Nitrogen input quality changes the biochemical composition of soil organic matter stabilized in the fine fraction: a long-term study

The chemical composition of soil organic matter (SOM) is a key determinant of its biological stability. Our objective in this study was to evaluate the effects of various sources of supplemental N on the chemical composition of SOM in the fine (<5 μm) mineral fraction. Treatments were fallow, maize/soybean in rotation, and continuous maize receiving no fertilizer (maize0N), synthetic fertilizer N (maize + N), or composted manure (maize + manure). The chemical structures in SOM associated with the fine fraction were determined using XANES spectroscopy at the C and N K-edges, which was assessed using multidimensional scaling. Analysis of amino sugar biomarkers were used to evaluate the fungal:bacterial contributions to the SOM. The addition of N to soils (i.e., maize + N, maize + manure, and maize/soybean treatments) resulted in the enrichment of proteinaceous compounds. Soils which did not receive supplemental N (i.e., fallow and maize0N treatments) were enriched in plant-derived compounds (e.g., aromatics, phenolics, carboxylic acids and aliphatic compounds), suggesting that decomposition of plant residues was constrained by N-limitation. Microbial populations assessed by amino sugar biomarker ratios showed that the highest contributions to SOM by bacteria occurred in the maize + manure treatment (high N input), and by fungi in the fallow treatment (low N input). The SOM in the maize + N and maize/soybean treatments was enriched in N-bonded aromatics; we attribute this enrichment to the abiotic reaction of inorganic N with organic C structures. The SOM in the maize + manure treatment was enriched in pyridinic-N, likely as a result of intense microbial processing and high SOM turnover. The presences of signals for ketone and pyrrole compounds in XANES spectra suggest their use as biomarkers for microbially transformed and stabilized SOM. The SOM in the maize + manure treatment was enriched in ketones which are likely microbial by-products of fatty acid catabolism. Pyrrole compounds, which may accumulate over the long term as by-products of protein transformations by an N-limited microbial community, were dominant in the fallow soil. A combination of molecular spectroscopy and biomarker analysis showed that the source of supplemental N to soil influences the stable C- and N-containing compounds of SOM in a long-term field study. Indeed, any increase in N availability allowed the microbial community to transform plant material into microbial by-products which occur as stable SOM compounds in the fine soil fraction.     

Sources of plant-derived carbon and stability of organic matter in soil: implications for global change

Alterations in forest productivity and changes in the relative proportion of above‐ and belowground biomass may have nonlinear effects on soil organic matter (SOM) storage. To study the influence of plant litter inputs on SOM accumulation, the Detritus Input Removal and Transfer (DIRT) Experiment continuously alters above‐ and belowground plant inputs to soil by a combination of trenching, screening, and litter addition. Here, we used biogeochemical indicators [i.e., cupric oxide extractable lignin‐derived phenols and suberin/cutin‐derived substituted fatty acids (SFA)] to identify the dominant sources of plant biopolymers in SOM and various measures [i.e., soil density fractionation, laboratory incubation, and radiocarbon‐based mean residence time (MRT)] to assess the stability of SOM in two contrasting forests within the DIRT Experiment: an aggrading deciduous forest and an old‐growth coniferous forest. In the deciduous forest, removal of both above‐ and belowground inputs increased the total amount of SFA over threefold compared with the control, and shifted the SFA signature towards a root‐dominated source. Concurrently, light fraction MRT increased by 101 years and C mineralization during incubation decreased compared with the control. Together, these data suggest that root‐derived aliphatic compounds are a source of SOM with greater relative stability than leaf inputs at this site. In the coniferous forest, roots were an important source of soil lignin‐derived phenols but needle‐derived, rather than root‐derived, aliphatic compounds were preferentially preserved in soil. Fresh wood additions elevated the amount of soil C recovered as light fraction material but also elevated mineralization during incubation compared with other DIRT treatments, suggesting that not all of the added soil C is directly stabilized. Aboveground needle litter additions, which are more N‐rich than wood debris, resulted in accelerated mineralization of previously stored soil carbon. In summary, our work demonstrates that the dominant plant sources of SOM differed substantially between forest types. Furthermore, inputs to and losses from soil C pools likely will not be altered uniformly by changes in litter input rates.     

