In the Pacific Northwest, commercial hybrid poplar (Populus generosa Henry × Populus canadensis Moench.) is managed at low stocking densities under irrigation for high-value timber production. The objectives of this study were to measure greenhouse gas emissions (CH4, CO 2 , and N 2 O) during intercropping of switchgrass (Panicum virgatum L.) with hybrid poplar; estimate losses of fertilizer-N as N 2 O, and estimate global warming potentials (GWP) of the intercrop. Cumulative above-ground biomass-C of the poplar monoculture (PM) closely matched the four year growing season (GS) soil CO 2 -C emissions, where aboveground biomass of the switchgrass monoculture (SM) and intercrop (IC) exceeded GS CO 2 -C emissions by 14.1 Mg C ha -1 . Soil CH 4 -C uptake was not significantly different between treatments, while GS N 2 O-N emissions for PM were ~80% lower than both IC and SM. N 2 O emissions factors averaged 0.7% of the applied N-fertilizer. Cumulative contributions of CO 2 emissions to GWP were offset by biomass-C resulting in a near zero balance (−5.1 Mg CO 2eq ha -1 ) for the PM, where, IC and SM sequestered significantly more CO 2 resulting in a net GWP of −42.5 and -32.2 Mg CO 2eq ha -1 , respectively. Intercropping with switchgrass can improve the net greenhouse gas balance of hybrid poplar. Continued research is needed on the effects of irrigated bioenergy production on GHG emissions in intercropped systems as they will become increasingly important as agricultural water use, water availability and quality are challenged by climate change.Abbreviations: DM, dry matter; GHG, greenhouse gas; GS, growing season; GWP, global warming potential; N EF , nitrogen emissions factor; PM, monoculture poplar; IC, poplar/ switchgrass intercrop; SM, switchgrass monoculture M ajor shifts in crop production will occur as farmers prepare to supply the demand for biomass feedstocks for production of renewable biofuels. These shifts will affect agroecosystem services related to water use, carbon storage, nutrient cycling and greenhouse gas (GHG) emissions that have direct effects on air, water, and soil quality (Popp et al., 2014). Perennial grasses are an important source of feed and fiber and are considered sustainable bioenergy crops because they have the capacity to produce large quantities of biomass, can be grown on marginal lands, improve soil quality and protect soils from erosion (Lemus and Lal, 2005;Sartori et al., 2006; Casler et al., 2009;Gelfand et al., 2013). Switchgrass (Panicum virgatum L.) is a prominent biomass crop for the US biofuel industry (Sanderson et al., 2007; Casler et al., 2009) because it yields well on a variety of soil types, is drought-tolerant, and has low fertility requirements (McLaughlin and Kszos, 2005;Lemus and Lal, 2005;Kimura et al., 2015). The root system of switchgrass has been shown to offset C losses and promotes C sequestration by adding a Core Ideas• A critical issue in the production of biofuels has been the competition with food crops• Intercropping switchgrass and hybrid poplar wa...