Life form-dependent nitrogen-phosphorous allocation strategies of leaf and fine root in a temperate natural forest under long-term nitrogen addition

Nitrogen deposition has increased rapidly in eastern China, which can affect the stoichiometric characteristics of plants. However, the effects of N addition on the nitrogen (N) and phosphorous (P) allocation strategies for various plant life forms (e.g. trees, shrubs, and herbs) have rarely been studied. This study evaluated the effects of N addition on N and P stoichiometry and their scaling exponents in the leaves and fine roots of the different life form groups. N and P concentrations in the leaves of shrubs and herbs increased under N addition treatments, whereas tree leaves exhibited a more stable response. In contrast, N addition has no significant effect on N and P concentrations in the fine roots of the three plant life forms. N addition accelerated the allocation of more N and P to the leaves than to the fine roots in all three life forms. Furthermore, the N-P allocation scaling exponents of the leaves and fine roots of the trees were equal to 1.0, indicating an isometric pattern. In contrast, the N-P allocation scaling exponents of shrubs and herbs were less than 1.0, indicating an allometric pattern. Although high N availability promoted shrubs and herbs to distribute P to leaves at a higher proportion than N, only slight effects were detected in tree leaves. These changes in N-P allocation patterns indicate that life forms and N addition levels work together to modulate plants allocation strategies. This suggests the importance of life form categories when evaluating N-P allocation strategies in forest plants.