mechanistic understanding of soil organic matter (SOM) transformations and stabilization of carbon in soil. Large amounts of carbon from organic fertilizers, root exudates, and crop residues are input into agricultural soils. Microbes then catalyze soil biogeochemical processes including carbon extracellular transformation, mineralization, and assimilation of resources that are later returned to the soil as metabolites and necromass. A systems biology approach for a holistic study of the transformation of carbon inputs into stable SOM requires the use of soil “omics” platforms (metagenomics, metatranscriptomics, metaproteomics, and metabolomics). Linking the data derived from these various platforms will enhance our knowledge of structure and function of the microbial communities involved in soil carbon cycling and stabilization. In this review, we discuss the application, potential, and suitability of different “omics” approaches (independently and in combination) for elucidating processes involved in the transformation of stable carbon in soil. We highlight biases associated with these approaches including limitations of the methods, experimental design, and soil sampling, as well as those associated with data analysis and interpretation.

Soils represent a major terrestrial carbon (C) reservoir and are herewith an important constituent of global climate change. They can act either as C sinks or as C sources, depending on the respective environmental conditions and management practices. At present, global forest ecosystems, including temperate European forests, are considered as C sinks. Whereas mineral topsoils under forest were found to be close to C saturation, it is assumed that especially subsoils provide large capacities for additional C storage in future. This is related to a high availability of sorption sites on mineral surfaces for organic matter compounds, which is frequently observed in laboratory experiments. However, sources of subsoil C are still under discussion and the potentials of mineral subsoils for C stabilization were not sufficiently investigated under field conditions so far. The crucial question remains, if forest subsoils can effectively retain and stabilize additional C inputs under the current environmental conditions, and thus contribute to mitigate global climate change. Therefore, this thesis aimed at evaluating (1) the role of the recent litter layer as a source for soil organic carbon (SOC), (2) at investigating dissolved organic carbon (DOC) dynamics as a controlling factor for organic C (OC) translocation in soils, and (3) the capability of subsoil to retain and stabilize fresh OC inputs. The objectives were addressed by conducting different 13C manipulation experiments in central European beech forests (Lower Saxony, Germany). This included (i) a 13C litter manipulation combined with DOC and CO2 monitoring at the Grinderwald subsoil observatories, (ii) injection of DO13C solution into three forest top- and subsoils, and (iii) burial and subsequent field incubation of 13C-coated minerals. Field approaches were complemented by (iv) laboratory sorption and desorption experiments. The field approaches comprised soil samplings down to the deep subsoil of > 100 cm and included recurring samplings for assessing the stability of the translocated OC. The soils in this thesis included Cambisols and Luvisols and the parent materials ranged from Pleistocene glacio-fluvial sand and Triassic upper red sandstone to Weichselian loess. Litter manipulation revealed that carbon inputs originating from the recent litter layer facilitated the actively cycling C pool in the mineral topsoil, but were not a major source of subsoil OC. Main acceptors of the litter-derived inputs in the soil were mineral surfaces, thereby forming mineral-associated organic carbon (MAOC). Migration of OC from the soil surface to the subsoil followed a sequence of sorption, microbial processing, and desorption cycles, resulting in a time offset until significant inputs reached the subsoil via DOC in the leaching soil solution. Bypassing this cascade of cycles in preferential flow paths caused a fast translocation into the subsoil, but DOC and CO2 monitoring suggest that these inputs were prone to microbial decomposition. Undersaturation of sorption capacities for OC binding in the forest subsoil, obtained in laboratory experiments, was not replicated under field conditions, since the injection of a 200 ppm DOC solution into subsoils did not result in additional C accumulation three months later. This suggests that under natural conditions, the stability of retained OC is more decisive for C accumulation in subsoils than the potential free sorption capacities based on laboratory experiments. But at a scale of years, the majority of fresh inputs of OC to top- and subsoils were not effectively preserved, neither in particulate form, nor in association with soil minerals. Accumulation of organic matter (OM) on mineral surfaces may even stimulate the activity of the microbial community due to an increased availability of easily accessible OM substrate, thus promoting decomposition and mobilization instead of stabilization. At present, the temperate forest top- and subsoils are neither sinks nor sources of C since they are situated in an equilibrium state of C inputs and C outputs, thereby maintaining their current C level through processes including sorption, microbial processing, and desorption. Additional C inputs likely promote mineralization and mobilization of fresh and also older SOC and thus cannot be effectively stabilized in forest subsoils. Hence it can be expected that the potential free capacities of forest subsoils for additional C uptake are not exploitable under the current environmental conditions and eventually, forest (sub)soils will not notably contribute to climate change mitigation. Upcoming investigations of subsoils and estimations of their OM stabilization potential should not rely on laboratory studies only, but rather integrate both laboratory and field approaches to obtain more precise insights into subsoil C dynamics., Böden stellen ein großes terrestrisches Kohlenstoff (C)-Reservoir dar und sind damit ein wichtiger Wirkungsfaktor beim globalen Klimawandel. Sie können entweder als C-Senken oder als C-Quellen fungieren, abhängig von den jeweiligen Umweltbedingungen und ihrer Bewirtschaftung. Gegenwärtig werden die globalen Waldökosysteme, einschließlich der gemäßigten europäischen Wälder, als C-Senken angesehen. Während die mineralischen Oberböden unter Wald nahezu C-gesättigt sind, wird angenommen, dass insbesondere die Unterböden große Kapazitäten für eine zusätzliche C-Speicherung in der Zukunft bieten. Dies hängt mit einer hohen Verfügbarkeit von Sorptionsplätzen an mineralischen Oberflächen für Komponenten der organischen Substanz zusammen, die in Laborexperimenten häufig beobachtet wird. Die Quellen des Unterboden-Cs werden jedoch noch diskutiert, und die Potenziale mineralischer Unterböden zur C-Stabilisierung wurden bisher unter Feldbedingungen nicht ausreichend untersucht. Die entscheidende Frage bleibt, ob Waldböden unter den derzeitigen Umweltbedingungen zusätzliche C-Einträge effektiv zurückhalten und stabilisieren können und damit einen Beitrag zur Abschwächung des globalen Klimawandels leisten können. Daher zielte diese Arbeit darauf ab, (1) die Rolle der rezenten Streuschicht als Quelle für organischen Kohlenstoff (SOC) im Boden zu untersuchen, (2) die Dynamik des gelösten organischen Kohlenstoffs (DOC) als kontrollierenden Faktor für die Verlagerung von organischem Kohlenstoff (OC) in Böden zu untersuchen und (3) die Fähigkeit des Unterbodens zu bewerten, frische OC-Einträge zurückzuhalten und zu stabilisieren. Die Ziele wurden durch die Durchführung verschiedener 13C-Manipulationsexperimente in mitteleuropäischen Buchenwäldern (Niedersachsen, Deutschland) verfolgt. Dazu gehörten (i) eine 13C-Streu-Manipulation in Kombination mit DOC- und CO2-Messungen in den Grinderwald-Unterboden-Observatorien, (ii) die Injektion von DO13C-Lösung in drei Ober- und –Unterböden unter Wald, und (iii) das Vergraben mit anschließender Feldinkubation von 13C-belegten Mineralen. Die Feldansätze wurden ergänzt durch (iv) Sorptions- und Desorptionsexperimente im Labor. Die Feldansätze umfassten Probenahmen bis in den tiefen Unterboden von > 100 cm und beinhalteten wiederkehrende Beprobungen zur Beurteilung der Stabilität des verlagerten OCs. Die Böden in dieser Arbeit umfassten Braunerden und Parabraunerden und die Ausgangsmaterialien reichten von pleistozänen glazi-fluviatilen Sand und triassischem Buntsandstein bis hin zu weichselzeitlichem Löss. Die Streu-Manipulation ergab, dass Kohlenstoffeinträge aus der rezenten Streuschicht den aktiv zirkulierenden C-Pool im mineralischen Oberboden unterstützten, aber keine wesentliche Quelle für OC im Unterboden darstellten. Die Hauptempfänger der Streu-bürtigen Einträge in den Boden waren mineralische Oberflächen, wodurch mineral-assoziierter organischer Kohlenstoff (MAOC) gebildet wurde. Die Migration von OC von der Bodenoberfläche in den Unterboden folgte einer Abfolge von Sorptions-, mikrobiellen Verarbeitungs- und Desorptionszyklen, was zu einer zeitlichen Verschiebung führte, bis signifikante Einträge den Unterboden, über DOC in der Bodenlösung, erreichten. Präferentieller Fluss umging diese Kaskade von Zyklen und führte zu einer schnellen Verlagerung in den Unterboden. Die DOC- und CO2-Messungen deuten aber darauf hin, dass diese Einträge anfällig für mikrobiellen Abbau waren. Die in Laborexperimenten ermittelte Untersättigung der Sorptionskapazitäten für die Bindung von OC in Waldunterböden konnte unter Feldbedingungen nicht repliziert werden, denn die Injektion einer 200 ppm DOC-Lösung in Unterböden führte drei Monate später nicht zu einer zusätzlichen C-Akkumulation. Dies deutet darauf hin, dass unter natürlichen Bedingungen die Stabilität des gebundenen OC in Unterböden entscheidender für die C-Akkumulation ist als die potenziellen freien Sorptionskapazitäten, die auf Laborexperimenten basieren. Auf einer Skala von Jahren wurde der Großteil der frischen Einträge von OC in Ober- und Unterböden jedoch nicht effektiv konserviert, weder in partikulärer Form noch in Verbindung mit Bodenmineralen. Die Akkumulation von organischer Substanz (OM) auf mineralischen Oberflächen könnte sogar die Aktivität der mikrobiellen Gemeinschaft aufgrund einer erhöhten Verfügbarkeit von leicht zugänglichem OM-Substrat stimulieren und so Zersetzung und Mobilisierung statt Stabilisierung fördern. Gegenwärtig sind die Ober- und Unterböden gemäßigter Wälder weder Senken noch Quellen für C, da sie sich in einem Gleichgewichtszustand von C-Einträgen und C-Austrägen befinden und somit ihr aktuelles C-Niveau durch Prozesse wie Sorption, mikrobielle Verarbeitung und Desorption aufrechterhalten. Voraussichtlich fördern zusätzliche C-Einträge eher die Mineralisierung und Mobilisierung von frischem und auch älterem SOC und können daher im Waldunterboden nicht effektiv stabilisiert werden. Folglich ist anzunehmen, dass die potenziellen freien Kapazitäten von Waldunterböden für eine zusätzliche C-Aufnahme unter den gegenwärtigen Umweltbedingungen nicht ausgenutzt werden können und Wald(unter)böden letztlich keinen nennenswerten Beitrag zur Abschwächung des Klimawandels leisten werden. Zukünftige Untersuchungen von Unterböden und Abschätzungen ihres OM-Stabilisierungspotenzials sollten sich nicht nur auf Laborstudien stützen, sondern sowohl Labor- als auch Feldansätze integrieren, um genauere Erkenntnisse über die C-Dynamik im Unterboden zu erhalten.

