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Soil nitrogen (N) is an important component in maintaining ecosystem stability, and the introduction of non-native plants can alter N cycling by changing litter quality and quantity, nutrient uptake patterns, and soil food webs. Our goal was to determine the effects of Bromus tectorum (C-3) invasion on soil microbial N cycling in adjacent non-invaded and invaded C-3 and C-4 native arid grasslands. We monitored resin-extractable N, plant and soil delta C-13 and delta N-15, gross rates of inorganic N mineralization and consumption, and the quantity and isotopic composition of microbial phospholipid biomarkers. In invaded C-3 communities, labile soil organic N and gross and net rates of soil N transformations increased, indicating an increase in overall microbial N cycling. In invaded C-4 communities labile soil N stayed constant, but gross N flux rates increased. The delta C-13 of phospholipid biomarkers in invaded C-4 communities showed that some portion of the soil bacterial population preferentially decomposed invader C-3-derived litter over that from the native C-4 species. Invasion in C-4 grasslands also significantly decreased the proportion of fungal to bacterial phospholipid biomarkers. Different processes are occurring in response to B. tectorum invasion in each of these two native grasslands that: 1) alter the size of soil N pools, and/or 2) the activity of the microbial community. Both processes provide mechanisms for altering long-term N dynamics in these ecosystems and highlight how multiple mechanisms can lead to similar effects on ecosystem function, which may be important for the construction of future biogeochemical process models.