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In a context of global change, soil has been identified as a potential carbon (C) sink, depending on land-use strategies. To detect the trends in carbon stocks after the implementation of new agricultural practices, early indicators, which can highlight changes in short timescales, are required. This study proposes the combined use of stable isotope probing and chemometrics applied to solid-state 13 C nuclear magnetic resonance (NMR) spectra to unveil the dynamics of the storage and mineralization of soil carbon (C) pools. We focused on light organic matter fractions isolated by density fractionation of soil water stable aggregates because they respond faster to changes in land use than the total soil organic matter (SOM). Samples were collected from an agricultural field experiment with grassland, continuous maize cropping, and ley grassland under temperate climate conditions. Our results indicated contrasting aggregate dynamics depending on land-use systems. Under our experimental conditions, grassland returns larger amounts of C as belowground inputs than maize cropping, evidencing a different distribution of light C fractions between aggregate classes. Coarse aboveground inputs from maize contributed mostly to larger macroaggregates. Land-use changes with the introduction of ley grassland provoked a decoupling of the storage and/or degradation processes after the grassland phase. The newly derived maize inputs were barely degraded during the first 3 years of maize cropping, whereas grassland-derived material was depleted. As a whole, results suggest large microbial proliferation as shown by 13 C NMR under permanent grassland, then reduced within the first years after the land-use conversion, and finally restored. The study highlighted a fractal structure of the soil, determining a scattered spatial distribution of the cycles of storage and degradation of soil organic matter related to detritusphere dynamics. As a consequence, vegetal inputs from a new land use are creating new detritusphere microenvironments that may be disconnected from the dynamics of C cycle of the previous land use. The formation of those different and unconnected microenvironments may explain the observed legacy effect of the previous land use, since each microenvironment type contributes separately to the overall soil C cycle. The effects of the new land use on the soil C cycle are delayed until the different detritusphere microenvironments remain unconnected, and the ones from the previous land use represent the predominant microenvironment type. Increasing knowledge of the soil C dynamics at a fine scale will be helpful in refining the prediction models and land-use policies. [ABSTRACT FROM AUTHOR]

Increased mineralization of the organic matter (OM) stored in permafrost is expected to constitute the largest additional global warming potential from terrestrial ecosystems exposed to a warmer climate. Chemical composition of permafrost OM is thought to be a key factor controlling the sensitivity of decomposition to warming. Our objective was to characterise OM from permafrost soils of the European Arctic: two mineral soils-Adventdalen, Svalbard, Norway and Vorkuta, northwest Russia-and a 'palsa' (ice-cored peat mound patterning in heterogeneous permafrost landscapes) soil in Neiden, northern Norway, in terms of molecular composition and state of decomposition. At all sites, the OM stored in the permafrost was at an advanced stage of decomposition, although somewhat less so in the palsa peat. By comparing permafrost and active layers, we found no consistent effect of depth or permafrost on soil organic matter (SOM) chemistry across sites. The permafrost-affected palsa peat displayed better preservation of plant material in the deeper layer, as indicated by increasing contribution of lignin carbon to total carbon with depth, associated to decreasing acid (Ac) to aldehyde (Al) ratio of the syringyl (S) and vanillyl (V) units, and increasing S/V and contribution of plant-derived sugars. By contrast, in Adventdalen, the Ac/Al ratio of lignin and the Alkyl C to O-alkyl C ratio in the NMR spectra increased with depth, which suggests less oxidized SOM in the active layer compared to the permafrost layer. In Vorkuta, SOM characteristics in the permafrost profile did not change substantially with depth, probably due to mixing of soil layers by cryoturbation. The composition and state of decomposition of SOM appeared to be site-specific, in particular bound to the prevailing organic or mineral nature of soil when attempting to predict the SOM proneness to degradation. The occurrence of processes such as palsa formation in organic soils and cryoturbation should be considered when up-scaling and predicting the responses of OM to climate change in arctic soils. [ABSTRACT FROM AUTHOR]

