Nitrogen (N) is an essential macronutrient for plant growth; however, excessive use of N fertilizers and complexities of the N cycle in soil cause negative environmental impacts. This imposes several challenges in controlling the N availability timing and losses. The objective of this study was to develop a biochar-based slow-release fertilizer (SRF) to reduce N loss and increase N use efficiency in crop production. We provided a laboratory-based assessment of several H3PO4 activated (5 and 15%) biochar-based SRFs, produced from different combinations of biochar to urea (1:2, 1:3, 1:4, and 1:6), calcium lignosulfonate (5%), and paraffin wax (10%). Characterization analyses (SEM–EDS, XRD, FTIR, and XPS) of developed SRFs suggest successful urea grafting onto biochar through both the urea amine N and carbonyl C═O modes, without urea crystal structure disruption. The SRFs were more efficient than uncoated urea (control): (1) urea released in aqueous medium was 61–90% in 4320 min for the SRFs versus 99.6% in 12 min for the control; (2) cumulative N leached from soil columns was 68–71% after 41 leaching events for SRF versus 99.9% after four leaching events for the control; and (3) NH3-N volatilization from soil was 0.2–0.9% for the SRFs versus 2% for the control. Inclusively, our results suggest that the developed SRFs are effective for reducing N loss from soil and provide larger quantities of NH4+-N to plants for a longer time (improved N use efficiency). We attribute this to that the developed SRFs are optimal for synchronizing with plant N uptake for providing better sustainability in modern agriculture.