Ice-rich permafrost has been subject to abrupt thaw and thermokarst formation in the past and is vulnerable to current global warming. The ice-rich permafrost domain includes Yedoma sediments that have never thawed since deposition during the late Pleistocene and Alas sediments that were formed by previous thermokarst processes during the Lateglacial and Holocene warming. Permafrost thaw unlocks organic carbon (OC) and minerals from these deposits and exposes OC to mineralization. A portion of the OC can be associated with iron (Fe), a redox-sensitive element acting as a trap for OC. Post-depositional thaw processes may have induced changes in redox conditions in these deposits and thereby affected Fe distribution and interactions between OC and Fe, with knock-on effects on the role that Fe plays in mediating present day OC mineralization. To test this hypothesis, we measured Fe concentrations and proportion of Fe oxides and Fe complexed with OC in unthawed Yedoma and previously thawed Alas deposits. Total Fe concentrations were determined on 1,292 sediment samples from the Yedoma domain using portable X-ray fluorescence; these concentrations were corrected for trueness using a calibration based on a subset of 144 samples measured by inductively coupled plasma optical emission spectrometry after alkaline fusion (R2 = 0.95). The total Fe concentration is stable with depth in Yedoma deposits, but we observe a depletion or accumulation of total Fe in Alas deposits, which experienced previous thaw and/or flooding events. Selective Fe extractions targeting reactive forms of Fe on unthawed and previously thawed deposits highlight that about 25% of the total Fe is present as reactive species, either as crystalline or amorphous oxides, or complexed with OC, with no significant difference in proportions of reactive Fe between Yedoma and Alas deposits. These results suggest that redox driven processes during past thermokarst formation impact the present-day distribution of total Fe, and thereby the total amount of reactive Fe in Alas versus Yedoma deposits. This study highlights that ongoing thermokarst lake formation and drainage dynamics in the Arctic influences reactive Fe distribution and thereby interactions between Fe and OC, OC mineralization rates, and greenhouse gas emissions.

