Regional Variation in Vegetation Biomass and CSDLv2-Derived Soil C:N:P Stoichiometry Across Chinese Fir Plantations of Different Ages: An Exploratory Comparison

Regional variations in vegetation biomass and soil carbon (C), nitrogen (N) and phosphorus (P) stocks across different age classes of subtropical plantations remain incompletely characterized. This study examined variation in vegetation biomass, combined vegetation–surface–soil C stocks, soil C, N, and P stocks, and soil ecological stoichiometry across 65 Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.) plantation plots in Hunan Province, China. The plantations were classified into five age classes: young (0–9 years), middle-aged (10–19 years), near-mature (20–24 years), mature (25–30 years), and over-mature (>30 years). Plot-level vegetation measurements were combined with spatially modeled soil properties for the upper 30 cm extracted from the China Soil Dataset for Land Surface Modeling version 2 (CSDLv2; 90 m resolution). Vegetation biomass and C stocks varied among age classes, with generally higher values in older plantations but a nonmonotonic pattern. Combined vegetation–surface–soil C stocks showed numerical variation among age classes, but the differences were not statistically significant. Soil accounted for the majority of the estimated combined C stocks, contributing approximately 76–89%, whereas vegetation contributed approximately 11–24%. Soil C, total N, and total P stocks, as well as soil C:N:P ratios, also varied numerically among age classes, but these differences were not statistically significant. These findings provide an exploratory regional comparison of vegetation biomass and C stocks alongside CSDLv2-derived soil attributes across Chinese fir plantations of different ages. Because the vegetation and soil data differ in source and spatial resolution, the combined C-stock estimates should be interpreted cautiously and do not establish age-related changes in ecosystem C partitioning or causal relationships between vegetation development and soil nutrient dynamics.