AbstractRoot morphology and production are important for soil nutrient acquisition and C sequestration, but these traits are poorly understood in the bioenergy crop elephantgrass [Pennisetum purpureum (L.) Schum.]. Our objective was to characterize root traits of elephantgrass receiving different nutrient management practices in comparison with bahiagrass (Paspalum notatum Flüggé) pasture grown in the southeastern U.S. Treatments were bahiagrass + 50 kg N ha−1 (BHG), and elephantgrass receiving either: 50 kg N ha−1 (E50), 50 kg N ha−1 + biochar (E50BC), 50 kg N ha−1 + lignocellulosic fermentation residual (E50FR), or 250 kg N ha−1 (E250). Roots were sampled annually for 4 yr (2013‐2016). Root C and N concentration were measured at termination (2016) of the study. Both crop species exhibited similar root length density (RLD) and root mass density (RMD) across all depths in 2014 and 2015. BHG root diameter was 55% greater than all elephantgrass treatments. By 2016, E50FR increased elephantgrass RLD and RMD in the shallow soil layers (< 0.2 m). Root N content was 15–39% lower for all elephantgrass treatments than BHG in the 0‐0.1 m depth, and 22–25% lower for E50 and E50BC in the 0.1‐ to 0.2‐m depth compared with BHG. Additionally, roots C content was 6% higher in the 0‐0.1 m compared with the 0.1‐ to 0.2‐m soil depth irrespective of treatment. Application of biochar and lignocellulosic fermentation residual as amendments produced a stimulatory effect on elephantgrass root growth in soil shallow layers, which could affect nutrient and water acquisition.