Influence of Pyrolysis Temperature and Production Method on Switchgrass Biochar for Use As a Soil Amendment.

Biochars are thought to form recalcitrant carbon and increase water retention in soils, while enhancing plant growth by promoting soil-nutrient adsorption. However, the magnitude of this is contingent upon production conditions and thermo-chemical conversion processes. Herein we aim at (i) characterizing switchgrass-biochar morphology, (ii) estimating water-holding capacity under increasing ratios of char: soil; and, (iii) determining nutrient profile variation as a function of pyrolysis conversion methodologies (i.e. continuous, externally-heated auger versus carbonized batch systems) for terminal use as a soil amendment. Auger system chars produced at 600°C had the greatest lignin portion by weight, indicating higher recalcitrance, whereas lower production temperatures (400ºC) yielded greater hemicellulose (i.e. greater mineralization promoting substrate). Under both pyrolysis methods, increasing soil application rates in our silt loam soil resulted in linear decreases in bulk density (g cm^-3). Increases in auger-char (400°C) applications, increased soil water-holding capacities; however application rates of >2 Mt ha^-1 are required. Whereas carbonized batch chars did not influence water-holding abilities (P>0.05). Biochar macro and micro nutrients tended to become sequestered in char fractions, as heating values increased in the batch system from 400-600°C; whereas nitrogen levels tended to decrease. Consequently, all chars are not inherently equal, in that varying operation systems, resident times, and production conditions can greatly impact uses as a soil amendment and overall rate for efficacy, and it cannot be assumed that all chars increase soil water-holding capacities, nutrient retention, and improve soil tilth.