Stability of C:N:P Stoichiometry in the Plant–Soil Continuum along Age Classes in Natural Pinus tabuliformis Carr. Forests of the Eastern Loess Plateau, China

Ecological stoichiometry is useful for revealing the biogeochemical characteristics of flows of nutrients and energy between plant and soil, as well as the important implications behind these ecological phenomena. However, the ecological stoichiometric linkages among leaf, litter, soil, and enzymes in the natural forests of the Loess Plateau remain largely unknown. Here, leaf, litter, and soil samples were collected from four age classes of natural Pinus tabuliformis Carr. (P. tabuliformis) to explore the deep linkages among these components. We measured the total carbon (C), total nitrogen (N), and total phosphorus (P) concentrations of leaf and litter, as well as the concentrations of soil organic C, total N, total P, nitrate N, ammonium N, available P, and the activities of β-1,4-glucosidase (a C-acquiring enzyme), β-1,4-N-acetylglucosidase (an N-acquiring enzyme), and alkaline phosphatase (a P-acquiring enzyme) in the topsoil (0–20 cm). The average leaf N:P was 6.9 indicated the growth of P. tabuliformis was constrained by N according to the relative resorption theory of nutrient limitation. The C:N, C:P, and N:P ratios in leaf, litter, and soil and the enzyme activity were not significantly different among age classes (p > 0.05). Litter C:N (43.3) was closer to the ratio of leaf C:N (48.8), whereas the litter C:P (257.7) was obviously lower than the ratio of leaf C:P (338.15). We calculated the stoichiometric homeostasis index (1/H) of leaf responses to soil elements and enzyme activities and found that the relationship between leaf C:P and soil C:P was homeostatic (p < 0.05), whereas the remaining indices showed the leaf stoichiometries were strictly homeostatic (p > 0.05). Correlation analysis showed both litter C:P and N:P were positively correlated with leaf and soil C:P, while the stoichiometric ratios of soil elements and enzymes were obviously irrelevant with leaf stoichiometries (p > 0.05). Partial least squares path modeling indicated that litter significantly changed soil element and enzyme characteristics through direct and indirect effects, respectively. However, soil elements and enzymes impacted leaf stoichiometries barely, which was further confirmed by an overall redundancy analysis. In summary, C:N:P stoichiometry within the plant–soil continuum revealed that natural P. tabuliformis is a relatively stable ecosystem in the Loess Plateau, where the element exchanges between plant and soil maintain dynamic balance with forest development. Further studies are needed to capture the critical factors that regulate leaf stoichiometry in the soil system.