Formation of Char-Like, Fused-Ring Aromatic Structures from a Nonpyrogenic Pathway during Decomposition of Wheat Straw

Fused-ring aromatics, important skeletal components of black carbon (BC), contribute to long-term carbon (C) sequestration in nature. They have previously been thought to be primarily formed by incomplete combustion of organic materials, whereas the nonpyrogenic origins are negligible. Using advanced solid-state ¹³C nuclear magnetic resonance (NMR), including recoupled long-range C–H dipolar dephasing, exchange with protonated and nonprotonated spectral editing (EXPANSE), and dipolar-dephased double-quantum/single-quantum (DQ/SQ) spectroscopy, we for the first time identify fused-ring aromatics that formed during the decomposition of wheat (Triticum sp.) straw in soil under aerobic, but not anaerobic conditions. The observed formation of polyaromatic units as plant litter decomposes provides direct evidence for humification. Moreover, the estimation of the annual flux of such nonpyrogenic BC could be equivalent to 3–12% of pyrogenic BC added to soils from all other sources. Our findings significantly extend the understanding of potential sources of fused-ring aromatic C and BC in soils as well as the global C cycle.