Potential alterations in the chemical structure of soil organic matter components during sodium hydroxide extraction

Authors: CHEN, XI - Anhui Agricultural University; JIN, MENGCAN - Anhui Agricultural University; XU, YUNJUN - Anhui Agricultural University; CHU, WENYING - Old Dominion University; Olk, Daniel - Dan; HU, JINGWEI - Anhui Agricultural University; JIANG, YANYU - Princess Anne High School; MAO, JINGDONG - Old Dominion University; GAO, HONGJIAN - Anhui Agricultural University; THOMPSON, MICHAEL - Iowa State University

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/8/2019
Publication Date: 9/5/2019
Citation: Chen, X., Jin, M., Xu, Y., Chu, W., Olk, D.C., Hu, J., Jiang, Y., Mao, J., Gao, H., Thompson, M.L. 2019. Potential alterations in the chemical composition of soil organic matter during NaOH extraction. Journal of Environmental Quality. 48(6):1578-1586. https://doi.org/10.2134/jeq2019.02.0077.
DOI: https://doi.org/10.2134/jeq2019.02.0077

Interpretive Summary: Carbon contributes positively to several soil properties, including nutrient cycling and retention and feeding microbial populations. Investigation of its roles often requires separation of younger portions from much older and stabilized portions. One approach for separating carbon portions involves washing the soil with a solution at high pH. This method has been criticized for potentially changing the forms of carbon during the washing. We found that some changes occurred in organic soils before and after extraction, but in many cases the changes were not large. These results suggest that modest changes can be expected when separating carbon portions using solutions at high pH. These results are useful to scientists who study the types of carbon present in soil and their responses to land management.

Technical Abstract: The use of dilute alkaline solutions, especially 0.1–0.5 M NaOH solutions, for the extraction of humic substances (HS) from soils has been criticized for potential chemical alteration of HS composition. However, the degree of alteration remains unquantified. By using elemental analysis and solid-state nuclear magnetic resonance (NMR) spectroscopy, this study investigated the effects of 0.1 M NaOH extraction on the chemical compositions of peat soils and also cellulose and lignin, as model compounds for crop residues. We found that NaOH dissolved small amounts of purified cellulose and lignin. The chemical composition of the NaOH-extractable cellulose fraction had substantially lower proportions of both O-alkyl C and anomeric C than did the original cellulose. Moreover, new aromatic C or olefin C, as well as carboxylate C, occurred in the fraction treated with 0.1 M NaOH. Compounds that were solubilized from the lignin by NaOH treatment were dominated by aromatic C and OCH3 functional groups. NMR spectra of the insoluble lignin were not substantially different from those of the untreated sample. After 0.1 M NaOH extraction of the peat samples, the proportions of aromatic C and aromatic C-O in the extractable material increased, whereas those of O-alkyl C and alkyl O-C-O decreased. In summary, the chemical composition of both the fresh cellulose and lignin and HS components of SOM were altered to some extent by 0.1 M NaOH extraction. These observations provide useful context when considering potential alterations in chemical composition when interpreting results gained by extracting of SOM using dilute NaOH solutions.