Characterization and agronomic evaluation of biochar amendments in a midwestern loam soil: effects on soybean yield, soil nutrient retention, and leachate quality

The development and application of biochar as a soil amendment has shown promising results for improving soil fertility, enhancing nutrient retention, and mitigating environmental degradation in agroecosystems. This dissertation investigates the physicochemical properties of biochar derived from different feedstocks and thermochemical processes, and evaluates their short-term effects on soybean (Glycine max) yield, soil nutrient dynamics, leachate composition, and plant nutrient uptake. The study combines biochar characterization with a controlled pot experiment to assess how feedstock type, production method, and application rate influence biochar behavior in soil. Chapter 2 focuses on the comprehensive characterization of two contrasting biochar types: wheat straw biochar (WSB) produced via slow pyrolysis at 400 °C and a commercial wood biochar (WB) produced via downdraft gasification. Proximate and elemental analyses, ATR-FTIR spectroscopy, surface area estimation, and molar ratio assessments were used to evaluate carbon stability, aromaticity, surface functionality, and nutrient content. The WSB biochar exhibited higher ash content, greater nutrient concentrations (e.g., total K, total P, NH4+-N), and higher H:C and O:C ratios, while WB demonstrated greater fixed carbon content and structural stability, reflecting the influence of both feedstock and highest heating temperature (HHT) on biochar physicochemical characteristics. Chapter 3 presents a 60-day experiment examining the impact of WSB and WB at 1% and 3% w/w application rates, alone and in combination with mineral fertilizer. Treatments were evaluated for effects on soybean shoot biomass, root traits, nodulation, pod production, yield, and plant nutrient uptake, alongside measurements of leachate ammonium, nitrate, dissolved phosphate, and potassium concentration. The WSB, particularly at the 3% (w/w) application rate, significantly enhanced pod number, pod dry weight, and shoot biomass, likely due to improved nutrient availability and water retention. In contrast, WB treatments exhibited lower leachate volumes and reduced nitrate losses, suggesting superior hydrological regulation and potential for mitigating nutrient leaching. Soil cation exchange capacity and nutrient retention also improved in biochar-treated soils, with notable differences between feedstocks in their effects on pH and cation retention. Collectively, the findings emphasize the importance of biochar feedstock selection and production conditions in determining its agronomic performance. The reported research contributes to the growing body of knowledge on the role of biochar role in sustainable soil management and provides guidance for optimizing biochar application strategies in Midwestern agricultural systems.