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Old world bluestems [OWB (Bothriochloa spp.)] are introduced warm-season grass species commonly used for livestock grazing in the semi-arid Texas High Plains. Interseeding forage legumes into OWB-based pastures can reduce N fertilizer dependency because of symbiotic N-2 fixation, but little is known about their N contribution where water for irrigation is limited. A 4-year study (2007-2010) evaluated N concentration and soil particulate organic C (POC) and N (PON) of three OWB species interseeded with alfalfa (Medicago sativa L.), yellow sweetclover (Melilotus officinalis Lam.), or sainfoin (Onobrychis viciifolia Scop.) compared with 60 kg N ha(-1) fertilizer and no N (control). Irrigation (maximum of 250 mm year) was applied uniformly to all treatments. Yellow sweetclover rapidly increased grass N concentration and improved the energy efficiency of forage production more than the other two legumes while N benefits from alfalfa increased over the 4 years. Carbon and N uptake reflected both forage N concentration and production benefits from interseeded legumes and N fertilization. Soil POC and PON concentration was not affected by N fertilization but was increased by interseeding yellow sweetclover. Inclusion of appropriate legumes in this semi-arid region can increase soil POC, PON, and N concentration in the associated OWB grass as much or more than typical rates of N fertilization and cost less energy at irrigation levels of about 33% ETo. Results have implications for both animal nutrition and the environment. (C) 2014 Elsevier B.V. All rights reserved.