Effects of Different Soil Phosphorus Levels on the Physiological and Growth Characteristics of Phyllostachys edulis (Moso Bamboo) Seedlings

Soil phosphorus (P) availability is a critical factor affecting the productivity of Phyllostachys edulis (moso bamboo) forests. However, the mechanisms underlying the physiological and growth responses of moso bamboo to varying soil P conditions remain poorly understood. The aim of this study was to elucidate the adaptive mechanisms of moso bamboo to different soil P levels from the perspectives of root morphological and architectural plasticity, as well as the allocation strategies of nutrient elements and photosynthates. One-year-old potted seedlings of moso bamboo were subjected to four P addition treatments (P1: 0, P2: 25 mg·kg−1, P3: 50 mg·kg−1, P4: 100 mg·kg−1) for one year. The biomass of different seedling organs, root morphological and architectural indices, and the contents of nitrogen (N), P, and non-structural carbohydrates in the roots, stems, and leaves were measured in July and December. P addition increased the root length (by 113.8%), root surface area (by 146.5%), root average diameter (by 14.8%), root length ratio of thicker roots (diameter > 0.9 mm), number of root tips (by 31.9%), fractal dimension (by 5.6%), P accumulation (by 235.8%), and contents of starch (ST) and soluble sugars (SS), while it decreased the specific root length (by 31.7%), root branching angle (by 1.9%), root topological index (by 4.8%), root length ratio of finer roots (diameter ≤ 0.3 mm), SS/ST, and N/P. The root–shoot ratio showed a downward trend in July and an upward trend in December. Our results indicated that moso bamboo seedlings tended to form roots with a small diameter, high absorption efficiency, and minimal internal competition to adapt to soil P deficiency and carbon limitation caused by low P. Under low-P conditions, moso bamboo prioritized allocating photosynthates and P to roots, promoting the conversion of starch to soluble sugars to support root morphological and architectural plasticity and maintain root growth and physiological functions. Sole P addition eliminated the constraints of low P on moso bamboo growth and nutrient accumulation but caused imbalances in the N/P.