Extent of Pyrolysis Impacts on Fast Pyrolysis Biochar Properties

A potential concern about the use of fast pyrolysis rather than slow pyrolysis biochars as soil amendments is that they may contain high levels of bioavailable C due to short particle residence times in the reactors, which could reduce the stability of biochar C and cause nutrient immobilization in soils. To investigate this concern, three corn (Zea mays L.) stover fast pyrolysis biochars prepared using different reactor conditions were chemically and physically characterized to determine their extent of pyrolysis. These biochars were also incubated in soil to assess their impact on soil CO2 emissions, nutrient availability, microorganism population growth, and water retention capacity. Elemental analysis and quantitative solid-state C-13 nuclear magnetic resonance spectroscopy showed variation in O functional groups (associated primarily with carbohydrates) and aromatic C, which could be used to define extent of pyrolysis. A 24-wk incubation performed using a sandy soil amended with 0.5 wt% of corn stover biochar showed a small but significant decrease in soil CO2 emissions and a decrease in the bacteria: fungi ratios with extent of pyrolysis. Relative to the control soil, biochar-amended soils had small increases in CO2 emissions and extractable nutrients, but similar microorganism populations, extractable NO3 levels, and water retention capacities. Corn stover amendments, by contrast, significantly increased soil CO2 emissions and microbial populations, and reduced extractable NO3. These results indicate that C in fast pyrolysis biochar is stable in soil environments and will not appreciably contribute to nutrient immobilization.