Soil stoichiometry and carbon storage in long-term afforestation soil affected by understory vegetation diversity

• Soil stoichiometry positively related on understory vegetation diversity.
• Afforestation significantly influences SOC and TN stocks in deep soil.
• Long-term afforestation could greatly affect soil RCN, RCP, and RNP ratios.

The afforestation of abandoned land could offer opportunities to sequester soil organic carbon (SOC), promote nutriment elements cycling, improve plant diversity in the plantation understory and provide ecosystem services. The objectives of this study were to identify plant diversity in the plantation understory, quantify the changes in SOC and total nitrogen (TN) storage in deep soil, assess the SOC, TN, and total phosphorus (TP) stoichiometries, and investigate their relationships in the Loess Plateau Region (LPR) undergoing long-term afforestation. Soil samples were collected at a soil depth of 0–200 cm under 30-yr old Robinia pseudoacacia L. and adjacent abandoned sites, and SOC, TN and TP were determined in different soil depth. Additionally, plant composition and diversity in the plantation understory were evaluated. The results showed that land subjected to long-term afforestation had greater plant coverage, plant density, richness index (R) and Shannon–Wiener diversity (H) compared to abandoned land communities (P < 0.05). SOC, TN and TP contents in afforested sites were significantly increased in surface soil (0–30 cm) as well as in the underlying soil (100 cm) compared to the corresponding abandoned land sites (P < 0.05) in most cases. Meanwhile, SOC, TN, and TP stoichiometry in afforested areas were higher than those of abandoned lands and significantly related to understory vegetation diversity (P < 0.05). In addition, lands subjected to long-term afforestation effectively increased SOC and TN storages compared to abandoned land at soil depths of 0–30 cm and 100–200 cm and were also significantly related to understory vegetation diversity (P < 0.05). These findings demonstrating that afforestation not only affects SOC and TN stocks in surface soil, but also strongly influences that in deep soil. And it is also indicating that long-term afforestation could greatly affect soil RCN, RCP, and RNP ratios.