The aim of this work was to compare the carbon (C) mineralization kinetics of three biochars (Formosan ash (Fraxinus formosana Hayata), ash biochar, Makino bamboo (Phyllostachys makino Hayata), bamboo biochar, and lead tree (Leucaena leucocephala (Lam.) de. Wit), lead tree biochar) applied with two addition rates (2 and 5 wt %) in three excessive compost-fertilized (5 wt %) soils (one Oxisols and two Inceptisols), and to ascertain the increasing or decreasing effect of biochar and soil type in the presence of excessive compost. The study results of 400 days incubation indicated that, in general, the potential of the three biochars for C sequestration is similar in the three studied soils. The presence of excessive compost stimulated the co-mineralization of the more labile components of biochar over the short term (first two months). The potential of biochar addition for neutralizing soil pH and regulating the release of Al from soil for preserving soil organic carbon (SOC) might be the important mechanisms in biochar-compost interactions, especially in the presence of excessive compost. Overall, 5% application rate of three high temperature-pyrolysis biochars showed the less detriments to studied soils. In these incubations of biochar, excessive compost, and soil, it is a decreasing effect overall, that is, the enhanced storage of both biochar-C and SOC, which is expected as a long-term carbon sequestration in soil. The recorded direction and magnitude of effect, both are strongly influenced by biochar and soil type. When co-applied with excessive compost, the negative (reducing CO2 release) effect with increasing biochar application rates was eliminated.

Despite the importance of subsoil carbon (C) deposition by deep-rooted crops in mitigating climate change and maintaining soil health, the quantification of root C input and its microbial utilization and stabilization below 1 m depth remains unexplored. We studied C input by three perennial deep-rooted plants (lucerne, kernza, and rosinweed) grown in a unique 4-m deep RootTower facility. 13C multiple pulse labeling was applied to trace C flows in roots, rhizodeposition, and soil as well as 13C incorporation into microbial groups by phospholipid fatty acids and the long-term stabilization of microbial residues by amino sugars. The ratio of rhizodeposited 13C in the PLFA and amino sugar pools was used to compare the relative microbial stability of rhizodeposited C across depths and plant species. Belowground C allocation between roots, rhizodeposits, and living and dead microorganisms indicated depth dependent plant investment. Rhizodeposition as a fraction of the total belowground C input declined from the topsoil (0–25 cm) to the deepest layer (360 cm), i.e., from 35%, 45%, and 36%–8.0%, 2.5%, and 2.7% for lucerne, kernza, and rosinweed, respectively, where lucerne had greater C input than the other species between 340 and 360 cm. The relative microbial stabilization of rhizodeposits in the subsoil across all species showed a dominance of recently assimilated C in microbial necromass, thus indicating a higher microbial stabilization of rhizodeposited C with depth. In conclusion, we traced photosynthates down to 3.6 m soil depth and showed that even relatively small C amounts allocated to deep soil layers will become microbially stabilized. Thus, deep-rooted crops, in particular lucerne are important for stabilization and storage of C over long time scales in deep soil.

An improved identification of the environmental variables that can be used to predict the content of soil organic carbon (SOC) stored belowground is required to reduce uncertainties in estimating the response of the largest terrestrial carbon reservoir to environmental change. Recent studies indicate that some metal cations can have an active role in the stabilization of SOC, primarily by coordinating the interaction between soil minerals and organic matter through cation bridging and by creating complexes with organic molecules when their hydration shells are displaced. The effective cation exchange capacity (CEC eff.) is a measure that integrates information about available soil surfaces to which metal cations are retained. Therefore, we critically tested the relationship between CEC eff. and SOC content using regression analyses for more than 1000 forest sites across Switzerland, spanning a unique gradient of mean annual precipitation (640–2500 mm), elevation (277–2207 m a.s.l), pH (2.8–8.1) and covering different geologies and vegetation types. Within these sites, SOC content is significantly related to CEC eff., in both topsoils and subsoils. Our results demonstrate that, on a pH-class average, in Swiss forest topsoils ( 5.5, between 59 and 83% of subsoil CEC eff. originates from exchangeable calcium, whereas in acidic soils exchangeable aluminum contributes between 21 and 44% of the CEC eff. Exchangeable iron contributes to less than 1% of the variability in CEC eff. Overall this study indicates that in Swiss forests subsoils, CEC eff. strongly reflects the surface of soil minerals to which SOC can be bound by metal cations. The strength of the relationship between CEC eff. and SOC content depends on the pH of the soil, with the highest amount of variation of SOC content explained by CEC eff. in subsoils with pH > 5.5.

Frontiers in Forests and Global Change, 3, ISSN:2624-893X

Specific surface area can be a strong predictor of organic carbon (SOC) contents in soils. Specific surface area can be estimated reliably and cost‐effectively from water adsorption by air‐dry soil samples, but SOC itself can also adsorb water. For estimating the mineral component of specific surface area, it is, therefore, necessary to exclude water‐adsorption by SOC. Here, we refer to “apparent specific surface area” for measurements that include water adsorption by both mineral soil and SOC. We used a mathematical approach to estimate water adsorption by SOC so that this component can be subtracted from measurements of apparent specific surface area. We used a dataset of apparent specific surface area and soil carbon at seven depths from 50 soil cores collected from a research farm in the Manawatu region in New Zealand. Both apparent specific surface area and SOC content decreased with soil depth with very high correlation (r² = 0.98). We estimated the SOC contribution to apparent specific surface area from the slope of the relationship between changes in apparent specific surface area and SOC content. For our soils, the SOC contribution to apparent specific surface area was estimated as 0.43 ± 0.02 m² mgC⁻¹. This parameter allows apparent specific surface area measurements to be corrected for the water adsorption by SOC to calculate the functionally relevant mineral specific surface area. HIGHLIGHTS: Soil surface area can be estimated from the H₂O content of air‐dry soil but SOC also adsorbs H₂O. We developed a mathematical approach to estimate water adsorption by SOC. We estimated the contribution of SOC to apparent specific surface area as 0.43 ± 0.02 m² mgC⁻¹. Mineral specific surface area can be inferred by subtracting SOC‐based H₂O adsorption.

