Soil C:N:P stoichiometry of typical coniferous (Cunninghamia lanceolata) and/or evergreen broadleaved (Phoebe bournei) plantations in south China

Chinese fir (Cunninghamia lanceolata), a fast-growing conifer, has been widely planted in south China and is commonly managed in successive monocultures, resulting in severe ecosystem degradation. To recover the degraded lands, forest managers are now replacing C. lanceolata monocultures with slow-growing broadleaved trees, including Phoebe bournei. However, whether P. bournei prevents soil degradation and improves soil fertility are poorly known. In this research, we studied the effects of monoculture and mixed culture of C. lanceolata x P. bournei (experiment 1) and the effects of P. bournei stand age (monoculture and early, middle, and late stages) (experiment 2) on soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), and dissolved organic matter (i.e., DOC, DON, and DOP), and C:N:P stoichiometry at different soil depths in different seasons. In experiment 1, we found that SOC, TN, and TP at 0-30 cm depth or at one or two of the soil depths (i.e. 0-10, 10-20, and 20-30 cm) were significantly higher in the P. bournei monoculture and the C. lanceolata x P. bournei mixed culture than in the C. lanceolata monoculture. The C:N ratio at 10-20 cm depth and the C:P and N:P ratios at 0-30 cm depth were significantly higher in the P. bournei monoculture than in the C. lanceolata monoculture. The C:P and N:P ratios at 0-30 cm depth were significantly higher in the C. lanceolata x P. bournei mixed culture than in the C. lanceolata monoculture. Among the three kinds of plantations in experiment 1, soil DOC and DON were generally lowest in the C. lanceolata monoculture. In experiment 2, SOC and TN at 0-45 cm soil depth increased but TP decreased with the stage (age) of the P. bournei forest. C:N, C:P, and N:P ratios were often higher in the late or middle stages than in the early stage or in the P. bournei monoculture. DON at 0-45 cm depth was lower in the late stage than in the other three stages of P. bournei forests. The results indicate that the P. bournei plantation maintained higher soil C, N, and P levels than the C. lanceolata plantation, suggesting that forest conversion from C. lanceolata to P. bournei monoculture or to C. lanceolata x P. bournei mixed culture should help prevent soil degradation in south China. Although soil C sequestration increases with the age of P. bournei stands, limitation of forest growth by N and P also increases with the age of P. bournei stands. These results increase our understanding of soil C:N:P stoichiometry in subtropical forest ecosystems and offer insight into subtropical forest management.

Automated summary

The Chinese fir plantation maintained higher soil C, N, and P levels than the Cunninghamia lanceolata plantation, suggesting that forest conversion from Cunninghamia lanceolata to Phoebe bournei monoculture or to Cunninghamia lanceolata x Phoebe bournei mixed culture should help prevent soil degradation in south China. Soil C sequestration increases with the age of Phoebe bournei stands, but limitation of forest growth by N and P also increases with the age of Phoebe bournei stands. These results enhance our understanding of soil C:N:P stoichiometry in subtropical forest ecosystems and provide insights into subtropical forest management.