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

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

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

Plant species, not climate, controls aboveground biomass O2:CO2 exchange ratios in deciduous and coniferous ecosystems

The oxidative ratio (OR) is the O2:CO2 ratio associated with photosynthesis, respiration, and other ecosystem gas exchange processes and can be reported on the scale of an individual leaf, an ecosystem, up to the entire terrestrial biosphere. The OR of the terrestrial biosphere is used to partition anthropogenic CO2 between oceanic and terrestrial carbon sinks, and the ease of measurement of this property on smaller scales suggests its potential for other uses. However, controls on the natural variation of OR are not understood in either organic matter pools or fluxes, and this lack of basic information limits the use of the tracer. Here we assess the annual variability of the OR of photosynthesis over ~decade for two temperate forests, one coniferous and one deciduous, and show that the photosynthetic OR signature is strongly dominated by plant species. We determined the OR of this flux by measuring the OR of carbon pools that close on annual or shorter timescales (leaves and individual tree rings), via solid‐state 13C NMR spectroscopy and elemental analysis. Leaf litter OR is different between coniferous and deciduous forests, but tree bole OR is constant between species. There was no significant change in leaf litter OR with time, nor any correlations between leaf litter OR and temperature or precipitation. During this time growing season precipitation varied by 95% from the time period average, and growing season temperature by 22%, demonstrating that on the decadal scale photosynthetic OR is invariant over significant shifts in these climate parameters

High carbon losses from oxygen-limited soils challenge biogeochemical theory and model assumptions

Oxygen (O2) limitation contributes to persistence of large carbon (C) stocks in saturated soils. However, many soils experience spatiotemporal O2 fluctuations impacted by climate and land-use change, and O2-mediated climate feedbacks from soil greenhouse gas emissions remain poorly constrained. Current theory and models posit that anoxia uniformly suppresses carbon (C) decomposition. Here we show that periodic anoxia may sustain or even stimulate decomposition over weeks to months in two disparate soils by increasing turnover and/or size of fast-cycling C pools relative to static oxic conditions, and by sustaining decomposition of reduced organic molecules. Cumulative C losses did not decrease consistently as cumulative O2 exposure decreased. After >1 year, soils anoxic for 75% of the time had similar C losses as the oxic control but nearly threefold greater climate impact on a CO2-equivalent basis (20-year timescale) due to high methane (CH4) emission. A mechanistic model incorporating current theory closely reproduced oxic control results but systematically underestimated C losses under O2 fluctuations. Using a model-experiment integration (ModEx) approach, we found that models were improved by varying microbial maintenance respiration and the fraction of CH4 production in total C mineralization as a function of O2 availability. Consistent with thermodynamic expectations, the calibrated models predicted lower microbial C-use efficiency with increasing anoxic duration in one soil; in the other soil, dynamic organo-mineral interactions implied by our empirical data but not represented in the model may have obscured this relationship. In both soils, the updated model was better able to capture transient spikes in C mineralization that occurred following anoxic–oxic transitions, where decomposition from the fluctuating-O2 treatments greatly exceeded the control. Overall, our data-model comparison indicates that incorporating emergent biogeochemical properties of soil O2 variability will be critical for effectively modeling C-climate feedbacks in humid ecosystems.This article is published as Huang, Wenjuan, Kefeng Wang, Chenglong Ye, William C. Hockaday, Gangsheng Wang, and Steven J. Hall. "High carbon losses from oxygen‐limited soils challenge biogeochemical theory and model assumptions." Global Change Biology 27, no. 23 (2021): 6166-6180. doi:10.1111/gcb.15867. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes

Characterization of Slow-Pyrolysis Bio-Oils by High-Resolution Mass Spectrometry and Ion Mobility Spectrometry

Bio-oils produced from biomass pyrolysis are an attractive fuel source that requires significant upgrading. Before upgrade strategies can be developed, the molecular composition of bio-oils needs to be better understood. In this work, oily and aqueous fractions of bio-oils produced by slow pyrolysis of two feedstocks, pine shavings (PS) and corn stover (CS), were analyzed by negative electrospray ionization (ESI)-Orbitrap and ion mobility-time-of-flight mass spectrometry (IM-TOF-MS). Analyte ion signal was observed primarily between <i>m</i>/<i>z</i> 80 and 450 in the mass spectra of these samples. Mass defect analysis and collision-induced dissociation (CID) experiments performed on mobility-separated ions indicated a high degree of homology among bio-oil samples produced from both feedstocks. Oxygen-rich species having between 1 and 9 oxygen atoms and with double bond equivalents (DBEs) ranging from 1 to 15 were identified, indicating that catalytic upgrading will likely be required if slow-pyrolysis bio-oils are to be utilized as fuel. IM-MS and IM-MS/MS analysis of ions belonging to select CH₂-homologous series suggest that mass-mobility correlations and post-ion mobility CID mass spectra may be useful in defining structural relationships among members of a given Kendrick mass defect series

Chemical and Isotopic Thresholds in Charring: Implications for the Interpretation of Charcoal Mass and Isotopic Data

Charcoal plays a significant role in the long-term carbon cycle, and its use as a soil amendment is promoted as a C sequestration strategy (biochar). One challenge in this research area is understanding the heterogeneity of charcoal properties. Although the maximum reaction temperature is often used as a gauge of pyrolysis conditions, pyrolysis duration also changes charcoal physicochemical qualities. Here, we introduce a formal definition of charring intensity (CI) to more accurately characterize pyrolysis, and we document variation in charcoal chemical properties with variation in CI. We find two types of responses to CI: either linear or threshold relationships. Mass yield decreases linearly with CI, while a threshold exists across which % C, % N, and δ¹⁵N exhibit large changes. This CI threshold co-occurs with an increase in charcoal aromaticity. C isotopes do not change from original biomass values, supporting the use of charcoal δ¹³C signatures to infer paleoecological conditions. Fractionation of N isotopes indicates that fire may be enriching soils in ¹⁵N through pyrolytic N isotope fractionation. This influx of “black N” could have a significant impact on soil N isotopes, which we show theoretically using a simple mass-balance model