International audience; Aims: Mineral-associated organic matter, mainly derived from microbial by-products, persists longer in soil compared to particulate organic matter (POM). POM is highly recalcitrant and originates largely from decomposing root and shoot litter. Theory suggests that root traits and growth dynamics should affect carbon (C) accumulation into these different pools, but the specific traits driving this accumulation are not clearly identified.Methods: Twelve herbaceous species were grown for 37 weeks in monocultures. Root elongation rate (RER) was measured throughout the experiment. At the end of the experiment, we determined morphological and chemical root traits, as well as substrate induced respiration (SIR) as a proxy for microbial activity. Carbon was measured in four different soil fractions, following particle-size and density fractionation.Results: Root biomass, RER, root diameter, hemicellulose content and SIR (characteristic of N2-fixing Fabaceae species), were all positively correlated with increased C in the coarse silt fraction. Root diameter and hemicellulose content were negatively correlated with C in the POM fraction, that was greater under non N2-fixing Poaceae species, characterized by lignin-rich roots with a high carbon:nitrogen ratio that grew slowly. The accumulation of C in different soil pools was mediated by microbial activity.Conclusions: Our results show that root traits determine C input into different soil pools, mediated primarily by microbial activity, thus determining the fate of soil organic C. We also highlight that C in different soil pools, and not only total soil organic C, should be reported in future studies to better understand its origin, fate and dynamics.

Tillage intensive cropping practices have deteriorated soil physical quality and decreased soil organic carbon (SOC) levels in rice–growing areas of South Asia. Consequently, crop productivity has declined over the years demonstrating the need for sustainable alternatives. Given that, a field experiment was conducted for six years to assess the impact of four tillage based crop establishment treatments [puddled transplant rice followed by conventional tillage in wheat/maize (CTTPR–CT), non–puddled transplant rice followed by zero–tillage in wheat/maize (NPTPR–ZT), zero–till transplant rice followed by zero–tillage in wheat/maize (ZTTPR–ZT), zero–tillage direct seeded rice followed by zero–tillage in wheat/maize (ZTDSR–ZT)], two residue management treatments [residue removal, residue retention (~33%)], and two cropping systems [rice–wheat, rice–maize] on soil aggregation, carbon pools, nutrient availability, and crop productivity. After six years of rotation, in top 0.2 m soil depth, zero–till crop establishment treatments (ZTTPR–ZT and ZTDSR–ZT) had higher (p, Highlights • Conservation tillage increased soil aggregation over conventional tillage. • Zero-tillage treatments increased labile carbon stabilization over conventional tillage. • Residue retention increased soil aggregation, soil available P, K and Zn over residue removal. • Zero-tillage with residue retention increased crop yields over conventional tillage practice.

8 páginas. 4 figuras.- 2 tablas.- referencias.- Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jenvman.2019.109567, A detailed and global quantitative assessment of the distribution of pyrogenic carbon (PyC) in soils remains unaccounted due to the current lack of unbiased methods for its routine quantification in environmental samples. Conventional oxidation with potassium dichromate has been reported as a useful approach for the determination of recalcitrant C in soils. However, its inaccuracy due to the presence of residual non-polar but still non-PyC requires additional analysis by 13C solid-state nuclear magnetic resonance (NMR) spectroscopy, which is expensive and time consuming. The goal of this work is to examine the possibility of applying infrared (IR) spectroscopy as a potential alternative. Different soil type samples (paddy soil, Histic Humaquept, Leptosol and Cambisol) have been used. The soils were digested with potassium dichromate to determine the PyC content in environmental samples. Partial Least Squares (PLS) regression was used to build calibration models to predict PyC from IR spectra. A set of artificially produced samples rich in PyC was used as reference to observe in detail the IR bands derived from aromatic structures resistant to dichromate oxidation, representing black carbon. The results showed successful PLS forecasting of PyC in the different samples by using spectra in the 1800–400 cm−1 range. This lead to significant (P, José M. De la Rosa and Marco A. Jiménez-González thank the Spanish Ministry of Economy and Competitiveness (MINECO, Spain) for funding the “Ramón y Cajal” post-doctoral contract (RYC2014-16338) and the pre-doctoral FPI fellowship (BES-2014-069238) respectively. Nicasio T. Jiménez-Morillo was funded by the scholarship PosDoc/Por 3O/OUT17 (FCT, Portugal).

The electrochemical performance of porous silicon (pSi) stabilized via thermal decomposition of acetylene gas is investigated for the first time. In this study, pSi undergoes two thermal treatments at either 525 or 800 °C, which result in hydrogen-terminated thermally hydrocarbonized pSi (THCpSi) and hydroxyl-terminated thermally carbonized pSi (TCpSi), respectively, the latter upon dipping in hydrofluoric acid to activate the surface termination. Electrochemical characterization, using cyclic voltammetry, chronocoulometry, and electrochemical impedance spectroscopy in the presence of several redox pairs, [Fe(CN) 6 ] 3/4− , [Ru(NH 3 ) 6 ] 2/3+ , and hydroquinone/quinone, is used to demonstrate the versatility and high stability to degradation of carbon-stabilized pSi nanostructures and their excellent electrochemical performance. Added to the large surface area, adjustable pore morphology and tailorable surface chemistry of THCpSi and TCpSi, these nanostructures demonstrate fast electron-transfer kinetics, providing key advantages over conventional carbon electrodes. The versatile surface chemistry of THCpSi and TCpSi offer various possibilities to introduce multiple functional groups depending on the nature of the bioreceptor to be immobilized. For proof of principle, the use of a THCpSi-based immunosensor to detect MS2 bacteriophage is demonstrated by means of electrochemical impedance spectroscopy, showing a detection limit of 4.9 pfu mL −1 . Carbon-stabilized pSi structures represent a new class of nanostructured electrodes for biosensing applications Refereed/Peer-reviewed

© 2016 Elsevier Ltd. The importance of lignin as a recalcitrant constituent of soil organic matter (SOM) remains contested. Associations with iron (Fe) oxides have been proposed to specifically protect lignin from decomposition, but impacts of Fe-lignin interactions on mineralization rates remain unclear. Oxygen (O2) fluctuations characteristic of humid tropical soils drive reductive Fe dissolution and precipitation, facilitating multiple types of Fe-lignin interactions that could variably decompose or protect lignin. We tested impacts of Fe addition on13C methoxyl-labeled lignin mineralization in soils that were exposed to static or fluctuating O2. Iron addition suppressed lignin mineralization to 21% of controls, regardless of O2availability. However, Fe addition had no effect on soil CO2production, implying that Fe oxides specifically protected lignin methoxyls but not bulk SOM. Iron oxide-lignin interactions represent a specific mechanism for lignin stabilization, linking SOM biochemical composition to turnover via geochemistry.

