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Corn production in the US Midwest has the potential to generate a large amount of crop residue for bioenergy production. However, unconstrained harvesting of crop residues is associated with a long-term decline in soil quality. Biochar applications can mitigate many of the negative effects of residue removal but data and economic analyses to support decision making are lacking. To explore sustainable and profitable practices for residue harvesting in central Iowa we used 11 years of soil, crop yield, and management data to calibrate the Agricultural Production Systems sIMulator (APSIM) biochar model. We then used the model to evaluate how different biochar types and application rates impact productivity and environmental performance of conventional corn and corn-soybean cropping systems in Iowa under different N fertilizer application rates and residue harvesting scenarios. A cost-benefit analysis was also employed to identify the economically optimal biochar application rate from both producer and societal perspectives. Modeling results showed for both continuous corn and corn-soybean rotations that as biochar application rate increased (from 0 to 90 Mg ha-1) nitrate leaching decreased (from 2.5 to 20 %) and soil carbon levels increased (from 8 to 115 %), but there was only a small impact on corn yields (from -2.6 to 0.6 %). The cost-benefit analysis revealed that public benefits, evaluated from decreased nitrate leaching and increased soil carbon levels, significantly outweighed the private revenue accrued from crop yield gains, and that a biochar application rate of 22 Mg ha-1 was more cost-effective (per ton) compared to higher biochar rates. Overall, this study found that applying biochar once at a rate of 22 Mg ha-1 allows for the sustainable annual removal of 50% of corn residue for 32 years, is profitable for farmers even with minimal impact on grain yield, and beneficial to society through reduced nitrate leaching and increased soil organic carbon levels.

The long-term impact of biochar on soil properties and agronomic outcomes is influenced by changes in the physical and chemical properties of biochars that occur with time (aging) in soil environments. Fresh biochars, however, are often used in studies because aged biochars are generally unavailable. Therefore, a need exists to develop a method for rapid aging of biochars in the laboratory. The objectives of this study were to compare the physicochemical properties of fresh, laboratory-aged (LA), and field-aged (FA) (≥3 yr) biochars and to assess the appropriateness of a laboratory aging procedure that combines acidification, oxidation, and incubations as a mimic to field aging in neutral or acidic soil environments. Twenty-two biochars produced by fast and slow pyrolysis, and gasification techniques from five different biomass feedstocks (hardwood, corn stover, soybean stover, macadamia nut shells, and switchgrass) were studied. In general, both laboratory and field aging caused similar increases in ash-free volatile matter (% w/w), cation and anion exchange capacities, specific surface area, and modifications in oxygen-containing surface functional groups of the biochars. However, ash content increased for FA (18-195%) and decreased for LA (22-74%) biochars, and pH decreased to a greater extent for LA (2.8-6.7 units) than for FA (1.6-3.8 units) biochars. The results demonstrate that the proposed laboratory aging procedure is effective for predicting the direction of changes in biochar properties on field aging. However, in the future we recommend using a less aggressive acid treatment.

Proximate analysis is widely used to determine moisture, volatile matter (VM), fixed carbon (FC) and ash content of biochars. The original ASTM D1762-84 method was developed to assess quality of hardwood charcoal for use as fuel. We have developed a modified proximate analysis method to assess quality of diverse biochars for use as soil amendments. We determined that a N 2 purge is necessary during both moisture and VM determination to avoid errors associated with sample oxidation. We assessed a range of boundary temperatures (350-950°C) for separating VM and FC, and determined that 800°C is the minimum temperature required to distinguish between VM and FC in biochars. Furthermore, correlation between VM/FC and molar H/C org ratios suggests that VM/FC ratios are a useful measure of biochar stability. Use of the proposed modified method is encouraged to reduce variance in analytical results among studies.

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Core Ideas Crop rotations including switchgrass were found to build soil organic C and N Biochar amendments increased soil C and N, pH, and gravity drained water content Total soil C and N increases with biochar age suggest negative priming Biochar and switchgrass offset negative effects of biomass harvesting on soil quality Corn residue removed from Midwestern farms is a large potential source of biomass for cellulosic bioenergy production in the US long‐term harvesting of biomass, however, may lead to the degradation of soil quality unless management practices that compensate for the removal of biomass are used. In this study, biochar amendments and long‐term crop rotations that include triticale and switchgrass with corn and soybeans were hypothesized to reduce the negative effects of biomass harvesting on soil quality. Chemical breakthrough curves, measured for intact soil cores indicate that crop rotations that include switchgrass or triticale increased both retardation and dispersivity relative to conventional rotations and biochar amendments decreased dispersivity relative to controls. Across all crop rotations, there was an increase in total soil C and N, soil C/N ratio, pH and gravity drained water content, and a decrease in bulk density for soils treated with biochar relative to no‐biochar controls. No significant effect of biochar age on soil physical properties was measured in 2014 but significant increases with biochar age were found for total soil C and N in 2016, suggesting a synergistic interaction (negative priming). Continuous switchgrass stands were found to build soil organic C and N, increase retention of plant available P and K, and lower bulk density relative to the continuous corn cropping system. The results suggested that soil biochar amendments and crop rotations that included switchgrass helped mitigate some of the adverse effects of biomass harvesting on soil quality.

This dissertation was made possible through the support and input of many people and funding from multiple sources. I would like to thank my major professor, David Laird, for giving me the opportunity to conduct this research, for challenging me, and always striving to make me a better scientist, writer, and communicator. Many thanks to my committee members, Rick Cruse, Sotirios Archontoulis, Robert Horton, and Jerry Hatfield, for their guidance, insight, and continuous encouragement throughout the course of my PhD. Also, thank you to my colleagues, the faculty, and staff in the Department of Agronomy for providing me with the resources and support I needed to advance in my degree and for making my time at ISU a great experience. Special thanks to members of my lab, in particular, Samuel Rathke, Rivka Fidel, Santanu Bakshi, and Chumki Banki for their assistance with and input in various aspects of my research. To the many friends I have made in Iowa-Thank you! It is impossible to mention all those who have contributed to my success and have been a part of my 'Iowa experience', but some people include:

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