Soil nitrogen (N) fixation, driven by microbial reactions, is critical to support the entrance of nitrogen in nutrient poor and pioneer ecosystems. However, how and why N fixation and soil diazotrophs evolve as forests develop remain poorly understood. Here, we used a 60-year forest rewilding chronosequence and found that soil N fixation activity gradually decreased with increasing forest age, experiencing dramatic drops of 64.8% in intermediate stages and 93.0% in the oldest forests. Further analyses revealed loses in diazotrophic diversity and a significant reduction in the abundance of important diazotrophs (e.g., Desulfovibrio and Pseudomonas) ) as forest develops. This reduction in N fixation, and associated shifts in soil microbes, was driven by acidification and increases in N content during forest succession. Our results provide new insights on the life history of one of the most important groups of soil organisms in terrestrial ecosystems, with consequences for understanding the buildup of nutrients as forest soil develops.

Free-living diazotrophs in the soil contribute substantially to nitrogen input in forest ecosystems through dinitrogen fixation. Dynamic forest communities provide heterogeneous habitats for soil diazotrophs, but the ecological processes of diazotrophic community assembly and their association with forest development are not fully understood. Using a chronosequence established based on the afforestation history of the forest ecosystem, we evaluated changes in dinitrogen fixation and diazotrophs associated with forest development over more than 60 years, from initial to mature community. The potential for biological dinitrogen fixation was assessed by employing both the rate of biological dinitrogen fixation (NFR) and quantitative PCR detection of the nifH gene. Furthermore, assembly processes, interactions among diazotrophic communities, and environmental regulatory pathways were analyzed to uncover potential strategies for diazotrophic adaptation in diverse forest ecosystems. Our results showed that the NFR gradually decreased with increasing forest age, experiencing dramatic drops of 64.8% in the mature stage and 93.0% in the over-mature stage. The deterministic selection of community assembly strengthened with forest development, triggering inefficient dinitrogen fixation by soil diazotrophs. Further analysis revealed a loss of diazotrophic diversity, reduced abundance of important diazotrophs (e.g., Desulfovibrio and Pseudomonas), and weakened community interactions in older forests. These successional processes of diazotrophic community were strongly influenced by environmental synergies, in which edaphic factors (pH and nitrate nitrogen) modulated dinitrogen fixation. These results highlight that forest development suppresses the potential for dinitrogen fixation in the soil. Future monitoring of soil health should consider temporal variations in biological dinitrogen fixation to maintain plant productivity and nitrogen sustainability.

Rhizosphere microbes play important roles in plant tolerance to abiotic stresses. Plants of different genetic backgrounds acquire stress resistance by assembling specific rhizosphere microbial communities or typical beneficial microbiota. However, the molecular mechanisms by which plants recruit microbiota during plant acclimation to environmental stresses are unclear. Here, we investigated transcription pattern in three poplar genotypes, namely Populus davidiana x P. bolleana Loucne (SXY), P. deltoides x P. euramericana 'Nanlin 895' (NL895) and P. alba x P. glandulosa '84K' (84K), and their effect on the rhizosphere microbial community under salinity. The results showed that SXY exhibited salt tolerance characterized by the best performance of photosynthesis and antioxidant system upon salt stress, while salt stress severely damaged the growth and membrane system of 84K. 16S rRNA sequencing revealed the lowest rhizobacterial community diversity associated with SXY compared to 84K and NL895, implying strong enrichment of certain bacterial taxa by the salt tolerant cultivar. Specifically, SXY recruited higher abundances of Hydrogenophaga and Pseudomonas. Furthermore, RNA-seq analysis of roots combined with weighted gene co-expression network analysis (WGCNA) identified module eigengene (ME)yellow, a module positively correlated to the SXY-enriched OTUs. KEGG analysis revealed significant enrichment of lipid metabolic and amino acid biosynthetic pathways in MEyellow. Twenty-four genes selected from the pathways enriched in MEyellow showed the highest expression levels in SXY. Our results suggest a "gene expression-rhizosphere-microbiota-salt tolerance" regulatory process in the poplar under salinity and provide new insights into the mechanisms by which plants shape the soil microbiome to improve salt tolerance.

连栽导致土壤退化是制约杉木初级生产力实现的重要障碍因素，而土壤对病原菌的抑制能力决定着植物能否有效抵御病原菌侵害，是人工林土壤地力状况的重要表现。以一代、二代、三代杉木人工林和天然次生林为对象，采用平板隔空、直接对峙的方法，分析了不同代际杉木林土壤细菌群落对尖孢镰刀菌和立枯丝核菌的抑制能力。进一步利用高通量测序技术，研究了杉木林土壤细菌群落影响土壤抑病能力的生态过程。结果表明：土壤磷元素随连栽呈显著积累趋势，而土壤pH和有机质（SOM）等含量随连栽代数的增加而下降，但这些下降指标在三代杉木林与天然林土壤间无显著差异。而杉木连栽导致土壤对病原菌的抑制能力逐代降低，天然林土壤较杉木人工林对病原菌具有显著的高抑制能力。同时杉木连栽显著改变了土壤细菌群落组成，而对群落整体α-多样性影响较小，说明土壤中一些关键类群对杉木连栽响应的敏感性高于整体细菌群落的变化。进一步利用随机森林模型预测与回归分析，揭示了杉木连栽引起的土壤一些关键细菌类群丰度的降低是土壤抑病能力下降的重要原因，这些类群主要受土壤pH、SOM、TP等土壤理化因子的调控。由此，杉木长期连栽会引起土壤微环境失衡，致使土壤抑制病原菌能力下降，从而增加病原菌危害杉木林健康，不利于人工林生产力的提升和可持续经营。

为了解国内外人工林土壤地力的研究进展和发展趋势,采用文献计量学方法,以中国知网(CNKI)和Web of Science核心合集数据库相关文献为数据源,利用VOSviewer、HistCite等工具,从国家、机构、作者、研究热点方面对2021年之前的有关文献进行了计量分析.结果表明:国内外该领域年发文量呈逐渐增长趋势,国际上发文量前3位的国家是中国、美国和巴西;中国最先在该领域进行研究且与国际的合作交流较多,文献总被引频次位居前列;国内该领域发文机构主要为涉林高校及中国科学院下属研究所,国际上美国俄亥俄州立大学发表的文献质量明显突出,引用频次较高;现阶段国内外在该领域的研究主要围绕土壤微生物群落组成、土壤养分含量以及与土壤地力维持的关系,但多处在定量描述阶段,很少涉及微生物功能代谢与人工林下有机物质周转以及对土壤地力维持的影响机制,研究深度亟待提升.总之,国内外同行合作交流、学科间交叉融合以及新生研究力量的补充是未来有效提升该研究领域的关键所在.