Formation of char-like, fused-ring aromatic structures from a nonpyrogenic pathway during decomposition of wheat straw

Authors: CHEN, XI - Anhui Agricultural University; YE, XINXIN - Anhui Agricultural University; CHU, WENYING - Old Dominion University; Olk, Daniel - Dan; CAO, XIAOYAN - Brandeis University; SCHMIDT-ROHR, KLAUS - Brandeis University; ZHANG, LIGAN - Anhui Agricultural University; THOMPSON, MICHAEL - Iowa State University; MAO, JINGDONG - Old Dominion University; GAO, HONGJIAN - Anhui Agricultural University

Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/12/2020
Publication Date: 2/25/2020
Citation: Chen, X., Ye, X., Chu, W., Olk, D.C., Cao, X., Schmidt-Rohr, K., Zhang, L., Thompson, M.L., Mao, J., Gao, H. 2020. Formation of char-like, fused-ring aromatic structures from a nonpyrogenic pathway during decomposition of wheat straw. Journal of Agricultural and Food Chemistry. 68(9):2607-2614. https://doi.org/10.1021/acs.jafc.9b06037.
DOI: https://doi.org/10.1021/acs.jafc.9b06037

Interpretive Summary: Soil organic matter plays important roles in many soil processes. Hence maintaining adequate levels of soil organic matter is a key objective of sustainable farming practices. One approach is to increase the stability of soil organic matter, and the composition of soil organic matter is thought to affect its duration in soil. One particularly stable form of soil organic matter has been claimed to form only through natural fires or artificial burning of organic materials that are then added to soil. In this study, we found that this stable form of soil organic matter can also form through natural decomposition of wheat straw. Therefore we demonstrate that the long-term stability of soil organic matter is less dependent on burnt materials than has been claimed. These results provide incentive to incorporate adequate amounts of crop residues into the soil in order to promote the formation of stable soil organic matter. This knowledge will benefit scientists who study soil organic matter and its cycling in soil and land managers who promote the accumulation and preservation of soil organic matter.

Technical Abstract: Fused-ring aromatic structures are important skeletal components of black carbon (BC) and contribute to long-term carbon (C) sequestration in soils and sediments. Since fused-ring aromatics have previously been thought to be primarily formed by incomplete combustion of organic materials, some methods used to quantify pyrogenic C in soils and sediments are based on the estimation of nonprotonated aromatic C or fused-ring aromatics, while non-pyrogenic origins are assumed to be negligible. However, in this study, using novel, advanced one- and two- dimensional solid-state 13C nuclear magnetic resonance techniques, we for the first time identify fused-ring aromatic structures that formed during the decomposition of wheat (Triticum sp.) straw in soils under aerobic condition, but not under anaerobic conditions. With increasing decomposition time, more and larger aromatic clusters were formed. The annual flux of this previously ignored non-pyrogenic BC, which is likely to be preserved in soils and sediments, could be equivalent to as much as one third of pyrogenic BC added to soils from all sources, significantly expanding our understanding of the potential sources of BC and the global C cycle. Our results suggest that common methods for estimating pyrogenic BC abundance by relying on quantification of nonprotonated C or fused-ring aromatic C could lead to misleading overestimation of the contribution of pyrogenic BC to soils. The finding also implies that the production, stocks, and fluxes of BC proposed in some literature should be re-estimated.