Aggregate composition directly affects the characteristics of soil microbial and nematode communities in a chronosequence of Chinese fir plantations

Abstract 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.