Divergent Effects of Monoculture and Mixed Plantation on the Trade-Off Between Soil Carbon and Phosphorus Contents in a Degraded Hilly Land

Carbon (C) and phosphorus (P) in soil are closely related to plantation types in afforestation practices. However, the trade-off between soil C and P in response to different restoration models on degraded hilly land is still not clear. In this study, four restoration patterns, including natural recovered shrubland (NS), Castanopsis hystrix plantation (CH), 10-species mixed plantation (10MX), and 30-species mixed plantation (30MX) were selected, and the physicochemical properties and readily oxidized carbon (ROC) in different layers of 1 m depth soil were measured to understand the effects of natural restoration and artificial afforestation on soil P and C pool and their trade-off on degraded hilly land in southern China. The results indicate that the total P (TP) content in each soil layer was observed to follow the order of CH > 10MX > 30MX > NS, with monoculture (CH) exhibiting higher levels of TP than mixed plantation. However, the soil C storage of NS (59.61 t hm−2) and 30MX (57.71 t hm−2) was similar, while 10MX boasted the highest C storage (64.99 t hm−2) of the four restoration patterns, with CH being the lowest (42.75 t hm−2). In deep soil layers (20–100 cm), the 10MX plantation presented the highest for both the C pool index (CPI) and C pool management index (CMI). Moreover, the structural equation model (SEM) revealed that the soil CMI was directly regulated by the levels of soil available P and total N, while soil C pool activity was directly influenced by soil pH. Thus, our study suggests that compared to mixed plantations, the monoculture plantation (CH) demonstrates lower P uptake and utilization, resulting in a higher soil P content. Furthermore, 10MX plantation showed a superior C fixation capacity over those with 30MX and monoculture plantations. These suggests that the trade-off between soil C and P contents was commonly observed among different plantation restoration patterns. Therefore, afforestation with different tree composition and nutrient regulation is necessary for maintaining the balance between soil C and P and keeping the sustainability of plantation management in the degraded hilly lands.