Reduced precipitation alters microbial availability and redistribution of rhizosphere carbon
Climate change intensifies grassland drying, which is predicted to decrease soil organic carbon (SOC) storage. Yet how reduced soil water affects the transport and processing of carbon in the rhizosphere -- a major source of SOC supporting microbial activities -- remains poorly resolved under field conditions. We traced living root-derived carbon (rhizodeposit C) into soils and microbes using {superscript 1}3CO2 labeling in a multi-year precipitation manipulation experiment in a California Mediterranean annual grassland dominated by Avena barbata. Under halved precipitation, season-averaged soil water content was 20% lower and soil matric potential more than threefold lower, without inducing detectable signs of plant stress. Quantitative stable isotope probing (qSIP) of rhizosphere microbial DNA revealed that up to 68% more bacterial and 13% more fungal taxa incorporated rhizodeposit C, and that microbial uptake of rhizodeposit C increased more over time under reduced precipitation, even as total microbial biomass and community composition were unchanged. {superscript 1}3C-informed co-occurrence networks of these consumers were larger and more connected under reduced precipitation. The surrounding soils contained less 13C, and modeled solute transport by soil water was 32-40% lower, implying that rhizodeposit C was retained and intensively processed nearer to roots rather than transported to the wider soil matrix. Our evidence supports that reduced precipitation concentrates rhizodeposit C near roots, increasing the number of distinct consumers and intensifying associations among them. This represents an overlooked mechanism linking precipitation reduction to rhizodeposit C cycling. These findings show that precipitation reduction may reshape rhizosphere C processing before apparent plant stress response, with potential consequences for the fate of rhizodeposit C in grassland soils.