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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. [ABSTRACT FROM AUTHOR]

Maintaining soil biological diversity is an effective approach in facilitating Chinese fir plantations' sustainable development. However, dynamic characteristics of soil microbial and nematode communities during Chinese fir planting remain unclear, particularly on the aggregate scales, which are the main sites of soil biological activity. Therefore, the research aims to explore how stand age (3, 9, 17, and 26 years) of Chinese fir plantations affects soil microbial biomass (including bacteria, fungi, and actinomycetes, based on the phospholipid fatty acid method), nematode abundances (including bacterivores, fungivores, plant parasites, and omnivores‐predators, based on the modified Baermann funnel method), and their diversity within aggregates of differing sizes (>2, 2–1, 1–0.25, and <0.25 mm). According to the findings, both aggregate size and stand age were major factors affecting soil microbes and nematodes in Chinese fir plantations. Specifically, soil microbial biomass (mainly bacteria), nematode abundances (except fungivores), and their diversity peaked within large macro‐aggregates (>2 mm), and the ranking order for stand ages was 17 > 9 ≈ 3 > 26 years. As the main fractions of soil aggregates in Chinese fir plantations, large macro‐aggregates had suitable internal environments for soil microbial propagation and nematode growth and mainly accumulated at 17 years, which caused the prosperity of soil microbes and nematodes at this stand age. Moreover, the partial least squares path model demonstrated that the influence of stand age on soil microbes and nematodes was indirect; but rather, the aggregate composition played a direct role in shaping their communities. Therefore, in order to maintain soil biological diversity, large macro‐aggregates should be protected during the growth of Chinese fir, especially at the late stage (from 17 to 26 years). Overall, these results could provide valuable insights into Chinese fir plantations' sustainable development. [ABSTRACT FROM AUTHOR]

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 (<0.25 mm) of the three stands were relatively limited by C and P. In contrast, microbial metabolism was more N-limited in macroaggregates (>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. [ABSTRACT FROM AUTHOR]