Characterization of agroforestry feedstocks and their biochars and their effects on soil greenhouse gas emissions in a floodplain soil

Abstract

The temperature at which biochar (biocharcoal) is produced is a major factor that determines its chemical and structural characteristics. The objectives of this research were to assess the effects of feedstock source among potential agroforestry materials and pyrolysis temperature on changes in biologically-labile and stable carbon and nitrogen forms in the original and pyrolyzed materials, and to compare several methods of chemical characterization and their utility in assessing changes in carbon and nitrogen forms. Three main analytical techniques (advanced solid-state [13]C NMR techniques, FTIR-DRIFTS analysis and total carbon and nitrogen analysis) were used to characterize biochar produced at 300 or 500[degrees]C from agroforestry feedstock (i.e., walnut (Juglans regia) hulls, switchgrass (Panicum virgatum) and cottonwood (Populus deltoids) feedstock). Results indicated that at 300[degrees]C, biochar was primarily composed of residues of biopolymers, such as lignin, cellulose and hemicellulose, while carbohydrates and most of the ligno-cellulosic features were lost when the temperature was increased to 500[degrees]C. These results confirmed other researchers' observations that sp[3] -- hybridized carbon was lost at 500[degrees]C and the biochars became enriched with non-protonated aromatic bridgehead carbons. Decreased protonated aromatic carbons and aromatic C-O functional groups and increased nonprotonated aromatic functional groups were observed with increased pyrolysis temperature. Higher pyrolysis temperatures resulted in biochars that predominantly consisted of aromatic functional groups, and promoted loss of lignin, cellulose and labile biopolymers and carbohydrates from the original feedstock.