Lignin is an aromatic plant compound that decomposes more slowly than other organic matter compounds; however, it was recently shown that lignin could decompose as fast as litter bulk carbon in minerals soils. In alpine Histosols, where organic matter dynamics is largely unaffected by mineral constituents, lignin may be an important part of soil organic matter ( SOM). These soils are expected to experience alterations in temperature and/or physicochemical parameters as a result of global climate change. The effect of these changes on lignin dynamics remains to be examined and the importance of lignin as SOM compound in these soils evaluated. Here, we investigated the decomposition of individual lignin phenols of maize litter incubated for 2 years in-situ in Histosols on an Alpine elevation gradient (900, 1300, and 1900 m above sea level); to this end, we used the cupric oxide oxidation method and determined the phenols' 13C signature. Maize lignin decomposed faster than bulk maize carbon in the first year (86 vs. 78% decomposed); however, after the second year, lignin and bulk C decomposition did not differ significantly. Lignin mass loss did not correlate with soil temperature after the first year, and even correlated negatively at the end of the second year. Lignin mass loss also correlated negatively with the remaining maize N at the end of the second year, and we interpreted this result as a possible negative influence of nitrogen on lignin degradation, although other factors (notably the depletion of easily degradable carbon sources) may also have played a role at this stage of decomposition. Microbial community composition did not correlate with lignin mass loss, but it did so with the lignin degradation indicators (Ac/Al)s and S/V after 2 years of decomposition. Progressing substrate decomposition toward the final stages thus appears to be linked with microbial community differentiation. [ABSTRACT FROM AUTHOR]

Soil microorganisms play a pivotal role in soil organic matter (SOM) turn-over and their diversity is discussed as a key to the function of soil ecosystems. However, the extent to which SOM dynamics may be linked to changes in soil microbial diversity remains largely unknown. We characterized SOM degradation along a microbial diversity gradient in a two month incubation experiment under controlled laboratory conditions. A microbial diversity gradient was created by diluting soil suspension of a silty grassland soil. Microcosms containing the same sterilized soil were re-inoculated with one of the created microbial diversities, and were amended with C labeled wheat in order to assess whether SOM decomposition is linked to soil microbial diversity or not. Structural composition of wheat was assessed by solid-state C nuclear magnetic resonance, sugar and lignin content was quantified and labeled wheat contribution was determined by C compound specific analyses. Results showed decreased wheat O-alkyl-C with increasing microbial diversity. Total non-cellulosic sugar-C derived from wheat was not significantly influenced by microbial diversity. Carbon from wheat sugars (arabinose-C and xylose-C), however, was highest when microbial diversity was low, indicating reduced wheat sugar decomposition at low microbial diversity. Xylose-C was significantly correlated with the Shannon diversity index of the bacterial community. Soil lignin-C decreased irrespective of microbial diversity. At low microbial diversity the oxidation state of vanillyl-lignin units was significantly reduced. We conclude that microbial diversity alters bulk chemical structure, the decomposition of plant litter sugars and influences the microbial oxidation of total vanillyl-lignins, thus changing SOM composition. [ABSTRACT FROM AUTHOR]

Cutins of plant shoots and suberins, mostly present in roots could contribute to significant portions of stable soil organic matter. Their biomarker potential, residing in their unique compositions in different plant types, has been used previously to infer sources of organic matter in sediments. These aliphatic plant biopolyesters contain specific biomarkers, which may be used for tracing their fate in soils and sediments, when combined with stable C isotope labelling. In order to evaluate the potential use of cutin and suberin biomarkers as indicators of shoot and root contributions from C and C plant origins, the objectives of this study were to 1) identify their constitutive monomers, which are specific for shoots and roots of maize (C) and wheat (C); 2) evaluate the C differences between maize and wheat biomarkers. Mid-chain hydroxy carboxylic acids were mainly found in the aboveground biomass, while α,ω-alkanedioic acids were only present in the roots. The differences in the isotopic composition of the specific monomers between wheat and maize plants (17-18‰ for shoot markers, 19‰ for root markers) were larger than those observed for bulk plant tissues and close to those observed for lignin-derived phenols in other studies. These differences should make it possible to differentiate and quantify the different types and sources of organic matter in sediments and soils. In particular, the molecular and isotopic signatures of cutins and suberins can be used to evaluate the specific dynamics of root vs shoot tissues in soils using C/C chronosequences. [ABSTRACT FROM AUTHOR]

