Abstract:To identify the adaptation strategies of plants in semi-arid areas to periodical changes of water and nutrient supply, a pot experiment was conducted to investigate plant growth, root morphological characteristics, N and P concentration under two soil moisture conditions (well-watered and post-drought rehydration) and two phosphorus (P) addition levels (0 and 0.2 g P2O5 per kg dry soil) of Old World bluestems at its jointing stage. Results showed that under re-watering after drought, effect of P addition on shoot and root biomass, total biomass, and root/shoot ratio of Old World bluestems have no significant difference, while total root length and root surface area increase 27.1% and 24.1%, specific root length and specific root area increase 18.3% and 15.9%, and root average diameter decrease 1.3%. P concentration in shoot, root and whole plant of Old World bluestems significantly increased 61.1%, 35.8% and 49.6%, respectively, and P accumulation significantly increased 68.6%, 52.0% and 61.3%, respectively, and, N:P ratio decreased significantly. In addition to N accumulation in shoot, N and P accumulation in shoot, root, and whole plant of Old World bluestems under different water and phosphorus treatments showed significant positive correlation with total root length and root surface area. The present study suggested that increases in root length and root surface area were the main adaptation strategies of Old World bluestems in response to periodical changes of water supply and phosphorus addition.