The hyphosphere—the region of soil that surrounds fungal hyphae—is a hotspot of interactions that shape terrestrial microbial community composition and function. For example, arbuscular mycorrhizal fungi (AMF) can consume more than 20% of plant photosynthetic carbon (C) and shape both the composition and functionality of surrounding hyphosphere microbial communities. In collaboration with surrounding biota, mycorrhizae also mobilize N and water and stimulate the formation of stable soil carbon. To evaluate the quantitative impact of such ‘cross-kingdom’ interactions—involving fungi, bacteria, archaea, protists, microfauna and viruses—our group uses stable isotope probing (SIP) and metagenomics/transcriptomics to assess taxon-specific growth and mortality. Taken together, our findings illustrate how plant-associated fungi support enhanced biotic activity, resilience to water limitation, and faster turnover of native soil organic matter and fresh plant photosynthates in the hyphosphere.