Soil organic matter molecular composition with long-term detrital alterations is controlled by site-specific forest properties

Forest ecosystems are important global soil carbon (C) reservoirs, but their capacity to sequester C is susceptible to climate change factors that alter the quantity and quality of C inputs. To better understand forest soil C responses to altered C inputs, we integrated three molecular composition published data sets of soil organic matter (SOM) and soil microbial communities for mineral soils after 20 years of detrital input and removal treatments in two deciduous forests: Bousson Forest (BF), Harvard Forest (HF), and a coniferous forest: H.J. Andrews Forest (HJA). Soil C turnover times were estimated from radiocarbon measurements and compared with the molecular-level data (based on nuclear magnetic resonance and specific analysis of plant- and microbial-derived compounds) to better understand how ecosystem properties control soil C biogeochemistry and dynamics. Doubled aboveground litter additions did not increase soil C for any of the forests studied likely due to long-term soil priming. The degree of SOM decomposition was higher for bacteria-dominated sites with higher nitrogen (N) availability while lower for the N-poor coniferous forest. Litter exclusions significantly decreased soil C, increased SOM decomposition state, and led to the adaptation of the microbial communities to changes in available substrates. Finally, although aboveground litter determined soil C dynamics and its molecular composition in the coniferous forest (HJA), belowground litter appeared to be more influential in broadleaf deciduous forests (BH and HF). This synthesis demonstrates that inherent ecosystem properties regulate how soil C dynamics change with litter manipulations at the molecular-level. Across the forests studied, 20 years of litter additions did not enhance soil C content, whereas litter reductions negatively impacted soil C concentrations. These results indicate that soil C biogeochemistry at these temperate forests is highly sensitive to changes in litter deposition, which are a product of environmental change drivers.     

Soil microbial diversity affects soil organic matter decomposition in a silty grassland soil

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 ¹³C 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 ¹³C nuclear magnetic resonance, sugar and lignin content was quantified and labeled wheat contribution was determined by ¹³C 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.     

Molecular-level changes in soil organic matter composition after 10 years of litter,root and nitrogen manipulation in a temperate forest

With climate change, forests are expected to receive increased inputs of carbon (C) and nitrogen (N) but it is unclear how this will modify forest C cycling and storage at the molecular-level. To investigate the response of forest soil organic matter (SOM) to changes in soil inputs, a study area was established in a Michigan hardwood forest as part of the Detrital Input and Removal Treatments (DIRT) network. Experimental treatments were comprised of both exclusions of detrital inputs (No Litter, No Roots, No Inputs) and additions of C and N (Double Litter, N-Addition, Double Litter?+?N, Wood). After 10 years of treatment, the soils were characterized using elemental analysis, molecular biomarker techniques, nuclear magnetic resonance spectroscopy, and microbial biomass C measurements. Although manipulation of detrital inputs did not significantly change the soil C and N content after 10 years, alterations in the cycling and distribution of SOM components were observed. Root exclusion enhanced SOM degradation, while doubling litter favoured the degradation of more labile forms of soil C such as unsaturated n-alkanoic acids and simple sugars. N-Addition and Double Litter?+?N increased the concentrations of extractable biomarkers, including aliphatic and cyclic lipids and compounds derived from cutin, suberin, and lignin. Microbial biomass C also varied with experimental litter input manipulations and N addition, and these data were consistent with the observed changes in SOM composition. Overall, the observed shifts in SOM chemistry after 10 years of manipulating ecosystem inputs highlight the sensitivity of natural systems to changes in amounts of C and N inputs from roots and litter, and N inputs from external sources.     

