Abstract: Switchgrass (Panicum virgatum) is a key bioenergy crop, valued in the bioenergy sector for its high biomass production and effective carbon (C) and nitrogen (N) fixation in soils. This study aims to examine the effects of N and phosphorus (P) addition on the ecological stoichiometry of litter and soil of energy plant switchgrass. The study compares the C, N, and P content and their stoichiometric ratios in the litter and soil of upland and lowland switchgrass with various N and P treatments, revealing the effects of ecotype and nutrient addition interactions on soil nutrient cycling, and plant nutrient uptake strategies. The results show that: (1) Under the N60 treatment, the total P content in the leaves of lowland switchgrass was 20.39% lower than that of upland, whereas under the N120 treatment, it was 25.03% higher, indicating that lowland switchgrass has a significantly greater P absorption capacity with N addition compared to upland switchgrass. (2) Under the N60 treatment, the leaf N∶P ratio for the P50 treatment was significantly lower than the control by 27.98%. In addition, the branch C∶N ratio for the N60 and N120 treatments were significantly lower than the control by 26.41% and 27.74%, respectively. The simultaneous addition of N and P significantly promoted the decomposition of litter and potentially increase the availability of N and P in the soil, greatly improving the nutrient exchange between plants and soil. (3) The biomass of lowland switchgrass under N60 and N120 treatments was significantly higher than the control, increasing by 31.07% and 29.39%, respectively, indicating enhanced growth performance and biomass accumulation with N supplementation. In summary, nutrient addition strategies that take into account the characteristics of different ecotypes are essential for optimizing the growth performance and ecological service functions of switchgrass.