Abstract
Abstract Below every cultivated field lies a civilization of unimaginable complexity. A single gram of productive agricultural soil harbors upwards of a billion bacterial cells, several kilometers of fungal hyphae, and a faunal community spanning nematodes, protozoa, mites and collembolans, all bound together in trophic relationships that have been taking shape for hundreds of millions of years. It is this community, not the chemistry of a dose of fertilizer, that ultimately determines how fertile a soil is, how stable a crop yield remains across years, and how resilient an agroecosystem is when a stressor arrives. And yet, the dominant tradition in agricultural sciences has treated soil as little more than a physical medium for root anchorage and a chemical reservoir for nutrient supply. Decades of intensive management have depleted soil organic matter, compressed food web complexity, and homogenized the microbial communities that drive elemental cycling, with consequences for agricultural resilience that are becoming impossible to ignore. Here, I review current understanding of soil microbial ecology across five interconnected themes: soil food web energy dynamics, rhizosphere plant-microbe interactions, arbuscular mycorrhizal fungal ecology, the biological regulation of nitrogen cycling, and the translation of soil biodiversity into ecosystem services. I posit that ecological theory already provides a sufficient conceptual basis for redesigning agroecosystems around microbial functionalities. I also identify where knowledge gaps remain significant and where the most urgent research investment is needed. Ultimately, I posit that the path to agricultural sustainability runs, unavoidably, through the soil microbiome.