Imbalance in nitrogen and phosphorus allocation between tree roots and leaves induced by nitrogen addition.

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The allocation of limiting elements, such as nitrogen (N) and phosphorus (P), in plant organs is essential for nutrient cycling between soil and plants (soil-plant nutrient cycling) and functional optimization in plant communities. Unprecedented inputs of anthropogenic N have caused drastic N and P imbalances in terrestrial ecosystems. However, the effects of N addition on the allocation strategies of N and P between plant organs remain unclear. In this study, we conducted a long-term, multilevel N addition experiment to investigate the allocation strategies for N and P in plant leaves and fine roots. We found that N addition significantly increased leaf N concentration, leaf P concentration, and leaf N:P ratios, while significantly decreasing fine root N concentration, fine root P concentration, and fine root N:P ratios. Additionally, we demonstrated a higher proportional increase of N in leaves and a lower proportional decrease of P in fine roots with N addition. Furthermore, our analyses revealed that N addition influenced the allocation of N and P between plant leaves and fine roots through changes in plant growth patterns and nutrient distribution strategies. These changes were driven by a significant increase in soil inorganic N concentration, a decrease in soil N cycling and a reduction in mycorrhizal symbiosis. Our findings suggest that N addition will likely lead to an imbalance between the N and P cycles in temperate forest ecosystems, due to the unequal allocation of N and P between tree roots and leaves. This imbalance may, in turn, have negative implications for the provision of ecosystem services.