Chemical composition and bioavailability of thermally altered Pinus resinosa (Red Pine) wood

Abstract

The residues remaining after incomplete combustion of vegetation (char) can contribute significantly to the carbon content of soils. Char C is considered biologically inert relative to other forms of organic C in soils; however, the degree of biological inertness is likely to be a function of the extent that the combustion residues were altered during thermal treatment. The relationship between changes in chemical composition and biological inertness of char C created in the laboratory by heating Pinus resinosa sapwood to temperatures between 70 and 350 °C was quantified. Heating at each temperature was maintained until the mass of the residual char material stabilised (±2%). Chemical composition of the chars was assessed by elemental analysis, solid-state 13C nuclear magnetic resonance (NMR) spectroscopy and diffuse reflectance infrared Fourier transform spectroscopy (DRIFT). The susceptibility of the heated sapwood to biological oxidation was quantified in a 120-day laboratory incubation. Thermal treatment at temperatures 200 °C induced significant variations in chemical composition. Changes in elemental contents and molar elemental ratios were consistent with an initial dehydration and the formation of unsaturated structures. The NMR and DRIFT data indicated that the changes in the chemical composition with increasing heating temperature included a conversion of O-alkyl C to aryl and O-aryl furan-like structures, consistent with results from work examining the chemical changes associated with thermal treatment of cellulose, the major component of wood. The chemical changes significantly reduced the ability of a microbial inoculum derived from decomposing Pinus resinosa wood to mineralise carbon contained in the charred samples. The C mineralisation rate constants decreased by an order of magnitude for wood heated to 200 °C.