Abstract: Lignins are amongst the most studied macromolecules in natural environments. During the last decades, lignins were considered as important components for the carbon cycle in soils, and particularly for the carbon storage. Thus, they are an important variable in many soil–plant models such as CENTURY and RothC, and appeared determinant for the estimation of the soil organic matter (SOM) pool-size and its stabilization. Recent studies challenged this point of view. The aim of this paper was to synthesise the current knowledge and recent progress about quantity, composition and turnover of lignins in soils and to identify variables determining lignin residence time. In soils, lignins evolve under the influence of various variables and processes such as their degradation or mineralization, as well as their incorporation into SOM. Lignin-derived products obtained after CuO oxidation can be used as environmental biomarkers, and also vary with the degree of degradation of the molecule. The lignin degradation is related to the nature of vegetation and land-use, but also to the climate and soil characteristics. Lignin content of SOM decreases with decreasing size of the granulometric fractions, whereas its level of degradation increases concomitantly. Many studies and our results suggest the accumulation and potential stabilization of a part of lignins in soils, by interaction with the clay minerals, although the mechanisms remain unclear. Lignin turnover in soils could be faster than that of the total SOM. Different kinetic pools of lignins were suggested, which sizes seem to be variable for different soil types. The mechanisms behind different degradation kinetics as well as their potential stabilization behaviour still need to be elucidated. [Copyright &y& Elsevier]

N has a controlling effect on litter biodegradation in the forest floor, while stabilization of organic matter in the mineral soil may be influenced by physical parameters related to soil texture. In this study, in order to understand the processes involved in soil organic matter (SOM) formation, the chemical composition of SOM was followed and evaluated with regards to N contents and soil texture. Samples were taken on sites covered with Norway spruce and displaying contrasting values of C/N ratios in the forest floor. The chemical structure of OM was characterized using solid-state CPMAS 13C and 15N nuclear magnetic resonance (NMR) spectroscopy, along with Proton Spin Relaxation Editing (PSRE) sequences. Four groups of sampling sites were defined based on the NMR spectra of Oh and A horizons. In each group displaying similar NMR characteristics, N content and soil texture could be highly different among sites. Some Oh horizons with similar NMR spectra had very different N contents. Highly humified OM in Oh horizons were observed mainly on sites with low N contents. Some A horizons with different soil texture displayed similar OM chemical structure. High contents of O-alkyl C in some A horizons could originate from higher fresh root material input. [Copyright &y& Elsevier]

