Background: Revealing the variations in soil aggregate-related organic carbon (OC) and labile organic carbon (LOC) fractions in a chronosequence of Chinese fir plantations plays an important role in better understanding the impact of soil carbon sink or source on the Chinese fir plantation ecosystem. In this study, soil samples in a depth of 0–20 cm were collected from Chinese fir plantations at different stand ages (0, 9, 17, and 26 years old) in Guangxi, China. With the optimal moisture sieving method adopted, the soil aggregates of 4 different sizes were obtained, including > 2-mm, 2–1-mm, 1–0.25-mm, and < 0.25-mm aggregates. Soil OC and LOC fractions were measured in the aggregates of different sizes. The LOC fractions included readily oxidizable carbon (ROC), particulate organic carbon (POC), microbial biomass carbon (MBC), water-soluble organic carbon (WOC), and mineralized organic carbon (MOC). Results: Soil aggregate stability, as indicated by the mean weight diameter (MWD), was the highest in the 17-year-old Chinese fir plantations and was significantly positively related (p < 0.05) to the concentrations of OC and LOC fractions (except for the ROC and MOC), with the POC in particular. As for all stand ages of Chinese fir plantations, the concentrations of soil OC and LOC fractions were significantly increased as the aggregate size decreased. Consequently, there were more OC and LOC fractions distributed in the < 0.25-mm aggregates. During the stand development, the concentrations of soil OC and LOC fractions first increased and then decreased, with the highest levels detected in the 17-year-old Chinese fir plantations, indicating that the 17-year-old Chinese fir plantations were conducive to the accumulation of soil OC and LOC fractions. Conclusion: After 17 years of planting, promoted soil carbon (especially for the POC) accumulation contributes significantly to enhancing soil aggregate stability for the Chinese fir plantations in Guangxi, China.

Soil ecological stoichiometry offers a tool to explore the distribution, cycling, limitation, and balance of chemical elements in tea plantation ecosystems. This study aimed to explore how soil organic C (OC) and nutrient contents (total N (TN), total P (TP), Ca²⁺, Mg²⁺, Fe²⁺, and Mn²⁺) as well as their stoichiometric ratios ($\mathrm{C}/\mathrm{N}$ , $\mathrm{C}/\mathrm{P}$ , $\mathrm{N}/\mathrm{P}$ , $\mathrm{Ca}/\mathrm{Mg}$ , and $\mathrm{Fe}/\mathrm{Mn}$ ) varied with tea plantation age (8, 17, 25, and 43 years) and soil depth (0–10, 10–20, 20–40, and 40–60 cm) within aggregates in southern Guangxi in China. Our results showed that tea plantation age and soil depth significantly affected soil nutrient stoichiometry in different sizes of aggregates. Among different ages of tea plantations, soil OC, TN, and TP contents as well as $\mathrm{C}/\mathrm{N}$ , $\mathrm{C}/\mathrm{P}$ , and $\mathrm{N}/\mathrm{P}$ ratios significantly decreased as the soil depth increased. In addition, soil Ca²⁺ and Mg²⁺ contents were significantly lower in the surface soil layer than the deeper soil layer, whereas soil Fe²⁺ and Mn²⁺ contents showed opposite trends, and no significant differences were detected in $\mathrm{Ca}/\mathrm{Mg}$ and $\mathrm{Fe}/\mathrm{Mn}$ ratios among different soil depths. At the 0–40 cm soil depth, continuous planting of tea corresponded to increases in soil OC, TN, Fe²⁺, and Mn²⁺ contents, whereas soil Ca²⁺ and Mg²⁺ contents significantly decreased over time. During the process of tea growth, the losses of soil Ca²⁺ and Mg²⁺, especially Ca²⁺ (as indicated by the decrease in the soil $\mathrm{Ca}/\mathrm{Mg}$ ratio), led to soil acidification, which reduced Fe²⁺ absorption and enhanced Mn²⁺ uptake by tea plants (as indicated by the increase in the soil $\mathrm{Fe}/\mathrm{Mn}$ ratio). In general, tea plantation age affected the variations of soil nutrient contents and stoichiometry, and such effects were more obvious at the 0–40 cm soil depth, in contrast to the 40–60 cm soil depth.

The nitrogen (N) demand of eucalyptus plantations is considered to be effectively supplemented by the addition of N‐fixing plants. However, the effects of intercropping combined with N fertilization on the enzyme activity, soil fertility, and bacterial relationships, such as bacterial‐mediated nutrient cycling, remain elusive. A pot experiment was performed to assess the effect of intercropping and fertilization on eucalyptus productivity, soil fertility, enzyme activity, and bacterial characteristics, in addition to any probable linkages between these factors and how the interactions promote eco‐friendly production in eucalyptus plantations. Intercropping eucalyptus combined with suitable N fertilization (3 g plant⁻¹) resulted in a higher Chao1 and Shannon diversity of bacterial communities; increased soil fertility, pH, enzyme activity, and eucalyptus productivity; as well as generated a stronger positive interaction between dominant bacterial taxa and soil environmental variables. According to the network and structural equation model (SEM) analysis, the interaction between dominant bacterial taxa and soil environmental variables was clearly linked to N application, and deficient or excess N fertilizer may exert a detrimental effect on eucalyptus productivity. This suggests that reasonable N application in intercropping is a potentially sustainable solution for maintaining eucalyptus productivity and soil quality under N‐limited conditions.

Nitrogen plays a crucial role in limiting plant growth and determining net primary productivity in forest ecosystems. However, variations and influencing factors of soil nitrogen distribution on the aggregate scale in pure and mixed Chinese fir (Cunninghamia lanceolata (Lamb.) Hook) plantations remain unclear. This study aimed to explore how soil aggregate composition, geometric mean diameter (GMD), mean weight diameter (MWD), total nitrogen (TN), total acidolyzable nitrogen (TAN), available nitrogen (AN), nonacidolyzable nitrogen (AIN), amino acid nitrogen (AAN), nitrate nitrogen (NO₃⁻-N), cidolyzable ammonia nitrogen (AMN), unknown-acidolyzable nitrogen (HUN), amino sugar nitrogen (ASN), and ammonium nitrogen (NH₄⁺-N) contents and stocks varied with aggregate size (>2, 1–2, 0.25–1, and <0.25 mm) and stand type [Chinese fir mixed with Michelia macclurei Dandy (CF + MM), Chinese fir mixed with Mytilaria laosensis Lecomte (CF + ML) and pure stands of Chinese fir (CF)] in 0–20 and 20–40 cm soil depth. Soil N content in different stand types of Chinese fir plantations decreased as the aggregate size increased, whereas the soil N stock exhibited the opposite trend. In contrast to CF soil, CF + MM and CF + ML soil displayed a significant increase in MWD, GMD, and aggregate-associated TN, AN, NO₃⁻-N, NH₄⁺-N, AIN, AAN, ASN, and AMN contents and stocks, especially CF + MM soil. Organic N was more sensitive to the response of aggregate size and stand type than mineral N. Redundancy analysis and Pearson’s correlation analysis indicated that the 0.25–1 mm aggregate proportion was the main controlling factor for the variations in soil N content and storage. Overall, this study contributed significantly to the promotion of the sustainable use of soil resources and reference information for the scientific management and sustainable development of Chinese fir forests.