Little is known about how the cycling of carbon (C) and fertilizer nitrogen (N) between crop tissues and soil pools is expressed in perennial grains such as intermediate wheatgrass (IWG, Thinopyrum intermedium (Host) Barkworth and Dewey), and whether these processes change within a growing season and across IWG stand age. To evaluate these ideas, we established three studies to quantify 1) root growth and decomposition, 2) C uptake and partitioning to crop-microbial-soil pools, and 3) N sources and N conservation in first-year IWG (IWG-1), second-year IWG (IWG-2), third-year IWG (IWG-3) and annual wheat (Wheat). These studies illuminated age-related changes in IWG root growth and decomposition, and C uptake and partitioning. In the 0 – 15 cm depth interval, IWG-1 new root growth was 1.7 times greater than that of IWG-2, and IWG-1 (14 – 17%) retained significantly more new C in roots than IWG-2 (6%) at the time of peak new C recovery. Conversely, IWG-2 root decomposition and utilization of new C by saprotrophic fungi was significantly greater than that of IWG-1, with the proportion of new soil microbial biomass C recovered from saprotrophic fungi increasing from 30% to 40% between IWG-1 and IWG-2. Together, these results indicated IWG transitioned from a system dominated by belowground C inputs and new root growth to one dominated by the loss of belowground C via root decomposition and heterotrophic decomposition across the first two years following establishment, in line with a system transitioning from an acquisitive to conservative growth strategy. The transition to a system dominated by the decomposition of belowground root inputs suggests IWG may develop a greater capacity for recycling N from root tissues to inorganic soil N pools, thereby lessening the need for external fertilizer N inputs. However, the proportion of tissue N sourced from fertilizer increased between the first and third IWG production year, suggesting reliance on fertilizer N increases with stand age. Optimizing IWG N management recommendations to adequately meet IWG N demands will be critical to reducing reliance on external fertilizer inputs and ensuring IWG remains a profitable and sustainable alternative to annual grains in agricultural landscapes.