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

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.