Soil organic matter (SOM) is a combination of materials having different origin and with different stabilization and decomposition processes. To determine the different SOM pools and their turnover rates, a silt loam-textured Luvisol from West Hungary was taken from the 0&ndash, 20 cm soil depth and incubated for 163 days. Maize residues were added to the soil in order to obtain natural 13C enrichment. Four different SOM fractions&mdash, particulate organic matter (POM), sand and stable aggregate (S + A), silt- plus clay-sized (s + c) and chemically resistant soil organic carbon (rSOC) fractions&mdash, were separated and analyzed using FT-IR, &delta, 13C, and 14C measurements. The mean residence time (MRT) of the new C and the proportion of maize-derived C in the fractions were calculated. The POM fraction was found to be the most labile C pool, as shown by the easily decomposable chemical structures (e.g., aliphatic, O-alkyl, and polysaccharides), the highest proportion (11.7 &plusmn, 2.5%) of maize-derived C, and an MRT of 3.6 years. The results revealed that the most stable fraction was the rSOC fraction which had the smallest proportion of maize-derived C (0.18 &plusmn, 2.5%) and the highest MRT (250 years), while it was the only fraction with a negative value of &Delta, 14C (&minus, 75.0 &plusmn, 2.4&permil, ). Overall, the study confirmed the hypothesis that the SOM associated with finer-sized soil particles decomposes the least, highlighting the significance of the fractionation process for more accurate determination of the decomposition processes of SOM pools.

Fractionation of soil organic carbon (SOC) is crucial for mechanistic understanding and modeling of soil organic matter decomposition and stabilization processes. It is often aimed at separating the bulk SOC into fractions with varying turnover rates, but a comprehensive comparison of methods to achieve this is lacking. In this study, a total of 20 different SOC fractionation methods were tested by participating laboratories for their suitability to isolate fractions with varying turnover rates, using agricultural soils from three experimental sites with vegetation change from C3 to C4 22-36 years ago. Enrichment of C4-derived carbon was traced and used as a proxy for turnover rates in the fractions. Methods that apply a combination of physical (density, size) and chemical (oxidation, extraction) fractionation were identified as most effective in separating SOC into fractions with distinct turnover rates. Coarse light SOC separated by density fractionation was the most C4-carbon enriched fraction, while oxidation-resistant SOC left after extraction with NaOCl was the least C4-carbon enriched fraction. Surprisingly, even after 36 years of C4 crop cultivation in a temperate climate, no method was able to isolate a fraction with more than 76% turnover, which challenges the link to the most active plant-derived carbon pools in models. Particles with density > 2.8 g cm(-3) showed similar C4-carbon enrichment as oxidation resistant SOC, highlighting the importance of sesquioxides for SOC stabilization. The importance of clay and silt sized particles (< 50 mu m) for SOC stabilization was also confirmed. Particle size fractionation significantly outperformed aggregate size fractionation, due to the fact that larger aggregates contain smaller aggregates and organic matter particles of various sizes with different turnover rates. An evaluation scheme comprising different criteria was used to identify the most suitable methods for isolating fractions with distinct turnover rates, and potential benefits and trade-offs associated with a specific choice. Our findings can be of great help to select the appropriate method(s) for fractionation of agricultural soils.

Legumes are used in forage agriculture to replace mineral N fertilizer by biological N fixation. We hypothesised that N addition through biological N fixation may have different effects on soil organic matter (SOM)112 quantity and composition compared to mineral N fertilization. In this study we introduced lucerne into grasslands and aimed to examine the resulting effects on C stocks and the molecular C signatures of roots and soil.Lucerne, cocksfoot and tall fescue were grown for five years as monocultures or mixtures. Grass monocultures were N-fertilised (SNF), while lucerne monoculture and mixtures received N through biological N-2 fixation (BNF). For root and soil samples from 30 cm depth, we analysed quantity and composition of (1) non-cellulosic neutral carbohydrates after acid hydrolysis and (2) lignin after CuO oxidation.Our data showed species-specific chemical signatures for roots extracted from soil. Lucerne presence increased the N content of roots and lowered their lignin/N ratio. Contrasting root input over four growing seasons did not impact SOM stocks, as they were similar in all treatments. We observed different chemical signatures for soil under mixtures as compared to lucerne monoculture. In particular, the state of SOM degradation was enhanced under mixtures.A greater input of higher quality litter to soils receiving BNF did not increase SOM storage. When lucerne was grown in mixture with grass, we observed lignin and carbohydrate signatures that indicated a more advanced state of degradation as compared to monocultures. Therefore, we suggest that the introduction of lucerne into grassland influences OM degradation more than its stabilization. After four growing seasons, SOM molecular signatures were not significantly influenced by type of N fertilization and showed little difference among the treatments despite species specific root composition.

A Humeomics sequential chemical fractionation coupled to advanced analytical identification was applied directly to soil for the first time. Humeomics extracted ~235% more soil organic carbon (SOC) than by the total alkaline extraction traditionally employed to solubilise soil humic molecules (soil Humeome). Seven fractions of either hydro- or organo-soluble components and a final unextractable humic residue were separated from soil. These materials enabled an unprecedented structural identification of solubilised heterogeneous humic molecules by combining NMR, GC-MS, and ESI-Orbitrap-MS. Identified molecules and their relative abundance were used to build up structure-based van Krevelen plots to show the specific contribution of each fraction to SOC. The stepwise isolation of mostly hydrophobic and unsaturated molecules of progressive structural complexity suggests that humic suprastructures in soil are arranged in multi-molecular layers. These comprised molecules either hydrophobically adsorbed on soil aluminosilicate surfaces in less stable fractions, or covalently bound in amorphous organo-iron complexes in more recalcitrant fractions. Moreover, most lipid molecules of the soil Humeome appeared to derive from plant polyesters rather than bacterial metabolism. An advanced understanding of soil humic molecular composition by Humeomics may enable control of the bio-organic dynamics and reactivity in soil.