Anthropogenic N deposition alters soil organic matter biochemistry and microbial communities on decaying fine roots

Fine root litter is a primary source of soil organic matter (SOM), which is a globally important pool of C that is responsive to climate change. We previously established that ~20 years of experimental nitrogen (N) deposition has slowed fine root decay and increased the storage of soil carbon (C; +18%) across a widespread northern hardwood forest ecosystem. However, the microbial mechanisms that have directly slowed fine root decay are unknown. Here, we show that experimental N deposition has decreased the relative abundance of Agaricales fungi (?31%) and increased that of partially ligninolytic Actinobacteria (+24%) on decaying fine roots. Moreover, experimental N deposition has increased the relative abundance of lignin‐derived compounds residing in SOM (+53%), and this biochemical response is significantly related to shifts in both fungal and bacterial community composition. Specifically, the accumulation of lignin‐derived compounds in SOM is negatively related to the relative abundance of ligninolytic Mycena and Kuehneromyces fungi, and positively related to Microbacteriaceae. Our findings suggest that by altering the composition of microbial communities on decaying fine roots such that their capacity for lignin degradation is reduced, experimental N deposition has slowed fine root litter decay, and increased the contribution of lignin‐derived compounds from fine roots to SOM. The microbial responses we observed may explain widespread findings that anthropogenic N deposition increases soil C storage in terrestrial ecosystems. More broadly, our findings directly link composition to function in soil microbial communities, and implicate compositional shifts in mediating biogeochemical processes of global significance.     

Long-term impacts of anthropogenic perturbations on dynamics and speciation of organic carbon in tropical forest and subtropical grassland ecosystems

Anthropogenic perturbations have profoundly modified the Earth's biogeochemical cycles, the most prominent of these changes being manifested by global carbon (C) cycling. We investigated long‐term effects of human‐induced land‐use and land‐cover changes from native tropical forest (Kenya) and subtropical grassland (South Africa) ecosystems to agriculture on the dynamics and structural composition of soil organic C (SOC) using elemental analysis and integrated ¹³C nuclear magnetic resonance (NMR), near‐edge X‐ray absorption fine structure (NEXAFS) and synchrotron‐based Fourier transform infrared‐attenuated total reflectance (Sr‐FTIR‐ATR) spectroscopy. Anthropogenic interventions led to the depletion of 76%, 86% and 67% of the total SOC; and 77%, 85% and 66% of the N concentrations from the surface soils of Nandi, Kakamega and the South African sites, respectively, over a period of up to 100 years. Significant proportions of the total SOC (46–73%) and N (37–73%) losses occurred during the first 4 years of conversion indicating that these forest‐ and grassland‐derived soils contain large amounts of labile soil organic matter (SOM), potentially vulnerable to degradation upon human‐induced land‐use and land‐cover changes. Anthropogenic perturbations altered not only the C sink capacity of these soils, but also the functional group composition and dynamics of SOC with time, rendering structural composition of the resultant organic matter in the agricultural soils to be considerably different from the SOM under natural forest and grassland ecosystems. These molecular level compositional changes were manifested: (i) by the continued degradation of O‐alkyl and acetal‐C structures found in carbohydrate and holocellulose biomolecules, some labile aliphatic‐C functionalities, (ii) by side‐chain oxidation of phenylpropane units of lignin and (iii) by the continued aromatization and aliphatization of the humic fractions possibly through selective accumulation of recalcitrant H and C substituted aryl‐C and aliphatic‐C components such as (poly)‐methylene units, respectively. These changes appeared as early as the fourth year after transition, and their intensity increased with duration of cultivation until a new quasi‐equilibrium of SOC was approached at about 20 years after conversion. However, subtle but persistent changes in molecular structures of the resultant SOM continued long after (up to 100 years) a steady state for SOC was approached. These molecular level changes in the inherent structural composition of SOC may exert considerable influence on biogeochemical cycling of C and bioavailability of essential nutrients present in association with SOM, and may significantly affect the sustainability of agriculture as well as potentials of the soils to sequester C in these tropical and subtropical highland agroecosystems.     

