Technical Abstract: Management interventions are needed to concurrently increase forage production and quality on native rangelands while improving the soil resource. Incorporation of alfalfa (Medicago spp.) into rangelands represents one such intervention, though information regarding the influence of alfalfa on key indicators of soil quality in rangelands is limited. The objective of this study was to quantify the effects of alfalfa interseeded into native rangeland on soil organic carbon (SOC) and total nitrogen (TN) for a mixed-grass rangeland site near Mandan, North Dakota, USA. Status of SOC and TN to a depth of 40 cm was determined over a 4 yr period (2001 and 2005) for three interseeded alfalfa cultivars (one hay-type alfalfa and two grazing-type alfalfas) as well as rangeland not seeded to alfalfa. Soil organic C and TN at the beginning of the study (2001) did not differ among treatments at any depth. In 2005, two interseeded alfalfa cultivars (Vernal and Anik) possessed greater SOC and TN than the non-alfalfa control, but only within the surface 10 cm. Stocks of SOC and TN within the surface 4000 Mg soil/ha increased significantly between 2001 and 2005 under the interseeded alfalfa treatments at rates ranging from 0.3 to 1.1 Mg C/ha/yr and 0.003 to 0.07 Mg N/ha/yr, respectively. Though results from this study are short-term, interseeding grazing- or hay-type alfalfas into rangelands of the northern Great Plains appears to be a viable management intervention for increasing soil C and N stocks.