Research on the variations in soil aggregate stability and ecological stoichiometry at aggregate scales by stand type is of great significance in investigating the distribution, limitation, balance, and cycling of organic carbon, nitrogen, and phosphorus (C-N-P). However, the effect of pure and mixed Chinese fir plantations on soil aggregate stability, organic carbon (OC), total nitrogen (TN), and total phosphorus (TP) stoichiometry characteristics at aggregate scales is still unclear. In this research, we explored the variations in soil aggregate mean weight diameter (MWD) and geometric mean diameter (GMD); soil OC, TN, and TP contents and stocks and the C:N:P ratios as affected by different stand types (mixed stands of Chinese fir and Mytilaria laosensis, mixed stands of Chinese fir and Michelia macclurei, and pure stand of Chinese fir); and aggregate size (2 mm) at 0–20 and 20–40 cm depths in subtropical China. The soil OC and TN contents, as well as C:N:P ratios declined as aggregate size increased, whereas the C-N-P stocks showed the opposite tendencies, which were more distributed in >2 mm aggregates. Mixed stands of Chinese fir and M. laosensis with Chinese fir and M. macclurei displayed significantly higher soil aggregate stability, aggregate-associated TP content, OC and TN contents and stocks, and C:N and C:P ratios than did pure stands of Chinese fir. Soil aggregate stability was significantly positively correlated with the C-N-P contents and stocks as well as the C:N and C:P ratios, especially the C:N ratio and TN content. Overall, this work offers further information for scientific management and sustainable development of Chinese fir plantations, soil OC and nutrient cycling with ecological stoichiometry in the global terrestrial ecosystem.

The selection of trees to be harvested is a core tenet of uneven-aged forest management; however, few studies have focused on the process of tree selection. A set of stand structural parameters (uniform angle index, W; mingling index, M; dominance index, U) based on tree neighbor-spatial relationships, are particularly suitable for expressing the structural characteristics of forest stands. Such indices were used to parameterize thinning in three plots (a-c, each 100 × 100 m2) in a Korean pine broad-leaved forest in northeastern China and one plot (h, 70 × 70 m2) in a pine-oak mixed forest in northwest China. Low-intensity single-tree selection was applied according to the principles of structure-based forest management (SBFM), i.e., to promote high mixture, obvious size differentiation, and random pattern with the aim of improving the overall structure of the managed plots. A group of thinning priority indices (v_ij, k_ij and z_ij) were calculated according to the bivariate distributions of the structural characteristics of harvested trees and stands before harvest. Our results demonstrated that v_ij, k_ij and z_ij adequately describe the spatial relationship between each tree and its nearest neighbors, and their combinations can be efficiently used to set thinning priorities on harvested trees with different structural characteristics. Their application can reduce the subjectivity of the selection process and improve the speed and accuracy of the choice of trees to be harvested in uneven-aged mixed forests.

Revealing the dynamics of soil organic carbon (C_org) and nutrients at aggregate scales is essential for improving our understanding of soil C_org mitigation and nutrient restitution in the Cunninghamia lanceolata plantations. In this study, soil C_org, total nitrogen (N_tot), available phosphorus (P_ava), and exchangeable cations (including calcium (Ca²⁺), magnesium (Mg²⁺), potassium (K⁺), and sodium (Na⁺)) received the analysis in aggregate fractions acquired from 0 to 10 cm and 10–20 cm depths in the three different stand types, namely mixed stands of Cunninghamia lanceolata and Michelia macclurei (stand A), Cunninghamia lanceolata and Mytilaria laosensis (stand B), and pure stand of Cunninghamia lanceolata (stand C). Soil aggregates were classified into macro-aggregates (>2 mm), meso-aggregates (2–0.25 mm), and micro-aggregates (<0.25 mm) by the dry-sieving process. The two mixed stands displayed higher level of soil aggregate stability than the pure stand. Moreover, micro-aggregates acted as the main fractions that carried soil C_org, N_tot, and P_ava, and both micro-aggregates and macro-aggregates referred to the main fractions that carried exchangeable cations. As for the soil nutrient stocks, only the exchangeable K⁺ stock of the pure stand dominated among the C_org and nutrient stocks, in addition, the rest of the other nutrient stocks of the mixed forests (stands A and B) took an advantage over the pure stand. Moreover, the C_org and nutrient stocks in stands A and B were reflected in the macro-aggregates, differently, those of stand C were mainly reflected in the micro-aggregates. Thus, selecting suitable broadleaf tree species mixed with Cunninghamia lanceolata can alleviate the reduction of soil aggregate stability and the loss of soil nutrients, thus promoting soil resources to be sustainably utilized and protecting soil quality and health in southern Guangxi of China.