Plants growing in the subtropical karst region of China are frequently subjected to periodical drought stress within a growing season. The available soil water is sufficient for plant transpiration needs for only 7-14 days following heavy rainfall exceeding soil field capacity, because of the shallow soils and highly porous limestone in this region. However, frequent precipitation in the form of extreme rainfall events allows the soils to remain wet before or after drought. Therefore, "karst drought" and strong fluctuations in water availability are becoming important selection pressure for vegetation growth and distribution in the karst region. In recent years, the concentration of atmospheric nitrogen compounds has been increasing sharply owing to human activities. Nitrogen deposition in a global context also has unpredictable impact on the ecological system in karst region. Thus,simultaneously exploring the response of plants to frequent alternation of drying and wetting and nitrogen deposition and their interaction in vulnerable karst vegetation is necessary; we used two Asteraceae annual herbs, Xanthium sibiricum and Bidens pilosa as the experimental materials. These two species were selected since they are native species,are widely distributed in the southwest karst area of China, and have a strong ability to adapt to the abiotic environment and to tolerate dry and barren conditions. We manipulated three factors (species, water treatment and nitrogen addition) in a factorial randomized design to determine how the two species respond to the different water treatments and nitrogen addition in terms of above- and belowground growth and biomass accumulation. The following results were obtained.Plant growth was markedly restrained by drought stress. Stem length, leaf area, total root length, root volume and aboveground biomass in both the species were significantly lower than thoes of the control whose soil water content was kept in 80%-90% of field capacity. However, the root to shoot ratio of both the species increased significantly. Re-watering after different periods of drought somewhat compensated for the decrease in the growth and biomass accumulation of the plants. However,the compensation effect was related to the drought period, and decreased with prolongation of drought. Nitrogen addition promoted plant growth and singnificantly increased the total and above- and belowground biomass. However, the stimulatory effect of nitrogen addition was weakened with increasing drought duration, which was probably related to the decreased soil moisture. Nitrogen addition also significantly affected the biomass allocation:it increased the root to shoot ratio and stimultaneously stimulated the above- and belowground growth of plants.