Application of solid surface fluorescence EEM spectroscopy for tracking organic matter quality of native halophyte and furrow-irrigated soils

Solid surface fluorescence excitation-emission matrix (EEM) is developed a potential method to characterize soil organic matter (SOM). Solid surface EEM spectroscopy with parallel factor analysis (PARAFAC) and hierarchical cluster analysis (HCA) is used to extract fluorescent components, to seek latent factors, and to investigate spatial distribution of SOM. Soil samples were collected from four native halophyte and two furrow-irrigated soil profiles, i.e. Comm. Salicornia europaea (CSE), Comm. Suaeda glauca (CSG), Comm. Kalidium cuspidatum (CKC), Comm. Sophora alopecuroides (CSA), corn fields (CFD), and wheat fields (WFD). SOM contained six fluorescent components: microbial/terrestrial fulvic-like fluorescent components (C1), tryptophan-like/lignin-derived phenol fluorescent components (C2), terrestrial humic-like fluorescent component (C3), lignin oxidative degradation by-products (C4 and C5), and amino acids (C6). The C 4 and C5 were the representative components of SOM within the CSE, CSG, CKC, CSA and CFD soil profiles, while the C2 and C6 were dominated within the WFD soil profile. The C4, C5, C1 and C2 were latent factors, and they could roughly distinguish SOM within the whole saline soil profiles except the CFD. A humification index (H/L) deduced from the fluorescent components, was very suitable to indicate humification levels of SOM. Humification levels of SOM within the halophyte soil profiles decreased with soil depth, but the opposite trends within the furrow-irrigated soil profiles. The H/L was closely correlated with exchangeable sodium percentage (ESP), and humification levels increased with the decreasing ESP. Soil surface EEM may not only indicate organic matter fractions of saline soils, but may be transferred to other types of landscape.     

Temperature response of litter and soil organic matter decomposition is determined by chemical composition of organic material

The global soil carbon pool is approximately three times larger than the contemporary atmospheric pool, therefore even minor changes to its integrity may have major implications for atmospheric CO₂ concentrations. While theory predicts that the chemical composition of organic matter should constitute a master control on the temperature response of its decomposition, this relationship has not yet been fully demonstrated. We used laboratory incubations of forest soil organic matter (SOM) and fresh litter material together with NMR spectroscopy to make this connection between organic chemical composition and temperature sensitivity of decomposition. Temperature response of decomposition in both fresh litter and SOM was directly related to the chemical composition of the constituent organic matter, explaining 90% and 70% of the variance in Q₁₀ in litter and SOM, respectively. The Q₁₀ of litter decreased with increasing proportions of aromatic and O‐aromatic compounds, and increased with increased contents of alkyl‐ and O‐alkyl carbons. In contrast, in SOM, decomposition was affected only by carbonyl compounds. To reveal why a certain group of organic chemical compounds affected the temperature sensitivity of organic matter decomposition in litter and SOM, a more detailed characterization of the ¹³C aromatic region using Heteronuclear Single Quantum Coherence (HSQC) was conducted. The results revealed considerable differences in the aromatic region between litter and SOM. This suggests that the correlation between chemical composition of organic matter and the temperature response of decomposition differed between litter and SOM. The temperature response of soil decomposition processes can thus be described by the chemical composition of its constituent organic matter, this paves the way for improved ecosystem modeling of biosphere feedbacks under a changing climate.     

Altered microbial community structure and organic matter composition under elevated CO2 and N fertilization in the duke forest

The dynamics and fate of terrestrial organic matter (OM) under elevated atmospheric CO₂ and nitrogen (N) fertilization are important aspects of long‐term carbon sequestration. Despite numerous studies, questions still remain as to whether the chemical composition of OM may alter with these environmental changes. In this study, we employed molecular‐level methods to investigate the composition and degradation of various OM components in the forest floor (O horizon) and mineral soil (0–15 cm) from the Duke forest free air CO₂ enrichment (FACE) experiment. We measured microbial responses to elevated CO₂ and N fertilization in the mineral soil using phospholipid fatty acid (PLFA) profiles. Increased fresh carbon inputs into the forest floor under elevated CO₂ were observed at the molecular‐level by two degradation parameters of plant‐derived steroids and cutin‐derived compounds. The ratios of fungal to bacterial PLFAs and Gram‐negative to Gram‐positive bacterial PLFAs decreased in the mineral soil with N fertilization, indicating an altered soil microbial community composition. Moreover, the acid to aldehyde ratios of lignin‐derived phenols increased with N fertilization, suggesting enhanced lignin degradation in the mineral soil. ¹H nuclear magnetic resonance (NMR) spectra of soil humic substances revealed an enrichment of leaf‐derived alkyl structures with both elevated CO₂ and N fertilization. We suggest that microbial decomposition of SOM constituents such as lignin and hydrolysable lipids was promoted under both elevated CO₂ and N fertilization, which led to the enrichment of plant‐derived recalcitrant structures (such as alkyl carbon) in the soil.     

