Long-term unreasonable management has led to the continuous decline of soil quality in Melaleuca alternifolia planting areas in southern China, and there is no effective way to improve its soil quality at present. In this study, residues of tea tree oil extraction were returned to the forest to explore its influence on soil quality. Therefore, four test groups (RT, residues were tiled; RS, residues were stacked; RDT, residues were decomposed and tiled; RDS, residues were decomposed and stacked) and one control group (CK, nothing was changed) were designed. We used one-way ANOVA and Pearson correlation analysis to detect 22 physical, chemical, and biological indicators of soil, and then used minimum data set (MDS) and principal component analysis (PCA) to evaluate soil quality. The results show that compared with the CK, BD and pH in the test groups decreases, while CP, TTP, SOM, AN, NN, AP, AK, CEC, MBC, MBN, MBP, catalase, urease, sucrase, and ACP increase or strengthen in different degrees, and the biomass increases by 5.3%~12.8%. The soil quality indexes (SQI) are RDT (0.616) > RT (0.546) > RDS (0.525) > RS (0.452) > CK (0.291). Significant correlation between SQI and biomass indicates that the indicators have high biological significance for the planting areas of Melaleuca alternifolia in the red soil region in southern China. These results show that residues could improve soil quality, and that the soil quality is different among different test groups. This study provides a new path for the management of Melaleuca alternifolia plantation.

The carbon sink capacity of forest ecosystem and its function of mitigating climate change have been confirmed. As two common coniferous species, Cunninghamia lanceolata (Lamb.) Hook. (C. lanceolata) and Pinus elliottii Engelmann (P. elliottii) are widely planted in southern China, and their carbon sink capacity has always been concerning. According to their diameter class, we randomly harvested 42 C. lanceolata trees and 38 P. elliottii trees from our entire study area, measured their carbon concentration, and constructed biomass models with DBH and tree height as variables. The biomass of the tree layer was estimated by measuring the DBH of all trees in the plots, and the biomass and carbon concentration of shrubs, herbs, dead wood and litter in the plot were measured by harvesting them. The results showed that the total biomass in C. lanceolata and P. elliottii plantations were 117.1 and 151.8 t·ha⁻¹; the biomass in the tree layer was 94.7 and 122.9 t·ha⁻¹; and in the other parts was 22.4 and 28.9 t·ha⁻¹, respectively. In addition, the total carbon densities in the C. lanceolata and P. elliottii plantation ecosystems were 166.3 and 198.6 t·ha⁻¹; the carbon densities in the soil were 108.1 and 124.6 t·ha⁻¹; and in the other parts, they were 58.2 and 74.0 t·ha⁻¹, respectively. These results indicate that there are significant differences in total biomass or total carbon storage between the two coniferous forest ecosystems, and net productivity and carbon sink capacity are higher in the P. elliottii plantation ecosystem. This study lays the foundation for the biomass estimation and carbon trading of these two coniferous forests in northern Hunan.