Effects of Mixing Hippophae rhamnoides and Pinus tabuliformis on Ecosystem Carbon, Nitrogen, and Phosphorus Sequestration and Storage Capacity in the Loess Hilly Region, China

Abstract

Mixed forests created by incorporating nitrogen-fixing tree species offer enhanced ecological advantages compared with forests consisting of only one type of tree species. These benefits include habitat rehabilitation and the promotion of biodiversity. Nevertheless, the impact of introducing nitrogen-fixing tree species on ecosystem carbon (C), nitrogen (N), and phosphorus (P) sequestration and storage capacity in the Loess Plateau of China remains inadequately explored. To examine changes in the sequestration and storage capacity of ecosystem C, N, and P, the mixed plantations of P. tabulaeformis and H. rhamnoides (HrPt) were selected as the research object, and the pure plantations of H. rhamnoides (Hr) and P. tabulaeformis (Pt) were selected as the control. The results indicated that in comparison to the pure forest, the ecosystem in HrPt significantly increased C and N stocks but decreased P stocks. In addition, C, N, and P stocks in the soil layer accounted for more than 60% of the C, N, and P stocks in the pure and mixed forest ecosystems compared with the vegetation layer. Moreover, HrPt significantly improved ecosystem C and N sequestration rates relative to the pure forest but decreased P sequestration rates. Furthermore, the soil physicochemical properties can be inferred from the redundancy analysis showing 66.79% and 0.06% in Hr, 44.84% and 0.06% in Pt, as well as 44.28% and 0.04% in HrPt, respectively. In conclusion, compared with the pure forest, the introduction of N-fixing tree species was more conducive to the accumulation of C and N. The results offer substantial significance for the scientific guidance of vegetation restoration in degraded landscapes and the stewardship of mixed forests in the Loess Hilly Region, providing essential data for nutrient storage in ecosystems.