Article pubs.acs.org/ac
Advanced Solvent Based Methods for Molecular Characterization of Soil Organic Matter by High-Resolution Mass Spectrometry Malak M. Tfaily,† Rosalie K. Chu,† Nikola Tolić,† Kristyn M. Roscioli,†,‡ Christopher R. Anderton,† Ljiljana Paša-Tolić,† Errol W. Robinson,† and Nancy J. Hess*,† †
Environmental Molecular Sciences Laboratory and Biological Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 99354, United States S Supporting Information *
ABSTRACT: Soil organic matter (SOM), a complex, heterogeneous mixture of above and belowground plant litter and animal and microbial residues at various degrees of decomposition, is a key reservoir for carbon (C) and nutrient biogeochemical cycling in soil based ecosystems. A limited understanding of the molecular composition of SOM limits the ability to routinely decipher chemical processes within soil and accurately predict how terrestrial carbon fluxes will respond to changing climatic conditions and land use. To elucidate the molecular-level structure of SOM, we selectively extracted a broad range of intact SOM compounds by a combination of different organic solvents from soils with a wide range of C content. Our use of electrospray ionization (ESI) coupled with Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS) and a suite of solvents with varying polarity significantly expands the inventory of the types of organic molecules present in soils. Specifically, we found that hexane is selective for lipid-like compounds with very low O/C ratios ( 0.5; methanol (MeOH) has higher selectivity toward compounds characterized with low O/C < 0.5; and hexane, MeOH, ACN, and H2O solvents increase the number and types of organic molecules extracted from soil for a broader range of chemically diverse soil types. Our study of SOM molecules by ESI FTICR MS revealed new insight into the molecular-level complexity of organics contained in soils. We present the first comparative study of the molecular composition of SOM from different ecosystems using ultra high-resolution mass spectrometry.
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varying C uptake by the terrestrial ecosystem under future climate conditions due to insufficient understanding and representation of these processes.3 Molecular characterization of SOM is a necessary first step to develop a more complete understanding of soil C processes and dynamics, their resulting feedbacks to climate change, and a reduction in the uncertainly of predictions of future climate scenarios modeled at the regional and global scales. The ability to address fundamental issues associated with biogeochemical cycling of C is limited because the molecular complexity of SOM compounds has prevented routine identification for a wide variety of soil types and ecosystems. Early studies of SOM mostly consisted of bulk analysis (i.e., the total weight of soil fractions or elements). Structural investigations of macromolecular SOM are now based on nondestructive spectroscopic techniques, such as NMR and Fourier transform-infrared (FT-IR), which provide semiquantitative analysis of the presence and identity functional groups and types of carbon bonds (aromatic C, alkyl C, Oalkyl-C, etc.).4 Identification of the molecular composition of
oils play an integral role in global environmental health acting as a carbon (C) and nutrient (N, P, S) reservoir for the exchange between plants, microorganisms, and the atmosphere.1 In the agricultural community it has long been recognized that the health of soils have immense socioeconomic impacts such as food and water security and erosion. Soil organic matter (SOM), a complex, heterogeneous mixture of above and belowground plant litter and animal and microbial residues at various degrees of decomposition, is a key reservoir for carbon (C) and nutrient biogeochemical cycling in soil based ecosystems. The critical role that SOM plays in the global C cycle and climate has been recognized for over 3 decades. Soils in the belowground ecosystem contain more than 3 times as much C as the atmosphere1 and natural soil CO2 emissions from the below ground ecosystem are nearly 7 times larger than anthropogenic emissions. Soil chemistry is intricately linked to the climate system through vegetation, microbial processes (respiration, organic matter formation, and decomposition), elemental (C, N, P), and hydrologic cycles.2 As a result, altered climate will have an effect on soil chemistry, processes, and dynamics. However, because C efflux from soils is largely in the form of greenhouse gases, CO2 and methane, any alteration in the soil C emissions can be a significant potential feedback to climate change. Current biogeochemical models predict widely © 2015 American Chemical Society
Received: January 10, 2015 Accepted: April 17, 2015 Published: April 17, 2015 5206
DOI: 10.1021/acs.analchem.5b00116 Anal. Chem. 2015, 87, 5206−5215
Article
Analytical Chemistry Table 1. Solvent Properties and Comparison of SRFA Extraction Results avg composition solvent
polarity index
dielectric constant (25 °C)
no. of peaks observed
no. of peaks assigned
O/C
H/C
DBEav
MeOH ACN hexane H2O composite composition
5.1 5.8 0 10.2
33 37.5 1.88 78.2
4626 5130 669 4946 4214
3932 3693 488 4027 4214
0.49 0.53 0.20 0.52 0.45
0.98 0.93 1.65 1.1 1.07
12.65 13.61 4.62 12.1 11.7
was a mix of fresh and decomposing sphagnum whereas the 75 cm depth sample contained mainly decomposed peat.15 Intermediate C soil samples were collected from two Alaskan soil cores from adjacent birch and spruce forests. Each core was divided into two sections: an organic section with higher C content and a mineral section with lower C content (Birch core, organic section, 5.0% C; mineral section, 1.8% C; Spruce core, organic section, 8.5% C; mineral section, 1.9% C; Table 2). Low C (1M) and mass measurement accuracy (
