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.

Assessing the limitations of microbial metabolic resources is crucial for understanding plantation soil quality and enhancing fertility management. However, the variation of microbial resource limitations at the aggregate level in response to changes in stands remains unclear. This research explores carbon (C), nitrogen (N), and phosphorus (P) limitations affecting microbial metabolism in bulk soils and aggregates in two mixed and one pure Chinese fir stands in subtropical China, analyzing resource limitations concerning soil carbon, nutrients, and microbial indicators. The results revealed that microbes in all aggregates of the pure stands and in the micro aggregates (2 mm) and small aggregates (2–0.25 mm) in the mixed stands. Additionally, in the mixed stands the proportion of soil macroaggregates increased, and that of micro aggregates decreased, resulting in a shift from C and P limitation to N limitation for bulk soil microbial metabolism. Redundancy analysis identified soil aggregate organic carbon and nutrient content as the main factors affecting microbial resource limitation, rather than their stoichiometric ratios. Pathway analysis further confirmed that soil nutrients and their stoichiometric ratios indirectly influenced soil microbe resource limitation by regulating microbial biomass, microbial respiration, and extracellular enzyme activities. Thus, the impact of mixed plantations on soil nutrients and microbial activity at the aggregate level may be crucial for maintaining land fertility and achieving sustainability.

Mixed cultivation with legumes may alleviate the nitrogen (N) limitation of monoculture Eucalyptus. However, how leaf functional traits respond to N in mixed cultivation with legumes and how they affect tree growth are unclear. Thus, this study investigated the response of leaf functional traits of Eucalyptus urophylla × Eucalyptus grandis (E. urophylla × E. grandis) and Dalbergia odorifera (D. odorifera) to mixed culture and N application, as well as the regulatory pathways of key traits on seedling growth. In this study, a pot-controlled experiment was set up, and seedling growth indicators, leaf physiology, morphological parameters, and N content were collected and analyzed after 180 days of N application treatment. The results indicated that mixed culture improved the N absorption and photosynthetic rate of E. urophylla × E. grandis, further promoting seedling growth but inhibiting the photosynthetic process of D. odorifera, reducing its growth and biomass. Redundancy analysis and path analysis revealed that leaf nitrogen content, pigment content, and photosynthesis-related physiological indicators were the traits most directly related to seedling growth and biomass accumulation, with the net photosynthetic rate explaining 50.9% and 55.8% of the variation in growth indicators for E. urophylla × E. grandis and D. odorifera, respectively. Additionally, leaf morphological traits are related to the trade-off strategy exhibited by E. urophylla × E. grandis and D. odorifera based on N competition. This study demonstrated that physiological traits related to photosynthesis are reliable predictors of N nutrition and tree growth in mixed stands, while leaf morphological traits reflect the resource trade-off strategies of different tree species.

The impacts of the successive planting of Eucalyptus on soil microbial communities and their underlying mechanisms remain unknown, limiting our understanding of its long-term effects on soil ecosystems. This study examined the 0–20 cm and 20–40 cm soil layers, investigating changes in soil bacterial and fungal communities after multiple plantings of Eucalyptus grandis × urophylla using high-throughput sequencing. Furthermore, we used the structural equation model (SEM) to analyze the relationships among soil active organic carbon (SAOC), enzyme activity, and microbial diversity. The study showed that the multigeneration successive planting of Eucalyptus significantly increased the soil bulk density and decreased the soil physicochemical properties and soil enzyme activities (p < 0.05). The soil’s dominant microbial compositions were unchanged in the two soil horizons, but the relative abundances of some dominant phyla (e.g., Crenarchaeota, Basidiomycota and Actinobacteriota) were affected by successive planting. The variability in the microbial community structure was influenced primarily by the soil water content (SWC) and organic carbon (p < 0.05). The microbial community diversity in the 20–40 cm horizon was significantly affected by multigeneration succession (p < 0.05). SWC was the core factor driving microbial community diversity. SEM results showed that multigeneration successive planting obviously limits SAOC fractions and enzyme activities, negatively affecting soil microbial diversity. Our study highlights the impact of the multigeneration successive planting of Eucalyptus on soil microbial community structure and suggests adjustments in forestry practices to mitigate soil degradation.