Soil Biodiversity and Soil Organisms: Microbes, Fauna, Ecosystem Functions, and Indicators of Soil Biological Health

Abstract Soil biodiversity—the diversity of microorganisms and animals living belowground—is increasingly recognized as a key driver of terrestrial ecosystem functioning. Soil microbes (bacteria, fungi, actinomycetes) and soil fauna (nematodes, earthworms, arthropods) regulate decomposition, nutrient cycling, soil structure formation, and plant productivity through complex food-web interactions. This narrative review synthesizes major soil organism groups and their functional roles, with emphasis on (i) how bacteria, fungi, and actinomycetes mediate carbon and nutrient transformations; (ii) how nematodes, earthworms, and arthropods influence soil food-web structure, aggregation, and bioturbation; and (iii) how soil biological health can be assessed using practical indicators ranging from microbial biomass and enzyme activities to nematode community indices and arthropod-based biological quality metrics. Evidence from global syntheses demonstrates that biodiversity loss belowground can reduce ecosystem multifunctionality and impair productivity. The review concludes with a pragmatic indicator framework and key research directions for integrating soil biodiversity into monitoring and climate-smart land management. Keywords: soil biodiversity, soil microbiome, fungi, bacteria, actinomycetes, nematodes, earthworms, arthropods, soil health indicators, soil multifunctionality