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The article presents a study on the investigation of speciation, oxidative state changes and long- and short-term molecular-level dynamics of organic sulfur (S) by using X-ray absorption near-edge structure (XANES) spectroscopy in the U.S. It notes the collection of soil samples from the upper 15 centimeters (cm) of undisturbed grasslands since 1931 and from cultivated fields since 1880 in the western part of the country. It demonstrates that synchrotron-based X-ray spectroscopic technique is an ideal nondestructive tool to identify the various oxidation states of S. It cites the three distinct groups of organosulfur compounds in the grassland-derived soils which include strongly reduced organic S, organic S in intermediate oxidation states, and strongly oxidized organic S.

Natural organic matter (NOM) is a highly active component of soils and sediments, and plays an important role in global C cycling. However, NOM has defied molecular-level structural characterization, owing to variations along the decomposition continuum and its existence as highly functionalized polyelectrolytes. We conducted a comprehensive systematic overview of spectral signatures and peak positions of major organic molecules that occur as part of NOM using near-edge x-ray absorption fine structure (NEXAFS) spectroscopy. The spectra of carbohydrates and amino sugars show resonances between 289.10 and 289.59 eV, attributed to 1s-3p/σ* transitions of 0-alkyl (C-OH) moieties. They also exhibited distinct peaks between 288.42 and 288.74 eV, representing C 1s-π*C=O from COOH functionalities. Amino acids produced a strong signal around 288.70 eV, which can be identified as a C 1s-π*C=O transition of carboxyl/carbonyl (COOH/COO-) structures. Spectral features near 285.29 eV were ascribed to C 1s-σ*C=C transition of ring structure of aromatic amino acids, while spectra between 287.14 and 287.86 eV were attributed to C 1s-π*C-H and C 1s-σ*C-H/3p Rydberg-like excitations from CH and CH2 groups. Phenols and benzoquinone produced strong resonances between 285.08 and 285.37 eV, attributed to the π* orbital of C (C 1s-π*C=O) atoms connected to either C or H (C-H) in the aromatic ring. The next higher excitation common to both phenols and quinone appeared between 286.05 and 286.35 eV, and could be associated with C 1s-π*C=O transitions of aromatic C bonded toO atom in phenols, and to C 1s-π*C=O transitions from aromatic C connected to 0 atom (C-OH) in phenols or to a CO in p-benzoquinone and some phenols with carbonyl structures, respectively. Nucleobases exhibited complex spectral features with pronounced resonances between 286.02 and 286.84 eV and between 288.01 and 288.70 eV. Molecular markers forblack C (benzenecarboxylic acid and biphenyl-4,4'-dicarboxylic acid) exhibit sharp absorption bands between 285.01 and at 285.43 eV, possibly from C 1s-π*C=C is-it transition characteristic of C-H sites or unsaturated C (C=C) on aromatic ring structures. These aromatic carboxylic acids also exhibit broad peaks between 288.35 and 288.48 eV, reflecting C 1s-π*C=O) transition of carboxyl functional groups bonded to unsaturated C. This investigation provides a more comprehensive NEXAFS spectral library of biogeochemically relevant organic C compounds. The spectra of these reference organic compounds reveal distinct spectral features and peak positions at the C K-edge that are characteristic of the molecular orbitals bonding C atoms. Detailed structural information can be derived from these distinctive spectral features that could be used to build robust peak assignment criteria to exploit the chemical sensitivity of NEXAFS spectroscopy for in situ molecular-level spatial investigation and fingerprinting of complex organic C compounds in environmental samples. [ABSTRACT FROM AUTHOR]

The article presents a study which determines the influence on biogeochemical dynamics and molecular-level speciation of soil sulfur. It cites that main component of the global biogeochemical sulfur cycle is soil environment. The soil environment acts as source and sink of several sulfur species and mediates oxidation state changes. The study uses elemental analysis and X-ray absorption near-edge structure (XANES) to investigate the long-term influences anthropogenically mediated transitions coming from natural to managed ecosystems.

