Corn growth and nitrogen nutrition after additions of biochars with varying properties to a temperate soil

The effects of biochar properties on crop growth are little understood. Therefore, biochar was produced from eight feedstocks and pyrolyzed at four temperatures (300°C, 400°C, 500°C, 600°C) using slow pyrolysis. Corn was grown for 46 days in a greenhouse pot trial on a temperate and moderately fertile Alfisol amended with the biochar at application rates of 0.0%, 0.2%, 0.5%, 2.0%, and 7.0% (w/w) (equivalent to 0.0, 2.6, 6.5, 26, and 91 t biochar ha⁻¹) and full recommended fertilization. Animal manure biochars increased biomass by up to 43% and corn stover biochar by up to 30%, while food waste biochar decreased biomass by up to 92% in relation to similarly fertilized controls (all P < 0.05). Increasing the pyrolysis temperature from 300°C to 600°C decreased the negative effect of food waste as well as paper sludge biochars. On average, plant growth was the highest with additions of biochar produced at a pyrolysis temperature of 500°C (P < 0.05), but feedstock type caused eight times more variation in growth than pyrolysis temperature. Biochar application rates above 2.0% (w/w) (equivalent to 26 t ha⁻¹) did generally not improve corn growth and rather decreased growth when biochars produced from dairy manure, paper sludge, or food waste were applied. Crop N uptake was 15% greater than the fully fertilized control (P < 0.05, average at 300°C) at a biochar application rate of 0.2% but decreased with greater application to 16% below the N uptake of the control at an application rate of 7%. Volatile matter or ash content in biochar did not correlate with crop growth or N uptake (P > 0.05), and greater pH had only a weak positive relationship with growth at intermediate application rates. Greater nutrient contents (N, P, K, Mg) improved growth at low application rates of 0.2% and 0.5%, but Na reduced growth at high application rates of 2.0% and 7.0% in the studied fertile Alfisol.