Electrospray and Photoionization Mass Spectrometry for the Characterization of Organic matter in Natural Waters: A Qualitative Assessment

Fourier-transform ion cyclotron resonance mass spectrometry (MS) has demonstrated potential to revolutionize the fields of limnology and chemical oceanography by identifying the individual molecular components of organic matter in natural waters. The use of MS for this purpose is made possible by the electrospray technique which successfully ionizes polar, nonvolatile organic molecules. Another recently developed ion source, atmospheric pressure photoionization (APPI), extends MS capabilities to less polar molecules. This article presents early results on the application of APPI MS to natural organic matter. We compare APPI MS and electrospray MS data for dissolved organic matter from Lake Drummond (Virginia, USA). Collectively, electrospray and APPI MS identify more than 6000 molecular species to which we assign unique molecular formulas. Fewer than 1000 molecular species are common to both electrospray and APPI mass spectra, indicating that the techniques are highly complementary in the types of molecules they ionize. Access to a broad range of molecules provided by combining APPI and electrospray has prompted a qualitative analysis. The goal is to assess the extent to which molecular MS data correspond with elemental (CHNOS) and structural characteristics determined by combustion elemental analyses and 13C nuclear magnetic resonance (NMR). Because the data obtained by these different methods are not directly comparable, we propose a novel data analysis procedure that facilitates their comparison. The bulk elemental composition calculated from electrospray MS data are in close agreement (±15%) with values determined by combustion elemental analysis. APPI and electrospray MS detect protein contributions in agreement with 13C NMR (6 wt %) but underestimate carbohydrates relative to 13C NMR. Nevertheless, MS results agree with NMR on the relative proportions of noncarbohydrate compounds in the organic matter: lignins \u3e lipids \u3e peptides. Finally, we use a molecular mixing model to simulate a 13C NMR spectrum from the MS datasets. The correspondence of the simulated and measured 13C NMR signals (74%) suggests that, collectively, the molecular species identified by APPI and electrospray MS comprise a large portion of the organic matter in Lake Drummond. These results add credibility to electrospray and APPI MS in limnology and oceanography applications, but further characterization of ion source behavior is fundamental to the accurate interpretation of MS data

Tradeoffs in soil carbon protection mechanisms under aerobic and anaerobic conditions

Oxygen (O2) limitation is generally understood to suppress soil carbon (C) decomposition and is a key mechanism impacting terrestrial C stocks under global change. Yet, O2 limitation may differentially impact kinetic or thermodynamic vs. physico-chemical C protection mechanisms, challenging our understanding of how soil C may respond to climate-mediated changes in O2 dynamics. Although O2 limitation may suppress decomposition of new litter C inputs, release of physico-chemically protected C due to iron (Fe) reduction could potentially sustain soil C losses. To test this tradeoff, we incubated two disparate upland soils that experience periodic O2 limitation—a tropical rainforest Oxisol and a temperate cropland Mollisol—with added litter under either aerobic (control) or anaerobic conditions for one year. Anoxia suppressed total C loss by 27% in the Oxisol and by 41% in the Mollisol relative to the control, mainly due to the decrease in litter-C decomposition. However, anoxia sustained or even increased decomposition of native soil-C (11.0% vs. 12.4% in the control for the Oxisol and 12.5% vs. 5.3% in the control for the Mollisol, in terms of initial soil C mass), and it stimulated losses of metal- or mineral-associated C. Solid-state 13C nuclear magnetic resonance spectroscopy demonstrated that anaerobic conditions decreased protein-derived C but increased lignin- and carbohydrate-C relative to the control. Our results indicate a tradeoff between physico-chemical and kinetic/thermodynamic C protection mechanisms under anaerobic conditions, whereby decreased decomposition of litter C was compensated by more extensive loss of mineral-associated soil C in both soils. This challenges the common assumption that anoxia inherently protects soil C and illustrates the vulnerability of mineral-associated C under anaerobic events characteristic of a warmer and wetter future climate

Decadal-scale litter manipulation alters the biochemical and physical character of tropical forest soil carbon

