Effects of nitrogen forms on adaptive strategies of Moso bamboo seedlings under low-phosphorus conditions

Abstract

Soil phosphorus loss frequently occurs in subtropical regions, leading to extremely low phosphorus levels in forest soils. This deficiency adversely affects soil fertility and hampers normal plant growth. Nitrogen fertilization can enhance phosphorus uptake under certain conditions. However, further research is needed to determine whether N fertilization under low-phosphorus conditions can improve P use efficiency, and to identify the most effective N form to achieve this purpose. This study explored the relationship between soil nutrient dynamics and seedling physiology, evaluating the effects of various nitrogen fertilizers on the growth of Phyllostachys edulis seedlings under low phosphorus conditions. A two-year pot experiment was conducted using four different forms of nitrogen (NO₃^--N, NH₄⁺-N, NO₃^--N + NH₄⁺-N, and organic-N) under two phosphorus treatment levels (deficient: 5.0 mg kg⁻¹ and sufficient: 20.0 mg kg⁻¹). Low phosphorus levels altered the nutrient environment of substrates and inhibited bamboo seedling growth, while increasing phosphorus distribution in the plant. Low phosphorus stress inhibited growth and altered amino acid composition. However, seedlings adapted through adjustments to the root-to-shoot ratio, modifications to root morphology, and coordination of the transport and distribution of nitrogen and phosphorus among roots, stems and leaves. This enhanced leaf electron transfer efficiency and redistributed nutrient resources. Furthermore, NH₄⁺-N fertilizer under low phosphorus conditions increased the root-to-shoot ratio, enhanced phosphorus and metal ion uptake by roots, improved nitrogen distribution and alleviated the inhibitory effects of low phosphorus stress. Org-N fertilizer demonstrated a distinct effect under low phosphorus conditions, particularly in enhancing nitrogen retention and improving root development. The use of Org-N resulted in increased amino acid content in bamboo leaves, promoting better photosynthetic efficiency compared to inorganic nitrogen treatments. However, its effects on phosphorus use efficiency were less pronounced than those observed with NH₄⁺-N. Conversely, a combined NO₃^--N and NH₄⁺-N fertilizer under sufficient phosphorus conditions improved photosynthetic capacity, nitrogen transport and metabolic efficiency, resulting in significantly higher total biomass. Therefore, nitrogen fertilizer forms should be carefully considered in bamboo seedling cultivation. A single ammonium nitrogen fertilizer is more suitable for bamboo seedlings in soils with severely low phosphorus, whereas a mixed inorganic nitrogen fertilizer is better for soils with adequate phosphorus. This study uniquely demonstrates how specific nitrogen forms can enhance phosphorus utilization efficiency in Moso bamboo seedlings under low phosphorus stress, providing critical insights for optimizing nutrient management in subtropical bamboo plantations.