Soil organic carbon stock and chemical composition in four plantations of indigenous tree species in subtropical China

Subtropical China has more than 60% of the total plantation area in China and over 70% of these subtropical plantations are composed of pure coniferous species. In view of low ecosystem services and ecological instability of pure coniferous plantations, indigenous broadleaf plantations are being advocated as a prospective silvicultural management for substituting in place of large coniferous plantations in subtropical China. However, little information is known about the effects of tree species conversion on stock and stability of soil organic carbon (SOC). The four adjacent monospecific plantations were selected to examine the effects of tree species on the stock and chemical composition of SOC using elemental analysis and solid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy. One coniferous plantation was composed of Pinus massoniana (PM), and the three broadleaf plantations were Castanopsis hystrix (CH), Michelia macclurei (MM), and Mytilaria laosensis (ML). We found that SOC stock differed significantly among the four plantations in the upper (0–10 cm) layer, but not in the underneath (10–30 cm) layer. SOC stocks in the upper (0–10 cm) layer were 11, 19, and 18% higher in the CH, MM, and ML plantations than in the PM plantation. The differences in SOC stock among the four plantations were largely attributed to fine root rather than aboveground litterfall input. However, the soils in the broadleaf plantations contained more decomposable C proportion, indicated by lower percentage of alkyl C, higher percentage of O-alkyl C and lower alkyl C/O-alkyl C ratio compared to those in the PM plantation. Our findings highlight that future strategy of tree species selection for substituting in place of large coniferous plantations in subtropical China needs to consider the potential effects of tree species on the chemical composition in addition to the quantity of SOC stock.