Effects of tree species mixture on soil organic carbon stocks and greenhouse gas fluxes in subtropical plantations in China

Indigenous broadleaf plantations are increasingly being developed as a prospective silvicultural approach for substituting coniferous plantations in subtropical China. Three plantations of monoculture and mixed Pinus massoniana and Castanopsis hystrix were selected to examine soil organic carbon (SOC) stocks and temporal and spatial patterns of the main greenhouse gases fluxes for understanding the effects of mixed forests on soil carbon and nitrogen (N) cycling processes. We found that SOC stock in 0–20 cm layer in the mixed plantation was 14.3% higher than that in the P. massoniana, and 8.1% higher than that in the C. hystrix plantations. Differences in SOC stock among the plantations were attributed to soil N stock and leaf litterfall input. Soil CO2 and N2O fluxes in the mixed plantation displayed the seasonal trends, while soil CH4 flux did not show the seasonal trend. The seasonal variations in soil CO2 and N2O emissions were positively related to soil temperature and moisture. Mean soil CO2 and N2O emissions (53.2 mg C m−2 h−1 and 5.21 μg N m−2 h−1, respectively) were significantly higher in the mixed plantation than in the P. massoniana plantation, while they were lower than in the C. hystrix plantation. Mean soil CH4 uptake (38.4 μg C m−2 h−1) was significantly higher in the mixed plantation than in the C. hystrix plantation, while it is similar to that in the P. massoniana plantation. Variations in soil CO2 flux among the plantations were influenced by fine root biomass, leaf litterfall mass, soil N stock and soil C:N ratio. Differences in soil N2O flux among the plantations could be attributed to the differences in soil N stock, soil NO3−−N content and soil C:N ratio. Soil respiration rate and soil NO3−−N content could account for variations in soil CH4 flux among the plantations. This study confirms that the mixed plantation has a higher SOC stock than the monoculture plantations, and there is an increase in amount of GHG absorbed by the soil of mixed plantations compared to C. hystrix plantations. Therefore, a mixture of C. hystrix versus P. massoniana, could be a better silvicultural approach for SOC sequestration than monoculture C. hystrix plantation for substituting P. massoniana plantations in subtropical China.

► The mixed plantation has the higher SOC stock than the monoculture plantations.
► There is an increase in amount of GHGs absorbed by the soil of mixed plantations compared to C. hystrix plantations.
► Mixed plantation could be a better silvicultural approach for SOC sequestration in subtropical China.