Litter type control on soil C and N stabilization dynamics in a temperate forest

While plant litters are the main source of soil organic matter (SOM) in forests, the controllers and pathways to stable SOM formation remain unclear. Here, we address how litter type (¹³C/¹⁵N‐labeled needles vs. fine roots) and placement‐depth (O vs. A horizon) affect in situ C and N dynamics in a temperate forest soil after 5 years. Litter type rather than placement‐depth controlled soil C and N retention after 5 years in situ, with belowground fine root inputs greatly enhancing soil C (x1.4) and N (x1.2) retention compared with aboveground needles. While the proportions of added needle and fine root‐derived C and N recovered into stable SOM fractions were similar, they followed different transformation pathways into stable SOM fractions: fine root transfer was slower than for needles, but proportionally more of the remaining needle‐derived C and N was transferred into stable SOM fractions. The stoichiometry of litter‐derived C vs. N within individual SOM fractions revealed the presence at least two pools of different turnover times (per SOM fraction) and emphasized the role of N‐rich compounds for long‐term persistence. Finally, a regression approach suggested that models may underestimate soil C retention from litter with fast decomposition rates.     

A consensus map of QTLs controlling the root length of maize

Despite their low carbon (C) content, most subsoil horizons contribute to more than half of the total soil C stocks, and therefore need to be considered in the global C cycle. Until recently, the properties and dynamics of C in deep soils was largely ignored. The aim of this review is to synthesize literature concerning the sources, composition, mechanisms of stabilisation and destabilization of soil organic matter (SOM) stored in subsoil horizons. Organic C input into subsoils occurs in dissolved form (DOC) following preferential flow pathways, as aboveground or root litter and exudates along root channels and/or through bioturbation. The relative importance of these inputs for subsoil C distribution and dynamics still needs to be evaluated. Generally, C in deep soil horizons is characterized by high mean residence times of up to several thousand years. With few exceptions, the carbon-to-nitrogen (C/N) ratio is decreasing with soil depth, while the stable C and N isotope ratios of SOM are increasing, indicating that organic matter (OM) in deep soil horizons is highly processed. Several studies suggest that SOM in subsoils is enriched in microbial-derived C compounds and depleted in energy-rich plant material compared to topsoil SOM. However, the chemical composition of SOM in subsoils is soil-type specific and greatly influenced by pedological processes. Interaction with the mineral phase, in particular amorphous iron (Fe) and aluminum (Al) oxides was reported to be the main stabilization mechanism in acid and near neutral soils. In addition, occlusion within soil aggregates has been identified to account for a great proportion of SOM preserved in subsoils. Laboratory studies have shown that the decomposition of subsoil C with high residence times could be stimulated by addition of labile C. Other mechanisms leading to destabilisation of SOM in subsoils include disruption of the physical structure and nutrient supply to soil microorganisms. One of the most important factors leading to protection of SOM in subsoils may be the spatial separation of SOM, microorganisms and extracellular enzyme activity possibly related to the heterogeneity of C input. As a result of the different processes, stabilized SOM in subsoils is horizontally stratified. In order to better understand deep SOM dynamics and to include them into soil C models, quantitative information about C fluxes resulting from C input, stabilization and destabilization processes at the field scale are necessary.     

