Analysis of the Chemically- and Thermally-Labile Biochar Fractions.

The new biochar module in the APSIM cropping systems model assumes that biochar C and N are divided into labile and recalcitrant fractions.  Currently, volatile matter and fixed carbon as determined by proximate analysis are used to estimate the size of the labile and recalcitrant fractions, respectively.  The long-term goal of our research is to assess the validity of using proximate analysis to quantify the biologically labile and recalcitrant biochar fractions and to develop a means of determining the C:N ratio of the labile fraction. The specific goal of this study was to evaluate various chemical fractionation schemes for biochar, which allow the C:N ratio of the labile fraction to be determined, and compare the results of the chemical fractionations with results for proximate analysis. Nine biochars produced from five biomass feedstocks (hardwood, corn stover, soybean stover, switchgrass and macadamia nut shells) using four pyrolysis techniques (gasification and slow, fast, and autothermal pyrolysis) were used in the study. The inorganic C fraction of biochar was determined by measuring CO₂ released on reaction with 1M HCl using a NaOH trap method. Acid and base labile organic C was progressively extracted in seven steps using hot water, 0.05M, 0.5M, 1M, 2M, 3M, and 6M either H₂SO₄ or NaOH. The fraction of biochar C oxidized with 15% H₂O₂, 0.33M KMnO₄ (pH 7.2), and acidified 0.16M K₂Cr₂O₇ will also be determined.  All extracted solutions will be analyzed for dissolved organic carbon and nitrogen and the residues will be evaluated separately by proximate and ultimate analysis. Volatile matter as determined by proximate analysis will be compared with the amounts of organic C extracted using hot water and the various H₂SO₄/NaOH/oxidant solutions. The C:N ratio of biochars will be compared before and after extraction with hot water and H₂SO₄/NaOH/oxidant solution.