Soil Carbon, Nitrogen, and Phosphorus Storages and Their Stoichiometry Due to Mixed Afforestation with Hippophae rhamnoides in the Loess Hilly Region, China

Mixed-species tree plantations have additional ecological benefits over single-species tree plantations, such as habitat restoration and increasing biodiversity. However, changes in the soil carbon, nitrogen, and phosphorus storages and stoichiometry after mixed afforestation with the N-fixing tree species under the "Grain for Green Project" in the Loess Plateau of China are not well understood. Typical restoration types, including the mixed plantations of Pinus tabuliformis with Hippophae rhamnoides (HrPt) and Robinia pseudoacacia with H. rhamnoides (HrRp), as well as the pure forests of P. tabuliformis (P-t) and R. pseudoacacia (R-p), were chosen to examine changes in the storages and stoichiometry of soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP) in 0-100 cm soil layers. The results showed that compared with the corresponding pure forest, HrRp significantly increased the SOC content in the 0-20 cm soil layer and the SOC storage in the 0-100 cm layer, while HrPt significantly increased the SOC content in the 0-10 cm layer, but there was no significant difference for SOC storage in the 0-100 cm layer between P-t and HrPt. Similarly, HrRp significantly increased the TN content in the 0-10 cm layer and the TN storage in the 0-100 cm layer, but there was no significant difference in TN storage between P-t and HrPt. Furthermore, HrRp significantly increased the TP content in the 0-100 cm layer and TP storage was higher than that of R-p, while there were no significant differences in TP content and storage between P-t and HrPt. In the 0-10 cm soil layer, HrRp significantly reduced C:N and increased N:P, but HrPt significantly increased C:P. In addition, compared with the pure forest, the soil physical and chemical properties had a stronger control effect on the soil storages and stoichiometric ratios in the mixed forests. In summary, compared with P. tabuliformis, the introduction of N-fixing tree species into the R. pseudoacacia forest was more conducive to the accumulation of SOC, TN, and TP reserves and the improvement of the N and P utilization efficiency. These results have important implications for the restoration of degraded vegetation and scientific management of mixed plantations on the Loess Plateau and can provide basic data for the assessment of soil quality at the regional scale.