Drought will increase in frequency and intensity in many areas of the world and has the potential to turn entire ecosystems from a sink to a source of C. Soil represents one of the largest C pools on earth, and small changes in the balance between inputs and outputs may have extreme consequences for total atmospheric CO2 concentrations. Outputs are determined by microbial decomposition of soil organic matter (SOM) which can be divided in pools of different inherent stability and turn-over. A major stable pool of C is represented by organo-mineral complexes (C bound to silt and clay), which is primarily controlled by plant-derived inputs to soil and soil microbes. Drought effects on plants, microbes and their interactions could cause changes to the stable pool of C, however information on this topic is lacking. In this thesis I: (i) reviewed and quantified drought-induced effects on soil respiration and microbial communities by meta-analysis; (ii) quantified the effects of drying and rewetting on wheat-derived C stabilization and N cycling; (iii) quantified and qualified drought-induced effects on root exudation of soybean and sunflower; (iv) examined drought effects to C stabilization in the field. Results show that drought can induce intense losses of C by increasing soil respiration following rewetting. Highest losses were produced in combination of intense drought and C-rich soils. At the same time drying and rewetting can cause intense stress on plants, reducing biomass and C inputs to soil. However plants can adopt different strategies to drought-induced changes which are reflected in different rates and quality of root exudates. In field drought did not change the size of the mineral-associated or more stable soil C, highlighting resistance of grassland soils. Specific microbial groups were linked to stable soil C at different depths and legumes were shown to be a key functional group in mediating drought effects and increasing stable C in soil.

International audience; Co-composting biowastes such as manures and biosolids can be used to stabilize carbon (C) without impacting the quality of these biowastes. This study investigated the effect of co-composting biowastes with alkaline materials on C stabilization and monitored the fertilization and revegetation values of these co-composts. The stabilization of C in biowastes (poultry manure and biosolids) was examined by their composting in the presence of various alkaline amendments (lime, fluidized bed boiler ash, flue gas desulphurization gypsum, and red mud) for 6 months in a controlled environment. The effects of co-composting on the biowastes' properties were assessed for different physical C fractions, microbial biomass C, priming effect, potentially mineralizable nitrogen, bioavailable phosphorus, and revegetation of an urban landfill soil. Co-composting biowastes with alkaline materials increased C stabilization, attributed to interaction with alkaline materials, thereby protecting it from microbial decomposition. The co-composted biowastes also increased the fertility of the landfill soil, thereby enhancing its revegetation potential. Stabilization of biowastes using alkaline materials through co-composting maintains their fertilization value in terms of improving plant growth. The co-composted biowastes also contribute to long-term soil C sequestration and reduction of bioavailability of heavy metals.

The global warming problem is one of the most serious ones for the human future. In accordance to solve this problem, the concept was tried as the searching examination, that is the general waste was carburized and the produced carbon was hardened with Phenol Resin. As the result, carburization went well and the carbon collection ratio was about 88%. But hardening did not go so well. Compression Strength was about 7~8N/mm2, which is about 1/5 of wooden material, and Specific Weight was about 1.0, which is twice of that. But the possibility for real use of this concept was confirmed. And some themes could be found for the following researches., 本研究では，上記システムが成立するための各種技術条件を求める探索的な研究である．

Greenhouse gas (GHG) emissions and their consequent effect on global warming are an issue of global environmental concern. Increased carbon (C) stabilization and sequestration in soil organic matter (SOM) is one of the ways to mitigate these emissions. Here we evaluated the role of nanoclays isolated from soil on C stabilization in both a C rich Andisols and C depleted Cambisols. Nanoclays were analyzed for size and morphology by transmission electron microscopy, for elemental composition and molecular composition using pyrolysis-GC/MS. Moreover, nanoclays were treated with H2O2 to isolate stable SOM associated with them. Our result showed better nanoclay extraction efficiency and higher nanoclay yield for Cambisol compared to Andisols, probably related to their low organic matter content. Nanoclay fractions from both soils were different in size, morphology, surface reactivity and SOM content. Nanoclays in Andisols sequester around 5-times more C than Cambisols, and stabilized 6 to 8-times more C than Cambisols nanoclay after SOM chemical oxidation. Isoelectric points and surface charge of nanoclays extracted from the two soils was very different. However, the chemical reactivity of the nanoclay SOM was similar, illustrating their importance for C sequestration. Generally, the precise C stabilization mechanisms of both soils may be different, with nanoscale aggregation being more important in Andisols. We can conclude that independent of the soil type and mineralogy the nanoclay fraction may play an important role in C sequestration and stabilization in soil-plant systems.

In a warmer world, soil CO2 emissions are likely to increase. There is still much discussion about which soil organic C (SOC) pools are more sensitive to increasing temperatures. While the temperature sensitivity of C stabilized by biochemical recalcitrance or by sorption to mineral surfaces has been characterized, few studies have been carried out on the temperature sensitivityexpressed as Q(10)of C physically protected inside soil macroaggregates (0.2-2mm). It has been suggested that increasing the availability of labile SOC by exposing C through macroaggregate crushing would decrease Q(10), i.e., the temperature dependence of soil CO2 emissions. To test this hypothesis, the temperature dependence of CO2 emissions from C physically protected in macroaggregates was measured through 21-d laboratory incubations of crushed and uncrushed soils, at 18 degrees C and 28 degrees C. 199 topsoil samples, acidic or calcareous, with SOC ranging from 2 to121gkg(-1) soil were investigated. The CO2 emissions were slightly more sensible to temperature than to C deprotection: about 0.3mgCg(-1)soil (=13 mgC g(-1) SOC) and 0.2 mgC g(-1) (=12mgC g(-1) SOC) were additionally mineralized, in average, by increasing the temperature or by disrupting the soil structure, respectively. The mean Q(10) index ratio of CO2 emitted at 28 degrees C and 18 degrees C was similar for crushed and uncrushed soil samples and equaled 1.6. This was partly explained because Q(10) of macro-aggregate-protected C was 1. The results did not support the initial hypothesis of lower temperature dependence of soil CO2 emissions after macroaggregate disruption, although a slight decrease of Q(10) was noticeable after crushing for soils with high amounts of macroaggregate-protected C. Field research is now needed to confirm that soil tillage might have no effect on the temperature sensitivity of SOC stocks.

