Optimal partitioning theory (OPT) suggests that plants should allocate relatively more biomass to the organs that acquire the most limited resources.The assumption of this theory is that plants trade off the biomass allocation between leaves,stems and roots.However,variations in biomass allocation among plant parts can also occur as a plant grows in size.As an altemative approach,allometric biomass partitioning theory (APT) asserts that plants should trade off their biomass between roots,stems and leaves.This approach can minimize bias when comparing biomass allocation pattems by accounting for plant size in the analysis.We analyzed the biomass allocation strategy of perennial Pennisetum centrasiaticum Tzvel in the Horqin Sand Land of northem China by treating samples with different availabilities of soil nutrients and water,adding snow in winter and water in summer.We hypothesized that P.centrasiaticum alters its pattern of biomass allocation strategy in response to different levels of soil water content and soil nitrogen content.We used standardized major axis (SMA) to analyze the allometric relationship (slope) and intercept between biomass traits (root,stem,leaf and total biomass) of nitrogen/water treatments.Taking plant size into consideration,no allometric relationships between different organs were significantly affected by differing soil water and soil nitrogen levels,while the biomass allocation strategy of P.centrasiaticum was affected by soil water levels,but not by soil nitrogen levels.The plasticity of roots,leaves and root/shoot ratios was ‘true in response to fluctuations in soil water content,but the plasticity of stems was consistent for trade-offs between the effects of water and plant size.Plants allocated relatively more biomass to roots and less to leaves when snow was added in winter.A similar trend was observed when water was added in summer.The plasticity of roots,stems and leaves was a function of plant size,and remained unchanged in response to different soil nitrogen levels.