Ecosystem C:N:P Stoichiometry and Carbon Stocks Along a Chronosequence of Malus pumila Orchards in North China

Understanding the dynamics of carbon (C), nitrogen (N), and phosphorus (P) stoichiometry and C stocks along a stand development chronosequence has been extensively studied in forest ecosystems. However, despite the global economic and ecological importance of apple orchards, such knowledge remains limited for these intensively managed perennial agroecosystems. We examined C, N, and P concentrations and stoichiometric ratios in tree tissues (root, stem, branch, foliage) and soils (0–100 cm depth), as well as ecosystem C stocks, across a chronosequence of 4, 8, 12, and 16 yr old Malus pumila orchards in the eastern Yan Mountains, Hebei Province, North China. The results showed that C concentrations exhibited no consistent age-dependent trend in tree tissues. In contrast, N and P concentrations in all tree tissues decreased significantly with stand age, while their C:N and C:P ratios increased. The leaf N:P ratios suggested progressive P limitation as orchards aged. In soil, C, N, and P concentrations first decreased and then increased along the chronosequence, with the highest values observed in the 16 yr stands. This U-shaped trajectory reflected the dynamic interplay between stand development and anthropogenic management. Intercropping and intensive fertilization in the 4 yr orchards initially elevated soil nutrient levels, while the cessation of intercropping and nutrient removal via fruit harvesting in the 8 yr stands led to a decline. Thereafter, accumulation of litter decomposition and root turnover, combined with continued organic matter inputs, progressively replenished soil nutrient pools in the 12 and 16 yr stands. The total ecosystem C stocks ranged from 70.80 to 136.13 Mg ha−1, initially declining from 4 to 8 years and then increasing at 12 and 16 years, with soil contributing 84.7–99.7% of the total. Plant and soil nutrient concentrations showed predominantly negative correlations, indicating weak coupling between tree and soil nutrient pools. Our findings demonstrated that stand age profoundly influenced C:N:P stoichiometry and C stocks in apple orchard ecosystems and that prolonged orchard development enhanced both tree biomass C and soil C stocks. These results provide a scientific basis for nutrient optimization and sustainable management of apple orchards in temperate regions.