Meulemans, Jabez2017-12-182017-12-182016-05http://hdl.handle.net/10125/51342M.S. University of Hawaii at Manoa 2016.Includes bibliographical references.Amendment of agricultural soils with biochar may sequester atmospheric carbon (C), affect soil greenhouse gas (GHG) emissions, and change crop yields; the resulting impact to agroecosystems’ net global warming potential (GWP), economic feasibility, and overall sustainability is highly relevant to tropical agriculture in Hawai‘i. To examine the use of biochar in Hawai‘i, field trials were studied for joint assessment of GWP and economic performance. A highly fertile Mollisol and an infertile Oxisol were amended with biochar and cultivated with no-till management of bioenergy feedstock (napier grass, var. bana) and conventional tillage of a food crop (sweet corn, Hawaiian Supersweet #9). Measurement of GWP included GHG emissions and C dynamics of crop biomass, soil, and biochar. Economic assessment combined traditional benefit-cost analysis (BCA) with full- cost accounting to include environmental costs of GWP, using net present value (NPV) as a metric for sustainability. The resulting hybrid BCA was tested under alternative scenarios wherein key variables were altered. Lastly, the relative importance of these variables in determining NPV was quantified using sensitivity analysis. Biochar amendment decreased soil GHG emissions in the Mollisol, but increased emissions in the Oxisol; concurrently, biochar increased napier grass yields by 14%, yet decreased sweet corn yields by 6%. These combined effects decreased GWP and increased NPV by as much as 73% in napier grass, resulting in a sustainable biochar system. In sweet corn, however, the best-case biochar scenario still decreased NPV by 31%—no matter how highly C was valued, corn yield decreases could not be outweighed by GWP improvements. In all, the most important factor was how biochar affected crop yields (ß=12.90±0.86), followed by GWP value (ß=10.01±1.12) and biochar investment cost (ß=7.88±0.01). For the average Hawaiian farmer, this means that investment in biochar should be carefully considered, despite its burgeoning popularity. This study showed that the best prospect for biochar amendment is for minimum-tillage crops, such as perennial bioenergy feedstocks, grown in naturally fertile soils.engThesis