Degradation and Preservation of Vascular Plant-derived Biomarkers in Grassland and Forest Soils from Western Canada

The total solvent extracts (TSE) of mineral and organic horizons of selected soils and overlying vegetation were analyzed using gas chromatography–mass spectrometry (GC–MS) to determine the composition of solvent-extractable (‘free’) lipids in soils and to study the degradation and possible preservation of vascular plant-derived molecular markers (biomarkers) in soils. Major compound classes in the TSE of soils and vegetation included a homologous series of aliphatic lipids (alkanoic acids, alkanols, alkanes), steroids, and terpenoids. Characteristic patterns of aliphatic and cyclic biomarkers derived from the overlying, native vegetation were recognized in the associated soil samples indicating the preservation of lipids from the external waxes of vascular plants in the soil organic matter (SOM). The observed biomarker patterns in the grassland soils (Brown Chernozems) were similar to the compounds identified in their major source vegetation, Western Wheatgrass. A similar composition of biomarkers was observed in Aspen leaves and the soil horizons of the forest–grassland transition soil (Dark Gray Chernozem). The Lodgepole Pine needles yielded a characteristic pattern of diterpenoids that was also detected in leaf litter and the O horizon of the associated forest soil (Brunisol). The results demonstrate that solvent extractable biomarkers derived from vascular plants maintain their characteristic pattern of aliphatic and cyclic lipids despite ongoing degradation processes and are thus valuable molecular markers for the determination of the sources of SOM. Furthermore, the abundance of aliphatic wax lipids in plant material and soils decreased at higher rates than the steroids and terpenoids indicating the preferential degradation of aliphatic over cyclic biomarkers. Most of the plant-derived steroids and terpenoids identified in the soils were unaltered, preserved biomolecules as observed in the source vegetation, but minor amounts of their degradation products were also present. Oxidation products of plant sterols are reported here for the first time in soils. The detected alteration products of steroids and diterpenoids are consistent with the oxidative degradation of free cyclic biomarkers in decomposing plant material and soils.     

Linking temperature sensitivity of soil organic matter decomposition to its molecular structure,accessibility, and microbial physiology

Temperature sensitivity of soil organic matter (SOM) decomposition may have a significant impact on global warming. Enzyme‐kinetic hypothesis suggests that decomposition of low‐quality substrate (recalcitrant molecular structure) requires higher activation energy and thus has greater temperature sensitivity than that of high‐quality, labile substrate. Supporting evidence, however, relies largely on indirect indices of substrate quality. Furthermore, the enzyme‐substrate reactions that drive decomposition may be regulated by microbial physiology and/or constrained by protective effects of soil mineral matrix. We thus tested the kinetic hypothesis by directly assessing the carbon molecular structure of low‐density fraction (LF) which represents readily accessible, mineral‐free SOM pool. Using five mineral soil samples of contrasting SOM concentrations, we conducted 30‐days incubations (15, 25, and 35 °C) to measure microbial respiration and quantified easily soluble C as well as microbial biomass C pools before and after the incubations. Carbon structure of LFs (<1.6 and 1.6–1.8 g cm^?3) and bulk soil was measured by solid‐state ¹³C‐NMR. Decomposition Q₁₀ was significantly correlated with the abundance of aromatic plus alkyl‐C relative to O‐alkyl‐C groups in LFs but not in bulk soil fraction or with the indirect C quality indices based on microbial respiration or biomass. The warming did not significantly change the concentration of biomass C or the three types of soluble C despite two‐ to three‐fold increase in respiration. Thus, enhanced microbial maintenance respiration (reduced C‐use efficiency) especially in the soils rich in recalcitrant LF might lead to the apparent equilibrium between SOM solubilization and microbial C uptake. Our results showed physical fractionation coupled with direct assessment of molecular structure as an effective approach and supported the enzyme‐kinetic interpretation of widely observed C quality‐temperature relationship for short‐term decomposition. Factors controlling long‐term decomposition Q₁₀ are more complex due to protective effect of mineral matrix and thus remain as a central question.     