Quacking aspen (Populus tremuloides Michx.) is an iconic species in western United States that offers multiple ecosystem services, including carbon sequestration. A shift in forest cover towards coniferous species due to natural succession, land management practices, or climate change may modify soil organic carbon (SOC) dynamics and CO2 emissions. The objectives of this study were to: (i) assess the effects of overstory composition on SOC storage and stability across the aspen-conifer ecotone, (ii) use Fourier transform infrared spectroscopy attenuated total reflectance (FTIR-ATR) to assess whether SOC storage is associated with preferential adsorption of certain organic molecules to the mineral surfaces, and (iii) develop models using near-infrared reflectance spectroscopy (NIRS) to predict aspen- and conifer-derived SOC concentration. Mineral soils (0 ��� 15 cm) were sampled in pure and mixed aspen and conifer stands in Utah and subjected to physical fractionation to characterize SOC stability (i.e., SOC protected against microbial decomposition), long term laboratory incubations (i.e., SOC decomposability), and hot water extractions (i.e., SOC solubility). Vegetation cover had no effect on SOC storage (47.0 �� 16.5 Mg C ha���1), SOC decomposability (cumulative released CO2-C of 93.2 �� 65.4 g C g���1 C), SOC solubility (9.8 �� 7.2 mg C g���1 C). Mineral-associated SOC (MoM) content was higher under aspen (31.2 �� 15.1 Mg C ha-1) than under mixed (25.7 �� 8.8 Mg C ha���1) and conifer cover (22.8 �� 9.0 Mg C ha���1), indicating that aspen favors long-term SOC storage. FTIR-ATR spectral analysis indicated that higher MoM content under aspen is not due to higher concentration of recalcitrant compounds (e.g., aliphatic and aromatic C), but rather to stabilization of simple molecules (e.g., polysaccharides) of plant or microbial origin. NIRS models performed well during calibration-validation stage (ratio of standard deviation of reference values to standard error of prediction (RPD) ��� 2). However, model performance decreased during independent validation (RPD = 1.2 ��� 1.6), probably due to the influence of soil texture, mineralogy, understory vegetation, and land history on SOC spectra. Further improvement of NIRS models could provide insight on SOC dynamics under potential conifer encroachment in semiarid montane forests.

Soil organic matter (SOM) results from a suite of microbial and geochemical processes that in combination convert carbon (C) of biological origin to stabilized, potentially long-lived materials. While it is well established that soil microorganisms are involved in the conversion of plant biomass into SOM, the conversion of microbial bodies themselves to stabilized soil organic materials is not well understood. Recent studies suggest that microbial products such as polysaccharides, amino acids, fatty acids, and a number of other biomolecules of microbial origin can remain in soils for long periods of time, and that microbial bodies play a much more important role as the precursors to SOM than previously considered. In this dissertation, I examine the flow of microbial carbon in two soil types as it is assimilated into the living microbial biomass under different climate regimes, how long it remains in the living microbial biomass, and ultimately the flow of this microbial C out of the living biomass as it is respired out as CO2 or is incorporated into SOM. The influence of soil type and climate were examined because both are key drivers of soil biogeochemical processes and strongly affect organic matter stabilization in soils. In Chapter 1, I report the results of a field study conducted to understand not only whether different microbial groups preferentially assimilate carbon from different microbial sources of C, but also whether climate and edaphic characteristics alter either the assimilation of necrotic microbial carbon and/or the length of residence of this carbon within the living biomass. This study followed the fate of 13C labeled dead microbial bodies for three years after they were injected into field soils located in a Temperate mixed-conifer forest and a Tropical wet forest. The 13C was subsequently assimilated into the standing microbial biomass and ultimately lost from the biomass over a 3 year period. In general, the saprophytic microbial groups preferentially assimilated carbon from their same groups or groups with similar molecular compositions; however, only the Gram-positive bacteria in the Tropical site and Gram-positive and Gram-negative bacteria in the Temperate site demonstrated an affinity for assimilating C from dead actinobacteria. At each harvest throughout the study, the Temperate soils retained more labeled 13C from the labeled necromass groups than the Tropical soils. The faster loss of labeled carbon from the living biomass in Tropical soils may have significant implications for the relative contributions of microbial biomass to the formation of SOM. As evidenced in Chapter 3, the retention time of C in living microbial biomass may be positively correlated to the sorption of microbial C to mineral surfaces in the heavy fraction (HF) of SOM. Since the HF is generally the longest-lived SOM pool, an increase in the proportion of microbial C sorbed to mineral surfaces in the HF due to longer residence times of C in the living biomass of Temperate soils may lead to a higher proportion of microbial C in the HF of Temperate soils which may then lead to longer residence times of microbial C in the SOM of Temperate soils. I also conducted a 1.5-year lab incubation designed to investigate the stability and fate of microbial cell materials by following the fate of added, labeled microbial cell C as it was assimilated into the living microbial biomass, respired out as CO2, and recovered in the SOM. Soil samples were collected from a Temperate mixed-conifer forest ecosystem and a Tropical wet forest ecosystem; a single common mixture of 13C labeled bodies was added to the soils, and the soils were incubated for 520 days under 3 different climate regimes (Mediterranean mixed conifer forest, Redwood forest, and Tropical forest). Results from the analyses of the stabilization of microbial C into operationally-defined organic matter fractions in two different soil ecosystems, and the fate of microbial C as it is assimilated into the living biomass, respired out as CO2, and stabilized in the SOM of a Tropical soil are presented in Chapters 2 and 3, respectively. The research presented in Chapter 2 addresses the interactions of climate and edaphic characteristics on the stabilization of microbial C into soil organic matter fractions in Temperate and Tropical soils. Both climate and soil type exerted significant influences on the total amount of 13C recovered in the incubated soils as well as the amount recovered in each of the three operationally-defined stabilized carbon pools: free light fraction (FLF), occluded light fraction (OLF), and heavy fraction (HF). The recovery of 13C was higher in the HF fraction than the OLF and FLF fractions in all but one soil-climate combination. The high recovery of 13C in the HF is consistent with the stabilization of microbial C through interactions with soil mineral surfaces. There was clear influence of climate on the 13C-OLF recoveries from Tropical soils as more 13C was stabilized under the Temperate climates (Mediterranean and Redwood) compared to the Tropical climate. In Chapter 3, the analyses were designed to ask whether longer residence times of carbon in the microbial biomass increased the association of microbial carbon with mineral surfaces in the heavy fraction of SOM. For this study, I monitored the fate of labeled dead carbon as it was added to Tropical soils under three climate regimes (Mediterranean, Redwood and Tropical), assimilated into the living saprophytic biomass, respired out as CO2, and recovered in the SOM. An increase in saprophytic fungi, actinobacteria, Gram-positive bacteria, Gram-negative bacteria, and unassigned lipid biomasses under the Mediterranean climate early in the incubation indicated that all four of these microbial communities temporarily responded favorably to the relatively cold, dry spring Mediterranean climate conditions. Interestingly, assimilation of the dead microbial carbon by actinobacteria was highest under the Tropical climate; this trend was primarily due to the atom% 13C excess of the actinobacterial cells, and not increases in actinobacterial biomass, indicating that while the presence of dead microbial carbon did not cause the actinobacterial communities to increase in size, the actinobacteria in this study did assimilate dead microbial carbon under the static warm, wet climate conditions. The results of Chapter 3 demonstrate that longer residence times of carbon in the microbial biomass may indeed increase the association of microbial carbon with mineral surfaces in the HF of SOM. Here, soils with the longest retention times of 13C in the living biomass and the lowest respiration rates stabilized the most labeled carbon in the HF, while soils with the lowest retention times of 13C in the living biomass and the highest respiration rates stabilized the least amount of labeled carbon in the HF. While the contribution of microbial bodies to soil organic matter have historically been overlooked, or have been considered to be negligible, the findings of this dissertation support recent research that shows that microbial bodies are central to organic matter stabilization and that soil type and climate influence the retention of microbial C in SOM. In Chapter 1, I found that Temperate mixed conifer soils retained higher amounts of microbial C in the living biomass than Tropical wet forest soils over a 3 year period. The research in Chapter 3 demonstrated that longer retention times of C in the living microbial biomass led to higher stabilization of microbial C in the the HF of SOM. Chapter 2 showed a higher retention of microbial C in the HF, as opposed to the FLF or OLF, is likely due to the stabilization of microbial C in the HF through interactions with mineral surfaces. Together, the chapters in this dissertation indicate that a higher proportion of microbial C in Temperate soils may be stabilized through association with soil mineral components than in Tropical soils; in Topical soils, C in the living biomass is lost at a faster rate, diminishing the amount of time that microbial C interacts with mineral surfaces, potentially lessening the proportion of microbial C sorbed to and stabilized on mineral surfaces.

