The Effects of Biochar Amendment to Soil on Bioenergy Crop Yield and Biomass Composition

This thesis research aims to improve the understanding of how pyrolysis of native feedstocks and biochar application to soil can be utilized to improve soil fertility, crop yield, and sequester carbon in the Southeastern U.S. An emphasis was placed on understanding how the pyrolysis conditions and feedstock material influence biochar characteristics and what effects biochar amendment has on soil and plant growth and composition.

Biochars were produced from switchgrass and pine wood feedstocks via fast pyrolysis at 450, 600, and 800 ˚C with a continuous auger pyrolysis system. Switchgrass-derived biochars had higher ash content, and as observed by elemental analysis, and Raman and X-ray diffraction spectroscopic techniques, underwent greater carbonization resulting in more aromatic carbon than pine-derived biochars. As pyrolysis temperature increased the aromaticity, crystallinity, and ash content increased. In addition, as indicated by FT-IR spectroscopy, biochars produced at low temperatures retained some feedstock characteristic surface functionality, and as temperature increased functionality decreased.

Upon biochar application to soil at a rate of 5 % (wt.), Mehlich-I extractable P, K, Mn, and exchangeable K significantly increased in soil. Cation extraction on pure biochar samples showed that, initially, biochars produced a high pyrolysis temperatures (800 ˚C) had greater exchangeable cations and cation exchange capacity (CEC); however, after 8 weeks of aging the CEC of low temperature (450 ˚C) biochar was significantly higher.

Plant growth experiments demonstrated that biochar application increased above-ground biomass yield in both switchgrass and sorghum by up to 25 %. Compositional analysis of sorghum grown in biochar amended soil demonstrated that as the biochar application rate increased the ash content of the plants increased. Response surface regression illustrated that the optimum temperature to produce biochar for soil amendment to maximize plant biomass yield is between 550 to 650 ˚C, and that as the application rate increased the plant biomass yield increased.

Biochar characteristics resulting in potential carbon sequestration (carbon aromaticity and crystallinity) and improving soil fertility and crop yield (increased ash content and CEC) were identified. It was concluded that switchgrass pyrolyzed at 600 ˚C best meets the balance between carbon sequestration and improving soil fertility and plant biomass yield.