Analytical pyrolysis of synthetic chars derived from biomass with potential agronomic application (biochar). Relationships with impacts on microbial carbon dioxide production

A set of 20 biochar samples produced from the pyrolysis of different biomass feedstocks with potential applications as soil amendments were investigated by pyrolysis coupled to gas chromatography–mass spectrometry (Py–GC–MS). The yields of 38 pyrolysis products representative of charred (e.g., benzene derivatives, PAHs, benzofurans) and partially charred biomass (e.g. derivatives of lignin phenols) were evaluated by Py–GC–MS. The estimated yields covered a wide interval (40–7700 g g−1) and were correlated with volatile matter (VM). The proportion of pyrolysis products associated with charred materials ranged from 49 to >99% and was positively correlated with benzene/toluene (B/T) ratios. The molecular distribution of partially charred materials reflected the original feedstock, with higher levels of protein fragments observed in samples with high initial nitrogen content. Both the abiotic and biotic production/consumption of CO2 was determined in original biochar incubated in water and soil–biochar systems. Large differences were observed in the net CO2 suppression/stimulation rates, with values between −50 and 2200 g g (char)−1 d−1 for biochar alone and biochar corrected rates from −19 to 690 g g (soil)−1 d−1 for amended soils. In general, increasing pyrolysis temperature yielded biochars with less intense Py–GC–MS pyrolysates, higher B/T ratios, and lower respiration rates. Biochars characterized by higher yields of proteins and cellulose-derived pyrolysis products (e.g. distiller grains, hardwood, mixed wood chips/manure) were associated with higher CO2 mineralization rates in the corresponding amended soils, particularly sugars. These results suggest that the stimulation in CO2 production could arrive from the residual non-charred material, and not be a true indication of more completely charred material, which typically had no effect or minimal suppression.