Condensation and Polymerization Explain the Humification of Lignin Into Aliphatic and Aromatic Structures in Soil

Soil organic matter (SOM) constitutes a global reservoir of carbon that is more than twice that of either atmospheric carbon or aquatic carbon; however, the manner in which it forms from degraded plant biomass is poorly understood. Some have recently questioned whether plant biomass is involved directly in SOM formation and suggest that it is microbial carbon that constitutes the main source of stable SOM. Such a view implies that above and below ground plant biomass is rapidly decomposed and mineralized. This view contrasts significantly with traditional ones that involve the transformation of plant biomass to recalcitrant humic materials fueled mainly by lignin. One of the main observations for the new viewpoint is that lignin phenols, biomarkers for lignin in plant biomass, become depleted rapidly in SOM and are thought to indicate that lignin, carbohydrates, proteins, and other plant biopolymers are depleted at the same rate. In the current study we demonstrate that lignin plays a pivotal role in the formation of geologically stable SOM and that the new viewpoint grossly miscalculates the input of microbial biomass. Promoted by the strong oxidation of prevalent reactive oxygen species (ROS) in soil, lignin is not completely mineralized but molecularly transformed to structures not recognizable by lignin phenol biomarker studies. We employ NMR and ultrahigh resolution mass spectrometry on a series of plant biopolymers, synthesized lignin, soil, and peat samples to demonstrate the effect of ROS transformations involving mainly hydroxyl radicals