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 (AE 2%). Chemical composition of the chars was assessed by elemental analysis, solid-state 13 C 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 5200 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 5200 C. #
[1] Black carbon (BC), the product of incomplete combustion of fossil fuels and biomass (called elemental carbon (EC) in atmospheric sciences), was quantified in 12 different materials by 17 laboratories from different disciplines, using seven different methods. The materials were divided into three classes: (1) potentially interfering materials, (2) laboratory-produced BC-rich materials, and (3) BC-containing environmental matrices (from soil, water, sediment, and atmosphere). This is the first comprehensive intercomparison of this type (multimethod, multilab, and multisample), focusing mainly on methods used for soil and sediment BC studies. Results for the potentially interfering materials (which by definition contained no fire-derived organic carbon) highlighted situations where individual methods may overestimate BC concentrations. Results for the BC-rich materials (one soot and two chars) showed that some of the methods identified
Abstract.Interactions between biochar, soil, microbes, and plant roots may occur within a short period of time after application to the soil. The extent, rates, and implications of these interactions, however, are far from understood. This review describes the properties of biochars and suggests possible reactions that may occur after the addition of biochars to soil. These include dissolution-precipitation, adsorption-desorption, acid-base, and redox reactions. Attention is given to reactions occurring within pores, and to interactions with roots, microorganisms, and soil fauna. Examination of biochars (from chicken litter, greenwaste, and paper mill sludges) weathered for 1 and 2 years in an Australian Ferrosol provides evidence for some of the mechanisms described in this review and offers an insight to reactions at a molecular scale. These interactions are biochar-and site-specific. Therefore, suitable experimental trials-combining biochar types and different pedoclimatic conditions-are needed to determine the extent to which these reactions influence the potential of biochar as a soil amendment and tool for carbon sequestration.
The stability of biochar carbon (C) is the major determinant of its value for long-term C sequestration in soil. A long-term (5 year) laboratory experiment was conducted under controlled conditions using 11 biochars made from five C3 biomass feedstocks (Eucalyptus saligna wood and leaves, papermill sludge, poultry litter, cow manure) at 400 and/or 550 °C. The biochars were incubated in a vertisol containing organic C from a predominantly C4-vegetation source, and total CO(2)-C and associated δ(13)C were periodically measured. Between 0.5% and 8.9% of the biochar C was mineralized over 5 years. The C in manure-based biochars mineralized faster than that in plant-based biochars, and C in 400 °C biochars mineralized faster than that in corresponding 550 °C biochars. The estimated mean residence time (MRT) of C in biochars varied between 90 and 1600 years. These are conservative estimates because they represent MRT of relatively labile and intermediate-stability biochar C components. Furthermore, biochar C MRT is likely to be higher under field conditions of lower moisture, lower temperatures or nutrient availability constraints. Strong relationships of biochar C stability with the initial proportion of nonaromatic C and degree of aromatic C condensation in biochar support the use of these properties to predict biochar C stability in soil.
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