Vertical and structural-phase controls on soil C:N:P stoichiometry in an old-growth Hyrcanian beech forest

Soil C:N:P relationships are widely used indicators of nutrient balance, organic matter quality, and biogeochemical functioning in forest ecosystems. However, their organization across structural phases and soil horizons in undisturbed temperate forests remains insufficiently documented. This study examined vertical and structural-phase variation in soil C:N:P stoichiometry within an old-growth Fagus orientalis forest the Hyrcanian region. We quantified C:N, C:P, and N:P ratios in paired organic and mineral soil samples, with stoichiometric relationships interpreted based on total C and N and Olsen-extractable (plant-available) P, across the initial, optimal, and decay structural phases of this contiguous old-growth stand. Within this forest, C:N and C:P ratios showed phase-associated differences in the organic horizon, generally declining from the initial to the decay phase, whereas N:P remained comparatively stable. In the mineral horizon, phase-associated differences were also observed for C:N and C:P; however, the magnitude of variation was smaller, and patterns were less consistent than in the organic layer. Strong vertical contrasts were observed, with consistently higher C:N and C:P ratios in the organic layer and comparatively conservative N:P patterns across horizons. Overall, vertical differentiation between soil horizons represented a stronger axis of variation than structural-phase differentiation within this old-growth system. These findings provide a high-resolution stoichiometric characterization of a long-protected Hyrcanian beech forest and may serve as a baseline for future comparisons in similare temperate beech forest ecosystems under low-disturbance conditions.