Soil depth matters: Shift in composition and inter-kingdom co-occurrence patterns of microorganisms in forest soils

Soil depth represents a strong physiochemical gradient that greatly affects soil-dwelling microorganisms. Fungal communities are typically structured by soil depth, but how other microorganisms are structured is less known. Here, we tested whether depth-dependent variation in soil chemistry affects the distribution and co-occurrence patterns of soil microbial communities. This was investigated by DNA metabarcoding in conjunction with network analyses of bacteria, fungi, as well as other micro-eukaryotes, sampled in four different soil depths in Norwegian birch forests. Strong compositional turnover in microbial assemblages with soil depth was detected for all organismal groups. Significantly greater microbial diversity and fungal biomass appeared in the nutrient-rich organic layer, with sharp decrease towards the less nutrient-rich mineral zones. The proportions of copiotrophic bacteria, Arthropoda and Apicomplexa were markedly higher in the organic layer, while patterns were opposite for oligotrophic bacteria, Cercozoa, Ascomycota and ectomycorrhizal fungi. Network analyses indicated more intensive inter-kingdom co-occurrence patterns in the upper mineral layer (0-5 cm) compared to the above organic and the lower mineral soil, signifying substantial influence of soil depth on biotic interactions. This study supports the view that different microbial groups are adapted to different forest soil strata, with varying level of interactions along the depth gradient.土壤深度作为极强的理化梯度，深刻影响着土壤栖居微生物群落。真菌群落通常随土壤深度呈现结构化分布，但其他微生物类群的深度分布模式尚不明晰。本研究旨在探究土壤化学性质随深度的变化是否会影响土壤微生物群落的分布及其共现模式。以挪威白桦林四个不同土壤深度层的样品为研究对象，本研究结合DNA元条形码技术（DNA metabarcoding）与网络分析手段，对细菌、真菌及其他微型真核生物进行了分析。结果显示，所有研究类群的微生物群落组成均随土壤深度呈现显著的周转变化。在营养丰富的有机层中，微生物多样性与真菌生物量显著更高，而向营养匮乏的矿质层则呈现急剧下降趋势。有机层中富营养型细菌、节肢动物门（Arthropoda）与顶复门（Apicomplexa）的占比显著更高；而寡营养型细菌、丝足虫门（Cercozoa）、子囊菌门（Ascomycota）及外生菌根真菌的占比则呈现相反的分布模式。网络分析结果表明，相较于上部有机层与下部矿质土层，上层矿质层（0-5 cm）的界间共现模式更为密集，这表明土壤深度对生物间相互作用具有显著影响。本研究证实了不同微生物类群适配不同森林土壤土层的观点，且类群间的相互作用强度随深度梯度呈现动态变化。