Chemical and seasonal controls on the dynamics of dissolved organic matter in a coniferous old-growth stand in the Pacific Northwest,USA

Soil organic matter (SOM) is the largest terrestrial C pool, and retention and release of dissolved organic matter (DOM) cause formation and loss of SOM. However, we lack information on how different sources of DOM affect its chemical composition, and how DOM chemical composition affects retention. We studied seasonal controls on DOM production and chemical controls on retention in soils of a temperate coniferous forest. The O horizon was not usually the dominant source for dissolved organic C (DOC) or N (DON) as has been reported for other sites. Rather, net production of both DOC and DON was often greater in the shallow mineral soil (0–10 cm) than in the O horizon. DOM production in the shallow mineral soil may be from root exudation as well as turnover of fine roots and microflora in the rhizosphere. In the field, the two acid fractions (hydrophobic and hydrophilic acids) dominated the soil solution at all depths. A major portion of net production and removal of total DOC within the soil column was explained by increases and decreases in these fractions, although a shift in chemical composition of DOM between the O and mineral soil horizons suggested different origins of DOM in these layers. A larger loss of the free amino fraction to deep soil water at this study site than at other sites suggested lower retention of labile DON. Field DOM removal measurements suggest that field-measured parameters may provide a good estimate for total DOM retained in mineral soil.     

Variations in microbial isotopic fractionation during soil organic matter decomposition

The soil microbial biomass (SMB) is known to participate in key soil processes such as the decomposition of soil organic matter (SOM). However, its contribution to the isotopic composition of the SOM is not clear yet. Shifts in the ¹³C and ¹⁵N natural abundances of the SMB and SOM fractions (mineralised, water soluble and non-extractable) were investigated by incubating an unamended arable soil for 6 months. Microbial communities were also studied using Fatty Acid Methyl Ester specific isotope analysis. The SMB was significantly ¹³C and ¹⁵N-enriched relative to other fractions throughout the incubation. However, significant isotopic variations with time were also observed due to the rapid consumption of relatively ¹³C-enriched water soluble compounds. The increase in the difference in SMB and water soluble ¹⁵N compositions as the water soluble C/N ratio decreased, indicated a shift from N assimilation to N dissimilation during the incubation. These changes also induced modifications of the microbial community structure. Once the system reached a steady-state (after 1 month), the isotopic trends appeared to corroborate those obtained in long term experiments in the field in that there was a constant microbial isotopic fractionation leading to a ¹³C and ¹⁵N enrichment of the SOM over the long-term. This work also suggests that caution must be exercised when interpreting short term incubation studies since perturbations associated with experimental set-up can have an important effect on C and N dynamics, microbial fractionation of ¹³C and ¹⁵N and microbial community structure.     

Effect of physical weathering on the carbon sequestration potential of biochars and hydrochars in soil

Physical weathering can modify the stability of biochar after field exposure. The aim of our study was to determine the potential carbon sequestration of the two chars at different timescales. We investigated the modification in composition and stability resulting from physical weathering of two different chars produced (i) at low temperature (250 °C) by hydrothermal carbonization (HTC); and (ii) at high temperature (1200 °C) by gasification (GS) using contrasting feedstocks. Physical weathering of HTC and GS placed on a water permeable canvas was performed through successive wetting/drying and freezing/thawing cycles. Carbon loss was assessed by mass balance. Chemical stability of the remaining material was evaluated as resistance to acid dichromate oxidation, and biological stability was assessed during laboratory incubation. Moreover, we assessed modification in potential priming effects due to physical weathering. Physical weathering induced a carbon loss ranging between 10 and 40% of the total C mass depending on the feedstock. This C loss is most probably related to leaching of small particulate and dissolved compounds. GS produced from maize silage showed the highest C loss. The chemical stability of HTC and GS was unaffected by physical weathering. In contrast, physical weathering strongly increased the biological stability of HTC and GS char produced from maize silage. After physical weathering, the half‐life (t_1/2) of GS was doubled but only slight increase was noted for those of HTC. During the first weeks of incubation, HTC addition to soil stimulated native soil organic matter (SOM) mineralization (positive priming effect), while the GS addition led to protection of the native SOM against biologic degradation (negative priming effect). Physical weathering led to reduction in these priming effects. Model extrapolations based on our data showed that decadal C sequestration potential of GS and HTC is globally equivalent when all losses including those due to priming and physical weathering were taken into account. However, at century scale only GS may have the potential to increase soil C storage.     

