Advanced Solvent Based Methods for Molecular Characterization of Soil Organic Matter by High-Resolution Mass Spectrometry

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 (<0.1); water (H2O) was selective for carbohydrates with high O/C ratios; acetonitrile (ACN) preferentially extracts lignin, condensed structures, and tannin polyphenolic compounds with O/C > 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.

Biogenic, urban, and wildfire influences on the molecular composition of dissolved organic compounds in cloud water

“…The detailed instrument parameters and analysis procedures, optimized during dissolved organic matter experiments, have been described previously (Kujawinski and Behn, 2006;Minor et al, 2012;Tfaily et al, 2015). A standard Bruker electrospray ionization (ESI) source was used to generate negatively charged molecular ions to target analysis of oxidized organic compounds.…”

“…In addition, observations of likely dinitroaromatics during all air mass influences, potentially from aqueous nitration of nitroaromatic compounds, have important implications because of their light-absorbing and mutagenic properties (Purohit and Basu, 2000;Zhang et al, 2011Zhang et al, , 2013. During wildfire influence, the cloud water showed evidence of aqueous SOA formation, including oligomer formation involving methylglyoxal Yasmeen et al, 2010) and aqueousphase reactions of syringol and guaiacol (Yu et al, 2014(Yu et al, , 2016. Monoterpene-derived organosulfates and organonitrates (Surratt et al, 2008) were observed in the cloud water during all air mass influences, similar to previous cloud water studies (Boone et al, 2015;Pratt et al, 2013).…”

“…Potential products of aqueous processing include carboxylic acids, esters, organosulfur compounds, polyols, amines, amino acids, and highly oxygenated oligomeric species (Blando and Turpin, 2000;Ervens et al, 2011). However, only limited ambient evidence of cloud processing involving high molecular weight organic compounds has been reported (Boone et al, 2015;Feng and Möller, 2004;Lee et al, 2011Lee et al, , 2012Pratt et al, 2013;Sagona et al, 2014;van Pinxteren et al, 2016;Zhao et al, 2013). A recent review found that more than 50 % of organic matter in fog and cloud water remains unspeciated (Herckes et al, 2013).…”

Molecular and Microscopic Insights into the Formation of Soil Organic Matter in a Red Pine Rhizosphere

“…After the mechanical separation of the material from the columns (Figure 2, 2. Mechanical separation), samples of bulk substrate (BS) and rhizospheric material (RS) were extracted in either water (Optima ® LC/MS grade, Fisher Scientific, Suwanee, GA, USA) or 100% methanol (Optima ® HPLC grade, Fisher Scientific, Suwanee, GA, USA) according to Tfaily et al [34] as follows: 0.5 g of BS or RS were weighed and divided evenly for water and methanol extraction, then mixed and shaken for 5 minutes. For RS, root samples were dipped in the solvent, manually stirred and gently shaken for 5 minutes, to minimize the portions of cut roots to be extracted in water or methanol.…”

“…We anticipated that (a) microbes would contribute to the new SOM pool by stabilizing root exudates and producing complex refractory C compounds. With our recent development of a solvent-based method for molecular SOM characterization by high resolution mass spectrometry [34], we also expected that (b) microbial residues will leave a stable signature of persistent OM on mineral surfaces that remain attached following a series of extractions with different polarity solvents. Furthermore, we hypothesized that (c) SOM composition would differ between rhizosphere soil and bulk soil.…”