Abstract: 13C contents of organic matter are changing during decomposition of plant material and stabilization as soil organic carbon (SOC). In this context, several studies showed 13C enrichment in soil as compared to vegetation for C3 forests, whereas depletion of 13C was frequently reported for C4 grassland soil as compared to C4 vegetation. These changes were often attributed to selective preservation and/or stabilization of specific organic compounds. This study investigates if changes in the chemical composition of OC and specifically lignin may explain the observed shifts in δ13C values from plant material to SOC. We analyzed aboveground biomass, roots and heavy organo-mineral fractions from topsoils in both, long-term stable C4 grasslands and C3 Araucaria forest situated nearby in the southern Brazilian highlands on soils with andic properties. The stable carbon isotope (12C/13C) composition was analyzed for total organic carbon (OCtot) and lignin-derived phenols. The bulk chemical composition of OC was assessed by solid-state 13C NMR spectroscopy while neutral sugar monomers were determined after acid hydrolysis. The shifts of the 13C/12C isotope signature during decomposition and stabilization (plant tissues versus soil heavy fractions) showed similar trends for VSC phenols and OCtot (13C depletion in C4 grassland soil and 13C enrichment in C3 forest soil compared to the corresponding vegetation). In this regard, the isotopic difference between roots and aboveground biomass was not relevant, but may become more important at greater soil depths. 13C depletion of VSC lignins relative to OCtot was higher in C3-biomass and C3-derived SOC compared to the C4 counterparts. As lignin contents of heavy fractions were low, in particular for those with C4 isotopic signature, the influence of lignin on OCtot δ13C values in grassland topsoils is presumably low. Rather, the presence of charred grass residues and the accumulation of alkyl C in heavy fractions as revealed by 13C NMR spectroscopy contribute to decreasing δ13C values from grass biomass to C4-derived heavy fractions. In forest topsoils, the accumulation of 13C depleted VSC lignin residues in heavy fractions counteracts the prevailing 13C enrichment of OCtot from plant biomass to heavy fractions. Nonetheless, non-lignin compounds with relatively high 13C contents like microbial-derived OC have a stronger influence on δ13C values of OCtot in forest soils than lignins or aliphatic biopolymers. The mineral-associated SOC is in a late phase of decomposition with large contributions of microbial-derived carbohydrates, but distinct structural and isotopical alterations of lignin between C4- and C3-derived heavy fractions. This may indicate different processes and/or extent of lignin (and SOM) biodegradation between C4 grassland and C3 forest resulting from other kind of decomposer communities in association with distinct types and amounts of plant input as source of SOM and thus, carbon source for microbial transformation. Our results indicate that the importance of lignin for δ13C values of OCtot was overestimated in previous studies, at least in subtropical C4 grassland and C3 forest topsoils. [Copyright &y& Elsevier]

A novel method coupling pyrolysis with mid-infrared spectroscopy (Py-MIRS) was developed to characterize soil organic matter (SOM) chemistry in soils. The pyrolyzer was interfaced to the MIR spectrometer by means of a Brill cell™ (CDS Analytica). The set-up generates pyrolysis fingerprint spectra from which individual pyrolysis products can be related to SOM bulk chemistry. Py-MIRS development involved the testing of experimental conditions like pyrolysis temperature (550, 700, 1000 °C), heating rate (20 °C s −1 and 20 °C ms −1 ) and time (15, 30 and 60 s) using reference standard compounds ranging from carbohydrates to phenols varying in chemical and structural composition like levoglusogan, gluten, tannin, syringol, pectin and leucine falling within different compound categories (carbohydrates, amino acids, proteins, phenols, etc.) as well as soil samples. Pyrolysis yields of prominent specific functional groups, like aliphatics (C H stretching at 2930 cm −1 ) and C C aromatics (1510 cm −1 ), varied with pyrolysis temperature, heating rate and time. The preferred settings for high pyrolysis yield and minimized secondary reactions were obtained at a pyrolysis temperature of 700 °C, heating rate of 20 °C ms −1 and heating time of 30 s. The suitability of Py-MIRS to detect changes in SOM composition was evaluated by comparing Py-MIRS results to Diffuse Reflectance Fourier Transform mid-Infrared Spectroscopy (DRIFTS) results. Soil samples taken from the Static Fertilization Experiment, Bad Lauchstädt, Germany (Chernozem) revealed a major SOM contribution of the peak at 1750 cm −1 (C O), followed by peaks at 950 (C H), 1510 (C C), 1176 (C H, O H) cm −1 , with smaller contributions from the 2930 (C H) and 3015 (CH 4 ) cm −1 peaks, apart from a dominant CO 2 peak. Using the preferred pyrolysis settings, Py-MIRS as well as DRIFTS results further indicated that soils receiving organic (e.g. farmyard manure) inputs were highly enriched in aliphatic groups, while their absence favored the accumulation of carboxyl and aromatic groups as well as polysaccharides. Py-MIRS allowed via semi-quantification of pyrolysis products a rapid monitoring of SOM bulk chemistry with a high degree of reproducibility. It was concluded that Py-MIRS represents a fast, effective and reproducible technique to characterize changes in the SOM bulk chemistry as a result of management practices. It also allows to minimize acknowledged constraints of other analytical techniques used to characterize SOM bulk chemistry such as mineral interferences and associated secondary reactions. [ABSTRACT FROM AUTHOR]