Soil organic carbon (SOC) is usually said to be well correlated with soil texture and soil aggregation. These relations generally suggest a physical and physicochemical protection of SOC within soil aggregates and on soil fi ne particles, respectively. Because there are few experimental evidences of these relations on tropical soils, we tested the relations of soil variables (SOC and soil aggregate contents, and soil texture) with the amount of SOC physically protected in aggregates on a set of 15 Malagasy soils. The soil texture, the SOC and water stable macroaggregate (MA) contents and the amount of SOC physically protected inside aggregates, calculated as the difference of C mineralized by crushed and intact aggregates, were characterized. The relation between these variables was established. SOC content was signifi cantly correlated with soil texture (clay+fi ne silt fraction) and with soil MA amount while protected SOC content was not correlated with soil MA amount. This lack of correlation might be attributed to the highest importance of physicochemical protection of SOC which is demonstrated by the positive relation between SOC and clay+fi ne silt fraction.

Physical fractionation is a widely used methodology to study soil organic matter (SOM) dynamics, but concerns have been raised that the available fractionation methods do not well describe functional SOM pools. We also examine the question whether physical fractionation techniques isolate ecologically meaningful, functionally relevant soil compartments. In this study we explore whether the kind of information that aggregate density fractionation (ADF) and particle size-density fractionation (PSDF) yield on soil OM dynamics is method-specific, similar, or complimentary. We do so by following the incorporation of a 15N label into mineral soils of two European beech forests a decade after its application as 15N labelled litter. Both density and size-based fractionation methods suggested that OM became increasingly associated with the mineral phase as decomposition progressed, within aggregates and onto mineral surfaces. Our results suggest that physical fractionation methods do isolate ecologically relevant functional soil subunits. However, scientists investigating specific aspects of OM dynamics are pointed towards ADF when adsorption and aggregation processes are of interest, whereas PSDF is the superior tool to research the fate of particulate organic matter (POM). Some methodological caveats were observed mainly for the PSDF procedure, the most important one being that fine fractions isolated after sonication can not be linked to any defined decomposition pathway or stabilisation process. This also implies that historical assumptions about the "adsorbed" state of carbon associated with fine fractions need to be re-evaluated. Finally, this work demonstrates that establishing a comprehensive picture of whole soil OM dynamics requires a combination of both methodologies and we offer a suggestion for an efficient combination of the density and size-based approaches.

International audience; Soils contain an abundance of nano-size particles. Because of their tendency to aggregate and associate with organic colloids, however, soil nanoparticles are difficult to obtain and characterize. Here we report on a simple and rapid method of extracting mesoporous nanomaterials from the clay fraction of an Andisol with narrow size distribution. The clay and nanoclay were characterized by elemental analysis, pyrolysis GC/MS, electron and atomic force microscopy, infrared spectroscopy, and electrophoresis. The nanoclay dominantly consists of hollow allophane spherules forming globular aggregates of about 100 nm in diameter. The nanoclay contains more organic matter (carbon and nitrogen) with a larger proportion of polysaccharides and nitrogen containing compounds, and has a lower isoelectric point, than the clay. Treatment with hydrogen peroxide causes a large decrease in the organic matter contents of both nanoclay and clay. The aggregates of allophane nanoparticles retain a significant amount (similar to 12%) of carbon against intensive peroxide treatment. Thus, besides playing an important role in carbon stabilization, these naturally occurring nanomaterials are potentially useful for developing a low-cost carbon sequestration technology. (C) 2010 Elsevier B.V. All rights reserved.