Response of carbon allocation strategies of psammophytes in southeastern Tibet to nitrogen and phosphorus additions

This study investigated the adaptive carbon allocation strategies of Hippophae rhamnoides L. subsp. yunnanensis Rousi, a psammophyte endemic to southeast Tibet, in response to experimental nitrogen (N) and phosphorus (P) enrichment. Five treatments were applied under controlled conditions: control (CK), low N addition (15 kg·hm⁻²·a⁻¹, N₁), high N addition (20 kg·hm⁻²·a⁻¹, N₂), low P addition (75 kg·hm⁻²·a⁻¹, P₁), and high P addition (80 kg·hm⁻²·a⁻¹, P₂). Results showed that: (1) Low nitrogen input enhanced the root-to-shoot ratio, branch starch content, and soluble sugar-to-starch ratio, reflecting preferential allocation of nonstructural carbohydrates (NSCs) to belowground organs, whereas high nitrogen treatment led to opposite trends; (2) Phosphorus enrichment reduced specific leaf area and the root-to-shoot ratio but increased soluble sugar and total NSC concentrations, reflecting preferential allocation of NSCs to aboveground tissues, while starch remained concentrated in belowground parts; (3) Both nitrogen and phosphorus treatments significantly elevated total NSC and structural carbohydrate (SC) pools at the whole-plant level. These findings indicate that nitrogen and phosphorus supplementation promotes the growth of H. rhamnoides, with carbon partitioning patterns modulated by nutrient type and dosage. Low nitrogen addition increased NSC accumulation in the root, whereas elevated nitrogen mimicked the effects of phosphorus addition, promoting carbon allocation toward aboveground tissues.