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Recent intensification of agriculture in face of the continuously increasing food demand has caused many environmental and socioeconomic problems. Excessive use of nitrogen (N) fertilizers is prevalent on many farms today. However, a large proportion of the N applied to agricultural soil contributes very little to crop production. Instead, a considerable amount of N is lost via leaching into groundwater, via runoff into surface water, and via nitrous oxide (N2O) emission into the atmosphere. As a result, the ecosystem is negatively impacted. Sustainable N management should aim at supplying sufficient N for optimum crop growth and development, while keeping losses to the environment to a minimum. Towards this goal, it is essential to enhance N retention and at the same time mitigate N2O emission in agricultural soil. Being the leader of one USDA project, I investigated the effects of different N management approaches that are based on the use of biochar, dicyandiamide (nitrification inhibitor) and polyacrylamide (superabsorbent hydrogel) in agricultural soils. It was found that a significantly higher amount of ammonium (NH4+) could be retained in the biochar-amended soil, comparing to the control soil. However, the retention of other ionic forms of N, such as nitrite (NO2-) and nitrate (NO3-), was barely improved. In fertilized soil, nitrification is a common process that transforms NH4+ into NO2- and eventually NO3-. To limit this transformation, the effects of one of the most commonly used nitrification inhibitor, i.e., dicyandiamide (DCD), was studied in combination with biochar. Based on the experimental data, mathematical models were developed to estimate the rate of nitrification impacted by the inhibition of persistently degrading DCD. It was discovered that the presence of biochar accelerated the biodegradation of both DCD and NH4+, but effectively reduced the spatial separation between the two. Depending on the soil properties and environmental conditions, the combined use of DCD and biochar can have versatile effects on nitrification inhibition. Also, considering the great amount of field data reported in the literature, statistical methods such as data synthesis and meta-analysis were also exploited to excavate the insightful information on the effectiveness of different soil amendments (e.g., biochar) in terms of N retention, crop production and N2O emission. Lastly, the recalcitrance of biochar, which is directly related to biochar's soil carbon sequestration potential, was also discussed. In order to quantify and compare the soil carbon sequestration potential of biochar produced from different conditions and feedstock types, a carbon-based recalcitrance index was developed using a state-of-the-art thermochemical technique.