Biochar, a charcoal created by pyrolysis of biomass, may be an effective means of storing carbon (C) in soils, but its effects on N cycling in semiarid, irrigated calcareous soils is little understood. This 3-yr field study measured net N mineralization (0- to 30-cm) and greenhouse gas emission fluxes under sprinkler irrigated corn (Zea mays L.) for Portneuf silt loam amended once in fall, 2008, with either stockpiled dairy manure (42 Mg ha^-1 dry wt.), hardwood-derived biochar  (22.4 Mg ha^-1), singly and in combination with manure, along with a control (yielding 4 treatments).  Net N mineralization was measured during the spring, early summer, and late summer periods using buried bags. When averaged across the 2 manure and 2 no-manure treatments, manure increased cumulative seasonal net N mineralization 1.63x on average, and increased seasonal CO₂ gas emissions 1.17x compared to the no-manure treatment group.  When added with manure, biochar increased net N mineralization 1.9x in spring 2009 and 1.7x in late summer 2010, relative to manure alone.   When added with manure, biochar increased 2009 cumulative seasonal CH₄ soil gas emissions, from -0.16 kg/ha for manure alone to 1.87 kg/ha for biochar + manure, but had little effect on cumulative seasonal CO₂ and N₂O gas emission flux.  Evidence indicates that biochar increases the mineralization rate of organic C added to these calcareous soils, particularly in the first two years following application.