Reducing nitrogen (N) pollution from agriculture remains a major challenge. Using crops that are N-conservative and retains N in the plant-soil system is one method to help mitigate N that would otherwise be lost to the environment. Native warm-season grasses are inherently N-use efficient and could also be a source of cellulosic biofuel feedstocks for renewable energy. Management for perennial bioenergy crops often recommends N fertilizer to increase and maintain yields. However, N addition may actually compromise some of the biological mechanisms that help conserve N in the plant-soil system. I investigated some of the mechanisms that conserve N resources in perennial grasses, with special attention to the effect of N addition on N conservation mechanisms and microbial associations. My objectives were 1) to understand the genetic and environmental controls and variation in the perennial plant N conservation strategies of N resorption and nitrogen-use efficiency, 2) assess the importance of the arbuscular-mycorrhizal fungal (AMF) associations in perennial grasses under varying soil N conditions and 3) quantify the amount of AMF-supplied N to plants under varying soil N conditions using 15N natural abundance techniques. Results showed that the N conservation mechanisms of N resorption and nitrogen-use efficiency were highly variable and the dominant strategy used by plants varied by species, ecotype, and site. N addition had no effect on the plant N conservation strategies measured, but soil nutrient ratios affected plant N conservation across eight restored grassland sites. N addition significantly decreased AMF abundance and function (i.e. nutrient transfer with host plant), and plant N correlated with increased AMF allocation to nutrient transfer structures within host roots. Differentiation of δ15N among plant, soil N and AMF fungal pools was higher than anticipated, leading to estimates of 34 to 100% of plant N transferred from AMF in the treatments receiving no N addition and a significant reduction in plant N transferred in high N addition treatments. When N is limited, AMF are able to supply N to plants in amounts comparable to recommended N fertilizer rates, highlighting that N fertilizer may be unnecessary in the management of perennial grasses for bioenergy production.