The karst ecosystems in southwest China are extremely fragile and vulnerable. The phenomenon of rocky desertification has become more serious, which causes karst soil to be scarcer and shallower. In addition, under the change of global rainfall pattern, the climate in SW China appears to have the trend of high temperature, low rainfall, and frequent occurrence of the continuous extreme drought events, which exacerbates the frequency and intensity of karst drought. Plants in karst regions have strong morphological plasticity and physiological regulation ability to minimize the impact of adverse environmental factors, such as shallow soil and drought. Meanwhile, in order to maximize the use of limited resources, plants in karst regions may have some ecological niche differentiation to reduce the inhibition of resource limitation on co-existence species. The nutritional characteristics play an important role in plant growth and physiological regulation, and might reveal clues about the adaptive and competitive mechanisms of plants in karst habit with shallow soil and karst drought. The gramineous perennial grass, Festuca arundinacea, is a deep-rooted plant as the root can be up to 40 cm depth in soil, while Lolium perenne, with root depth up to 25 cm, is a shallow-rooted plant. These two species are both introduced as pioneer grasses for re-vegetation in karst regions due to their high drought resistance, barren-tolerance, strong ecological adaptability, high yield and good economic benefit. They effectively improve the local ecological environment and help to alleviate rural poverty. Therefore, we chose these two species to study the effects of decline of soil thickness and/or water on the nutrient element contents, accumulation and distribution under different planting patterns. In our study, three factors, namely soil thickness (T_CK:15 cm, and T_S:5 cm), water (W_CK:120 mL/3 d and W_D:36 mL/3 d), and planting pattern (monoculture and mixture) were designed in a full three-way randomized block design. The results showed that:(1) Compared with control group (CK:T_CKW_CK), the C and N contents in aboveground part, root, and whole plant of both species increased (except in shallow soil group, S:T_SW_CK), while the P contents and their accumulations decreased in the shallow soil group (S:T_SW_CK), drought group (D:T_CKW_D), and shallow soil plus drought group (SD:T_SW_D) both in monoculture and in mixture. The root nutrient distribution ratio of F. arundinacea increased in monoculture but decreased in mixture at three low resource levels (S, D, SD). The root nutrient distribution ratio of L. perenne increased in the shallow soil group, but decreased in the drought group. (2) The C content in aboveground part, P content in root, and their accumulations of each part and root nutrient distribution ratio of F. arundinacea were significantly higher in mixture than in monoculture at control resource level (CK), while there was no-significant difference between mixture and monoculture for above traits of F. arundinacea at low resources levels. (3) At all resource levels, there was no significant difference between monoculture and mixture for all the above traits of L. perenne. The results indicated that both species had higher absorbing-resource and defense abilities by increasing C and N contents at low resource levels. In mixed planting, F. arundinacea could improve its competitiveness by adjusting element accumulation and distribution, while L. perenne might maintain a constant strategy of nutrient regulation in response to competition.