Abstract: A two-year field experiment was conducted to examine the effects of simulated yak and Tibetan sheep trampling (light, moderate, and heavy trampling) and simulated rainfall (low, average, and high) on sexual reproduction of Medicago ruthenica var. inschanica in a Tianzhu alpine meadow in Gansu Province. The results showed that the plant height, pod length, number of pods per square meter, seed number per pod, 1000-grain weight and seed yield of M. ruthenica decreased with the increases of trampling intensity and the decreases of rainfall. Under low rainfall level, heavy trampling decreased the pod number per square meter by >70% than un-trampling. The 1000-grain weight ofM. ruthenicaseeds under trampling was 13% lower than that of un-trampling. Under low rainfall level, the seed yield of M. ruthenicaunder light, moderate, and heavy trampling of Tibetan sheep was 60.1%, 13.8% and 56.7% higher than that under corresponding intensity of yak trampling, respectively. Rainfall, trampling and their interaction showed significant effects on the height ofM. ruthenica. Moreover, rainfall and trampling significantly affected the sexual reproduction indices of M. ruthenica; however, their interaction effect was not statistically significant. Overall, the negative effect of yak trampling on M. ruthenica sexualreproduction was higher than that of Tibetan sheep under same trampling intensity. The sexual reproduction of M. ruthenicawas inhibited by low rainfall level and heavy trampling.

Key words: biodiversity., microbial ecology, soil acidification, nutrient availability, nitrogen deposition