Utilization of legumes in place of fallow has been shown to decrease input costs and improve nitrogen use efficiency (NUE) in many crop rotations.  However, Northern High Plains dryland cash crop production relies on summer fallow for water storage and little is known about the impact of replacing summer fallow with legumes on soil and plant nutrient cycling. Furthermore, increased variability in winter moisture is forcing dryland farmers to convert to irrigated, legume-based production.  The overall goal of this research is to estimate the impact of legumes in dryland in transition crop rotations in Northern High Plains on relative contributions to agroecosystem carbon (C) and nitrogen (N). To accomplish this goal, we will assess greenhouse gas (GHG) emissions and soil and plant C and N in dryland and irrigated crop-legume rotations that incorporate perennial alfalfa and annual Austrian winter pea.  Our approach will utilize standard methodologies for carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) emissions, as well as soil N mineralization and plant residue decomposition.  To our knowledge, there are no inventories of GHG emissions for dryland and irrigated Northern High Plains systems, specifically, systems that incorporate legumes.  We hypothesize that legumes produce high GHG emissions during the legume growing season, however contributions from legume residues to the succeeding crop increases NUE and therefore, lowers GHG emissions in the long-term. The differences in legume biomass production between dryland and irrigated systems will determine the magnitude of GHG emissions and C and N contributions to soil organic matter.