© 2018 Elsevier Ltd Climate change and rising atmospheric carbon dioxide (CO2) concentrations are likely to alter tropical forest net primary productivity (NPP), potentially affecting soil C storage. We examined biochemical and physical changes in soil C fractions in a humid tropical forest where experimental litter manipulation changed total soil C stocks. We hypothesized that: (1.) low-density soil organic C (SOC) fractions are more responsive to altered litter inputs than mineral-associated SOC, because they cycle relatively rapidly. (2.) Any accumulation of mineral-associated SOC with litter addition is relatively stable (i.e. low leaching potential). (3.) Certain biomolecules, such as waxes (alkyl) and proteins (N-alkyl), form more stable mineral-associations than other biomolecules in strongly weathered soils. A decade of litter addition and removal affected bulk soil C content in the upper 5 cm by +32% and −31%, respectively. Most notably, C concentration in the mineral-associated SOC fraction was greater in litter addition plots relative to controls by 18% and 28% in the dry and wet seasons, respectively, accounting for the majority of greater bulk soil C stock. Radiocarbon and leaching analyses demonstrated that the greater mineral-associated SOC in litter addition plots consisted of new and relatively stable C, with only 3% of mineral-associated SOC leachable in salt solution. Solid-state13C NMR spectroscopy indicated that waxes (alkyl C) and microbial biomass compounds (O-alkyl and N-alkyl C) in mineral-associated SOC are relatively stable, whereas plant-derived compounds (aromatic and phenolic C) are lost from mineral associations on decadal timescales. We conclude that changes in tropical forest NPP will alter the quantity, biochemistry, and stability of C stored in strongly weathered tropical soils

Aromaticity and degree of aromatic condensation of char

The aromatic carbon structure is a defining property of chars and is often expressed with the help of two concepts: (i) aromaticity and (ii) degree of aromatic condensation. The varying extent of these two features is assumed to largely determine the relatively high persistence of charred material in the environment and is thus of interest for e.g. biochar characterization or carbon cycle studies. Consequently, a variety of methods has been used to assess the aromatic structure of chars, which has led to interesting insights but has complicated the comparison of data acquired with different methods. We therefore used a suite of seven methods (elemental analysis, MIR spectroscopy, NEXAFS spectroscopy, 13C NMR spectroscopy, BPCA analysis, lipid analysis and helium pycnometry) and compared 13 measurements from them using a diverse sample set of 38 laboratory chars. Our results demonstrate that most of the measurements could be categorized either into those which assess aromaticity or those which assess the degree of aromatic condensation. A variety of measurements, including relatively inexpensive and simple ones, reproducibly captured the two aromatic features in question, and data from different methods could therefore be compared. Moreover, general patterns between the two aromatic features and the pyrolysis conditions were revealed, supporting reconstruction of the highest heat treatment temperature (HTT) of char

Aromaticity and degree of aromatic condensation of char

The aromatic carbon structure is a defining property of chars and is often expressed with the help of two concepts: (i) aromaticity and (ii) degree of aromatic condensation. The varying extent of these two features is assumed to largely determine the relatively high persistence of charred material in the environment and is thus of interest for, e.g., biochar characterization or carbon cycle studies. Consequently, a variety of methods has been used to assess the aromatic structure of chars, which has led to interesting insights but has complicated the comparison of data acquired with different methods. We therefore used a suite of seven methods (elemental analysis, MIR spectroscopy, NEXAFS spectroscopy, ¹³C NMR spectroscopy, BPCA analysis, lipid analysis and helium pycnometry) and compared 13 measurements from them using a diverse sample set of 38 laboratory chars. Our results demonstrate that most of the measurements could be categorized either into those which assess aromaticity or those which assess the degree of aromatic condensation. A variety of measurements, including relatively inexpensive and simple ones, reproducibly captured the two aromatic features in question, and data from different methods could therefore be compared. Moreover, general patterns between the two aromatic features and the pyrolysis conditions were revealed, supporting reconstruction of the highest heat treatment temperature (HTT) of char.Keywords: Aromaticity, Stability, Aromatic condensation, Pyrolysis, Char, Biochar, Pyrogenic organic matter, Heat treatment temperatur

Mutation, purification and chemical studies on the tobacco necrosis virus

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Biochar Volatile Matter and Feedstock Effects on Soil Nitrogen Mineralization and Soil Fungal Colonization

Biochar has important biogeochemical functions in soil—first as a means to sequester carbon, and second as a soil conditioner to potentially enhance soil quality and fertility. Volatile matter (VM) content is a property of biochar that describes its degree of thermal alteration, which can have a direct influence on carbon and nitrogen dynamics in soil. In this study, we characterized the VM in biochars derived from two locally sourced feedstocks (corncob and kiawe wood) and evaluated the relationship of VM content to nitrogen transformations and culturable fungal biomass. Using 13C nuclear magnetic resonance (NMR) spectroscopy, we found that the VM content of biochar primarily consisted of alkyl (5.1–10.1%), oxygen-substituted alkyl (2.2–6.7%), and phenolic carbon (9.4–11.6%). In a series of laboratory incubations, we demonstrated that corncob biochars with high VM (23%) content provide a source of bioavailable carbon that appeared to support enhanced viable, culturable fungi (up to 8 fold increase) and cause nitrogen immobilization in the short-term. Corncob biochar with bioavailable VM was nitrogen-limited, and the addition of nitrogen fertilizer resulted in a four-fold increase in total hydrolytic enzyme activity and the abundance of culturable fungal colonies. In contrast, kiawe biochar with an equivalent VM content differed substantially in its composition and effect on these same biological parameters. Therefore, the rapid measurement of VM content is too coarse to differentiate chemical composition and to predict the behavior of biochars across feedstocks and production methods