减少降雨对杉木幼林土壤有机质组分及稳定性的影响

由于土壤有机质(SOM)化学结构上的异质性,其对于全球气候变化的响应变得难以预测.随着分子水平技术逐渐应用于SOM结构、来源及分解状态的研究,长久以来关于SOM组分及稳定性的问题可能将被解决.本研究通过两年的减少降雨(50%)处理,运用生物标志物技术,对杉木幼林SOM组分及分解程度进行研究,以探究降水格局的改变对亚热带杉木幼林SOM稳定性的影响.结果表明: 减少降雨处理显著降低了土壤中游离脂质的含量,分别降低了短链烷酸的62.8%和萜类及固醇类含量的19.1%,而对其他脂类无显著影响.尽管短期减少降雨处理并未影响土壤中木质素总量,却显著降低了紫丁香基和香草基的酸醛比值.因此,随着降雨格局的改变,可能加快SOM易分解组分分解.尽管难分解组分(木质素)相对稳定,但从长远来看,其稳定性还需持续监测.     

Dual,differential isotope labeling shows the preferential movement of labile plant constituents into mineral‐bonded soil organic matter

The formation and stabilization of soil organic matter (SOM) are major concerns in the context of global change for carbon sequestration and soil health. It is presently believed that lignin is not selectively preserved in soil and that chemically labile compounds bonding to minerals comprise a large fraction of the SOM. Labile plant inputs have been suggested to be the main precursor of the mineral‐bonded SOM. Litter decomposition and SOM formation are expected to have temperature sensitivity varying with the lability of plant inputs. We tested this framework using dual ¹³C and ¹⁵N differentially labeled plant material to distinguish the metabolic and structural components within a single plant material. Big Bluestem (Andropogon gerardii) seedlings were grown in an enriched ¹³C and ¹⁵N environment and then prior to harvest, removed from the enriched environment and allowed to incorporate natural abundance ¹³C–CO₂ and ¹⁵N fertilizer into the metabolic plant components. This enabled us to achieve a greater than one atom % difference in ¹³C between the metabolic and structural components within the plant litter. This differentially labeled litter was incubated in soil at 15 and 35 °C, for 386 days with CO₂ measured throughout the incubation. After 14, 28, 147, and 386 days of incubation, the soil was subsequently fractionated. There was no difference in temperature sensitivity of the metabolic and structural components with regard to how much was respired or in the amount of litter biomass stabilized. Only the metabolic litter component was found in the sand, silt, or clay fraction while the structural component was exclusively found in the light fraction. These results support the stabilization framework that labile plant components are the main precursor of mineral‐associated organic matter.     

Organic matter properties in soils afforested with Pinus radiata

Aims

Afforestation causes important alterations in SOM content and composition that affect the soil functions and C balance. The aim of this study was to identify the mechanisms that determine the changes in SOM composition following afforestation of grasslands.

Methods

The study included 4 chronosequences and 5 paired plots comprising pastures and land afforested with Pinus radiata. The SOM was characterized by ¹³C CP-MAS NMR spectroscopy and differential scanning calorimetry.

Results

During the first 10–20 year after afforestation, the changes in SOM content varied from slight gains to large losses (>40 %). The analyses revealed that even SOM compounds considered resistant to decomposition were degraded during this time. The SOM gains, observed 20 year after stand establishment, were favoured by the higher recalcitrance of pine litter and possibly by soil acidification. The concentrations of most SOM compounds, particularly the stable compounds, were higher at the end of the rotation. The low degree of protection, along with the favourable climatic conditions, may also explain the rapid decomposition of SOM, including resistant compounds, in these soils. DSC analysis complemented the information about SOM composition provided by other techniques.

Conclusions

The accumulation of stable SOM compounds at the end of the rotation suggests a longer soil C turnover in these afforested soils, which may alleviate the gradual loss of SOC in intensively managed forest soils.