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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.

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.

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.

© 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.

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.

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.