The Responses of C Allocation of New Needle and Fine Root Affected the Phosphorus Adaptation of Pinus massoniana Seedlings

Carbon (C) allocation plays a vital role in enabling plants to effectively cope with limited phosphorus (P) availability, especially in P-deficient subtropic. However, in what pattern did C allocated to different plant organs along a P gradient is still unclear. This study conducted a nutrient manipulation experiment on Pinus massoniana seedlings, an endemic species in subtropical China, to explore the variations in non-structural carbohydrate (NSC) fractions of needles and roots along a fact-based P gradient of 0–9.299 mg·kg−1 (0–4 times of average soil available P content in the major distribution area of P. massoniana). P promoted the relative growth rate of P. massoniana seedlings. Biomass showed no difference along the P gradient, but biomass proportion of fine root decreased with P addition. The C concentration in new needle increased with increasing substrate P content, but old needle showed an reverse trend. Old needle remained major C pool in low-P conditions, but more C would be allocated to new needle with P addition. Higher C was found in fine root in low-P groups. The NSC concentration in needles rose with P addition, but decreased in roots. NSC existed more in the form of soluble sugar in needles in low-P conditions, but more in the form of starch in fine root. In low-P conditions, P. massoniana seedlings regulated growth by preferentially allocating C to fine root and old needle, promoting absorption and maintaining a stable C pool, while exhibiting distinct NSC fraction patterns in fine root (starch) and needles (soluble sugar).