Soil degradation is one of the most serious threats to sustainable crop production in many tropical agroecosystems where extensification rather than intensification of agriculture has occurred. In the highlands of western Kenya, we investigated soil nitrogen (N) and phosphorus (P) constraints to maize productivity across a cultivation chronosequence in which land-use history ranged from recent conversion from primary forest to 100 years in continuous cropping. Nutrient treatments included a range of N and P fertilizer rates applied separately and in combination. Maize productivity without fertilizer was used as a proxy measure for indigenous soil fertility (ISF). Soil pools of mineral nitrogen, strongly bound P and plant-available P decreased by 82%, 31% and 36%, and P adsorption capacity increased by 51% after 100 years of continuous cultivation. For the long rainy season (LR), grain yield without fertilizer declined rapidly as cultivation age increased from 0 to 25 years and then gradually declined to a yield of 1.6 Mg ha−1, which was maintained as time under cultivation increased from 60 to 100 years. LR grain yield in the old conversions was only 24% of the average young conversion grain yield (6.4 Mg ha−1). Application of either N or P alone significantly increased grain yield in both the LR and short rainy (SR) seasons, but only application of 120 kg N ha−1 on the old conversion increased yield by >1 Mg ha−1. In both SR and LR, there was a greater average yield increment response to N and P when applied together (ranging from 1 to 3.8 Mg ha−1 for the LR), with the greatest responses on the old conversions. The benefit–cost ratio (BCR) for applying 120 kg N ha−1 alone was <1 except on the old conversions, while BCRs were>1 for applying 25 kg P ha−1 alone at all levels of conversion for both seasons. Application of both N (120 kg N ha−1) and P (25 kg P ha−1) on the old conversions resulted in the greatest BCRs. This study clearly indicates that maize productivity responses to N and P fertilizer are significantly affected by the age of cultivation and its influence on ISF, but that loss of productivity can be restored rapidly when these limiting nutrients are applied. Management strategies should consider ISF and economic factors to determine optimal N and P input requirements for achieving and sustaining profitable crop production on degraded soils. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Methodological constraints limit the extent to which existing soil aggregation models explain carbon (C) stabilization in soil. We hypothesize that the physical infrastructure of microaggregates plays a major role in determining the chemistry of the occluded C and intimate associations between particulate C, chemically stabilized C and the soil mineral matrix. We employed synchrotron-based scanning transmission X-ray microscopy (STXM) coupled with near-edge X-ray absorption fine structure (C 1s-NEXAFS) spectroscopy to investigate the nanoscale physical assemblage and C chemistry of 150-μm microaggregates from a Kenyan Oxisol. Ultra-thin sections were obtained after embedding microaggregates in a sulfur block and sectioning on a cryo-microtome at -55°C. Principal component and cluster analyses revealed four spatially distinct features: pore surfaces, mineral matter, organic matter, and their mixtures. The occurrence of these features did not vary between exterior and interior locations; however, the degree of oxidation decreased while the complexity and occurrence of aliphatic C forms increased from exterior to interior regions of the microaggregate. At both locations, compositional mapping rendered a nanoscale distribution of oxidized C clogging pores and coating pore cavities on mineral surface. Hydrophobic organic matter of aromatic and aliphatic nature, representing particulate C forms appeared physically occluded in 2- to 5-μm pore spaces. Our findings demonstrate that organic matter in microaggregates may be found as either oxidized C associated with mineral surfaces or aromatic and aliphatic C in particulate form. Using STXM and C 1s-NEXAFS we are for the first time able to resolve the nanoscale biogeocomplexity of unaltered soil microaggregates. [ABSTRACT FROM AUTHOR]

Small-scale heterogeneity of organic carbon (C) forms in soils is poorly quantified since appropriate analytical techniques were not available up to now. Specifically, tools for the identification of functional groups on the surface of micrometer-sized black C particles were not available up to now. Scanning Transmission X-ray Microscopy (STXM) using synchrotron radiation was used in conjunction with Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy to investigate nano-scale distribution (50-nm resolution) of C forms in black C particles and compared to synchrotron-based FTJR spectroscopy. A new embedding technique was developed that did not build on a C-based embedding medium and did not pose the risk of heat damage to the sample. Elemental sulfur (5) was melted to 220°C until it polymerized and quenched with liquid N2 to obtain a very viscous plastic S in which the black C could be embedded until it hardened to a noncrystalline state and was ultrasectioned. Principal component and cluster analysis followed by singular value decomposition was able to resolve distinct areas in a black carbon particle. The core of the studied biomass-derived black C particles was highly aromatic even after thousands of years of exposure in soil and resembled the spectral characteristics of fresh charcoal. Surrounding this core and on the surface of the black C particle, however, much larger proportions of carboxylic and phenolic C forms were identified that were spatially and structurally distinct from the core of the particle. Cluster analysis provided evidence for both oxidation of the black C particle itself as well as adsorption of non-black C. NEXAFS spectroscopy has great potential to allow new insight into black C properties with important implications for biogeochemical cycles such as mineralization of black C in soils and sediments, and adsorption of C, nutrients, and pollutants as well as transport in the geosphere, hydrosphere, and atmosphere. [ABSTRACT FROM AUTHOR]

Soil organic matter (SOM) is a fundamental component of soil and the global C cycle. We used C (1s) near-edge x-ray absorption fine structure (NEXAFS) and synchrotron-based Fourier transform infrared-attenuated total reflectance (FTIR-ATR) spectroscopy to speciate C and investigate the influence of land use on the composition of SOM in the humic substances extracted from clay and silt fractions. Soil samples were collected from natural forest, tea and Cupressus plantations and cultivated fields in Ethiopia. Carbon K-edge spectra revealed multiple C (1s) electron transitions in the fine structure of C NEXAFS region (284–290 eV) indicating the presence of aromatic-C. phenolic-C, aliphatic-C, carboxylic-C, and O-alkyl-C in the humic substances. It also exhibited good selectivity, where specific energy regions correspond In C in discrete functional groups. However, regions of slight overlap between 1s-3p/σ∗ transition of aliphatic-C and 1s-π∗ transition of carboxylic-C may not be excluded. Fourier transform infrared-attenuated total reflectance spectroscopy showed larger proportions of aromatic-C (25.5%, 21.9%) and asymmetric and symmetric aliphatic-C (19.7%, 15.2%) groups in the silt than in clay, respectively. However, smaller proportion of polysaccharides (19.3%, 11.5%) was obtained from the silt compared with clay. The proportions of phenols (20.7%, 20.4%), aliphatic deformation of CH2 or CH3 (13.1%, 14.5%), and carboxylic (9.8%, 8.3%) groups were of similar magnitude in both fractions. The proportion of polysaccharides de- creased in the order: natural forests plantations cultivated fields, while recalcitrant aromatic-C increased in the order: natural forest < plantation < cultivation. Therefore, C (1s) NEXAFS and synchrotron- based FTIR-ATR spectroscopy are powerful, nondestructive techniques that can potentially be used not only to identify and fingerprint complex structural characteristics of organic C macromolecules but also to investigate the impact of long-term anthropogenic management on the composition and biogeochemical cycling of organic C in terrestrial ecosystems. [ABSTRACT FROM AUTHOR]