Understanding nutrient allocation based on leaf nitrogen isotopes and elemental ratios in the karst region of Southwest China

It is important to understand nutrient-use-efficiency based on the interaction between nutrient allocation and plant growth. This study utilized mulberry (Monts spp.), an economically important plant for farmers, as an example, to investigate the effect of nutrient allocation on plant growth and its response to environmental conditions in the karst region of Southwest China. Leaf stoichiometric analysis and isotopic measurements were applied to reveal the causes and effects of the allocation of different nutrients. The results illustrated that the N:P ratios of mulberry leaves decreased as the mean temperature increased and plant tissues grew, exhibiting a high leaf N:P ratio (21.82 +/- 6.33) at the early stage of leaf expansion, and a low ratio (10.70 +/- 2.66) at the late stage of leaf development. The low leaf N:P ratios indicated that leaf development was largely limited by nitrogen availability as the plant grew. The slight difference of delta N-15 between leaf and root (leaf-root delta N-15, -0.30 parts per thousand) implied that the mulberry preferred ammonium-N at the early stage of leaf development, while the large difference of leaf-root delta N-15 (1.58 parts per thousand) implied that the mulberry preferred nitrate-N at the late stage. Although the ammonium uptake could improve P availability by increasing soil pH value deficiency at the early stage of leaf expansion, plant growth was constrained by P availability, given the relatively high P uptake from soils and the large P allocation in leaves. Moreover, the allocations of N and P were strongly correlated with other nutrients allocations. P exhibited strongly positive effects on Ca and K allocations under P deficiency at the early stage of leaf expansion, while N presented important impacts on them under N deficiency at the late stage. To summarize, this study highlighted the interactions between allocations of different nutrient components which are related to plant growth and environmental conditions in the karst system.