Tradeoffs in soil carbon protection mechanisms under aerobic and anaerobic conditions

Oxygen (O2) limitation is generally understood to suppress soil carbon (C) decomposition and is a key mechanism impacting terrestrial C stocks under global change. Yet, O2 limitation may differentially impact kinetic or thermodynamic vs. physico-chemical C protection mechanisms, challenging our understanding of how soil C may respond to climate-mediated changes in O2 dynamics. Although O2 limitation may suppress decomposition of new litter C inputs, release of physico-chemically protected C due to iron (Fe) reduction could potentially sustain soil C losses. To test this tradeoff, we incubated two disparate upland soils that experience periodic O2 limitation—a tropical rainforest Oxisol and a temperate cropland Mollisol—with added litter under either aerobic (control) or anaerobic conditions for one year. Anoxia suppressed total C loss by 27% in the Oxisol and by 41% in the Mollisol relative to the control, mainly due to the decrease in litter-C decomposition. However, anoxia sustained or even increased decomposition of native soil-C (11.0% vs. 12.4% in the control for the Oxisol and 12.5% vs. 5.3% in the control for the Mollisol, in terms of initial soil C mass), and it stimulated losses of metal- or mineral-associated C. Solid-state 13C nuclear magnetic resonance spectroscopy demonstrated that anaerobic conditions decreased protein-derived C but increased lignin- and carbohydrate-C relative to the control. Our results indicate a tradeoff between physico-chemical and kinetic/thermodynamic C protection mechanisms under anaerobic conditions, whereby decreased decomposition of litter C was compensated by more extensive loss of mineral-associated soil C in both soils. This challenges the common assumption that anoxia inherently protects soil C and illustrates the vulnerability of mineral-associated C under anaerobic events characteristic of a warmer and wetter future climate.This is the peer reviewed version of the following article: Huang, W., C. Ye, W. Hockaday, and S. J. Hall. "Tradeoffs in soil carbon protection mechanisms under aerobic and anaerobic conditions." Global change biology (2020), which has been published in final form at doi: 10.1111/gcb.15100. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions.</p

Molecular trade-offs in soil organic carbon composition at continental scale

The molecular composition of soil organic carbon remains contentious. Microbial-, plant-, and fire-derived compounds may each contribute, but do they vary predictably among ecosystems? Here we present carbon functional groups and molecules from a diverse spectrum of North American surface mineral soils, primarily collected from the National Ecological Observatory Network, quantified by nuclear magnetic resonance spectroscopy and a molecular mixing model. Soils varied widely in relative contributions of carbohydrate, lipid, protein, lignin, and char-like carbon, but each compound class had similar overall abundance. Three principal component axes explained 90% of the variance in carbon composition: the first showed a tradeoff between lignin and protein, the second showed a tradeoff between carbohydrate and char, and the third was explained by lipids. Reactive aluminum, crystalline iron oxides, and pH plus overlying organic horizon thickness best explained variation along each respective axis; these predictors were ultimately related to climate. Together, our data point to continental-scale tradeoffs in soil carbon molecular composition which are linked to environmental and geochemical variables known to predict carbon mass concentrations. Controversies regarding the genesis of soil carbon and its potential responses to global change can be partially reconciled by considering diverse ecosystem properties that drive complementary persistence mechanisms.This is a manuscript of an article published as Hall, Steven J., Chenglong Ye, Samantha R. Weintraub, and William C. Hockaday. "Molecular trade-offs in soil organic carbon composition at continental scale." Nature Geoscience (2020). doi: 10.1038/s41561-020-0634-x. Posted with permission.</p

Molecular trade-offs in soil organic carbon composition at continental scale

The molecular composition of soil organic carbon remains contentious. Microbial-, plant-, and fire-derived compounds may each contribute, but do they vary predictably among ecosystems? Here we present carbon functional groups and molecules from a diverse spectrum of North American surface mineral soils, primarily collected from the National Ecological Observatory Network, quantified by nuclear magnetic resonance spectroscopy and a molecular mixing model. Soils varied widely in relative contributions of carbohydrate, lipid, protein, lignin, and char-like carbon, but each compound class had similar overall abundance. Three principal component axes explained 90% of the variance in carbon composition: the first showed a tradeoff between lignin and protein, the second showed a tradeoff between carbohydrate and char, and the third was explained by lipids. Reactive aluminum, crystalline iron oxides, and pH plus overlying organic horizon thickness best explained variation along each respective axis; these predictors were ultimately related to climate. Together, our data point to continental-scale tradeoffs in soil carbon molecular composition which are linked to environmental and geochemical variables known to predict carbon mass concentrations. Controversies regarding the genesis of soil carbon and its potential responses to global change can be partially reconciled by considering diverse ecosystem properties that drive complementary persistence mechanisms