![]() ![]() However, the difference method only works for low ash biochars and biochars that contain SiO 2 as the only significant form of ash. Numerous studies 17, 18, 19, 20, 21, 22 have used the difference method for calculating the O content of biochars. For instance, the O content of biochar has been determined by difference from unity after subtracting the ash content and elemental C, H, N and S content as determined by the proximate and ultimate analysis methods 15, 16, respectively. Various analytical methods have been used to determine the O content of biochars. The International Biochar Initiative (IBI) reported various alpha methods including the use of O:C ratio 14 to assess biochar stability and biochar degradation patterns. Spokas 13 noted that most biochars have O:C ratios within 0.2–0.6 range with O:C ratios approaching 0.6 indicating the least stable biochars. Masiello 12 discussed the potential use of O:C ratios for assessing biochar stability in soils. 10 mentioned the importance of O to C ratio of fresh biochars as an index for the extent of charring and the O:C ratio can also be used to assess the extent of post-pyrolysis oxidation, which occurs as biochars age/weather in soil environments 11. The O:C ratio of the organic phase is a measure of the density of polar functional groups on biochar surfaces. Knowing the O:C elemental ratio of the organic phase apart from the O:C ratio of the whole biochar is important for many reasons. Thus, O and C are constituents of both the organic and inorganic phases in most biochar samples. Carbon is also the primary element in condensed aromatic structures, which dominate the organic phase of biochar while O is the key element in many polar organic functional groups on biochar surfaces, which influence biochar reactivity in soil environments. However, increasing the pyrolysis temperatures also promotes the formation of inorganic phases, particularly carbonates, which contain both O and C. The pyrolysis process favors the elimination of H and O over C from the organic phase and increasing the pyrolysis temperature drives the elimination of H and O towards completion 6, 7, 8. Amonette and Joseph 5 reported that the elemental content of biochar depends on the biomass feedstock whereas pyrolysis temperatures has a large influence on functional group and structural chemistry of biochars. The impact of soil biochar applications on C sequestration and various agronomic and environmental outcomes is influenced by the physical and chemical properties of the biochar. Soil biochar applications are also of interest because of their potential to positively impact soil quality and crop productivity, particularly when applied to degraded and otherwise problematic soils. Soil biochar applications are effective for C-sequestration because photosynthesis transfers C from the atmosphere to biomass and pyrolysis transforms some of the biologically labile biomass C into recalcitrant biochar C, which can stay in the soil for hundreds to thousands of years 1, 2, 3, 4. Interest in the use of biochar as a soil amendment to enhance soil quality and sequester C has increased substantially during the last decade. Environmental modelers are encouraged to use biochar H:C ratios. We also present evidence that acid washing removed 17% of the structural O from biochars and significantly changes O/C ratios. Novelty of this study includes a new method to predict organic O content of biochar using three easily measured biochar parameters- pyrolysis temperature, H/C molar ratio, and %biochar yield, and evidence indicating that the conventional difference method may substantially underestimate the organic O in biochar and adversely impact the accuracy of O:C ratios and van Krevelen plots. Here, we compare estimates of the O content for biochars produced from pure compounds (little or no ash), acid-washed biomass (little ash), and unwashed biomass (range of ash content). Biochar contains both organic and inorganic forms of O, and some of the organic O is converted to inorganic O (e.g., newly formed carbonates) when samples are ashed. However, accurately determining the organic O content of biochar is difficult. The organic O content of biochar is useful for assessing biochar stability and reactivity.
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