Determining C:N Ratios of the Labile Biochar Fraction.

Most models divide biochar into a labile fraction, which is assumed to be mineralized rapidly, and a recalcitrant fraction, which is assumed to be stable for hundreds or even thousands of years.  The C:N ratio of the labile fraction is important as this ratio determines whether the biochar immobilizes N or is a source of N fertility during the first few years after application as the labile fraction is mineralized. Volatile matter as determined by proximate analysis is commonly assumed to represent the labile fraction of biochar; however, proximate analysis does not provide a ready means of determining the C:N ratio of the volatile matter.  The goal of this study is to evaluate a proposed fractionation scheme for biochar, which allows the C:N ratio of the labile fraction to be determined. Laboratory controlled chemical and spectroscopic analyses will be reported for six biochars produced from four biomass feedstocks (hardwood, soybean, switchgrass and corn stover) using two pyrolysis techniques (slow and fast). All analyses will be done in quadruplicates and will be quantified with respect to total C of biochar. The inorganic C fraction of biochar is determined by CO₂ released on reaction with 1M HCl using a 1M NaOH trap. Labile organic C will progressively be extracted in four steps using hot water, 6M, 9M, and 12M H₂SO₄. 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 matter extracted using hot water and the various H₂SO₄ solutions. The C:N ratio and total N extracted with hot water and H₂SO₄ will be compared with the C:N ratio and total N of the original biochar and the residue after the H₂SO₄ treatments.