Potential Alterations in the Chemical Structure of Soil Organic Matter Components during Sodium Hydroxide Extraction

The use of alkaline solutions, especially 0.1 to 0.5 M NaOH solutions, for the extraction of humic substances (HS) from soils is controversial because of the potential for chemical and physical alteration of the organic materials in soils. Therefore, it is important to investigate the effects of NaOH extraction on the chemical structure of natural organic matter (NOM) in terrestrial ecosystems. By using solid‐state nuclear magnetic resonance, we investigated the impact of NaOH and water extractions on the chemical structure of insoluble and extractable fractions of three model components of NOM: cellulose, van Soest lignin, and peat. We found both sp2–hybridized and carboxyl C groups occurred in the NaOH‐extractable cellulose fraction relative to the bulk cellulose. Compounds solubilized from the lignin by treatment with 0.1 M NaOH were dominated by aromatic C‐O, aromatic C, and OCH3 functional groups, indicating that some free phenolic monomers were extracted by the alkaline solution. Compared with the bulk peat, the concentration of COO/NCO and sp2–hybridized C in NaOH‐reconstituted peat sample increased by 40 and 5%, respectively, whereas the concentration of carbohydrate C decreased by 14%. In summary, the chemical structures of cellulose, van Soest lignin, and the peat were altered to some extent by the 0.1 M NaOH extraction. These observations provide useful context in the consideration of potential alterations in chemical composition when one interprets results of HS extractions employing NaOH solutions. Future research will establish the proportion of these alterations that may result from an alternative, slower procedure for acidifying solutions after NaOH extraction. Core Ideas Some unintended chemical reactions may occur in SOM during alkaline extraction. Alkaline extraction may cause an increase of COO/NCO and sp2–hybridized C groups. Alkaline extraction may induce a decrease of O‐alkyl C and anomeric C groups.