Up to now stabilization of organic matter (OM) in soils due to mineral interactions has been assessed mainly by correlations between carbon and iron and/or aluminum oxides evidencing that metal oxides may be principal stabilization agents. The nature and morphology of stabilized OM are poorly known. Taking advantage of a field experiment, the aim of our study was to analyze the fate of 13C and 15N labeled root material at 30 and 90 cm depths after three years of incubation and to characterize the nature of OM stabilized by interactions with metal oxides. Our methodological approach included isolation of metal oxides by physical fractionation and visualization of their interaction with OM using NanoSIMS. We concentrated metal oxides in a fraction corresponding to our objectives: the heavy fraction (>3 g cm−3) of fine silt. NanoSIMS analyses of this fraction allowed us to locate unlabeled OM and OM either double labeled or carrying one single label in association with metal oxides. Our results suggest that decoupling of C and N may have happened during OM stabilization within the timeframe of the 3 year field experiment. Scanning electron microscopy (SEM) after NanoSIMS analyzes, indicated that 15N labeled OM at 90 cm were well-defined ovoid OM particles resembling to microbial cells in interaction with Fe, Al and Ti oxides. At 30 cm depth, OM associated with metal oxides was 13C and 15N labeled unstructured material, possibly derived from plant debris. We suggest that at the two soil depths under investigation different processes might be at work, leading to association of OM with mineral compounds of the isolated fraction: in upper soil layers, decomposed plant material may directly interact with metal oxides, whereas in deep mineral soil, OM could mainly interact with metal oxides after microbial turnover. Both types of interactions may be fairly stable as they persisted after ultrasonication and salt extraction. [ABSTRACT FROM AUTHOR]

Abstract: The vegetation of the southern Brazilian highlands in Rio Grande do Sul State is a mosaic of grassland (C4) and deciduous forests (C3) with the conifer Araucaria angustifolia. It was uncertain, whether the grasslands represent relics of drier periods in the Holocene or if they are the result of deforestation in recent times. We analyzed plant tissues from gramineous and woody species, organic surface layers, as well as soil organic matter of 13 Andosols and Umbrisols in grassland, shrubland, pine plantations and Araucaria forest for stable carbon isotope ratios (δ 13C) and 14C activity. The soil organic matter was separated into a free particulate organic matter (fPOM) and a heavy, organo-mineral fraction by density fractionation. All grassland soils have consistently δ 13C values of −18.7 to −14.3‰ typical for C4 grasses. In Araucaria forests and forest patches within grassland the δ 13C values of both, the fPOM throughout the soil and the organic surface layers, are characteristic for the present below- and above-ground input from C3 trees. The C3- and C4-derived SOC stocks reflect expansion of Araucaria forest on grassland, which started after 1300 yr BP. The youngest forests are found at the forest border and in forest patches. Grassland soils lose their typically black colour from the top downwards after shrub encroachment or establishment of forest as indicated by increasing melanic indexes which are closely related to the δ 13C values. The natural 13C depletion with depth in grassland soils counteracts the enrichment of 13C in the subsoils of present Araucaria forest. The results clearly indicate that current grasslands represent relics at least from the early and mid Holocene period (6000–8000 yr BP) and are not the result of recent deforestation. [Copyright &y& Elsevier]