Distinguished Lecture: Below ground dynamics of recently fixed plant carbon in a California annual grassland

Plant roots and the organisms that surround them are the primary source for stabilized soil organic carbon (SOC). This characteristic of the terrestrial C cycle is critical to C storage, soil water holding capacity, nutrient provision, and the management of soil health. While grassland soils have a large capacity to store organic carbon (C), few field-based studies have quantified the movement of plant-fixed C into soil and its persistence belowground over multiple years. We tracked the fate of plant-fixed C in the field following a five-day ¹³CO₂ labeling of a Northern California annual grassland, characterizing the soil C pools over a period of two years. Our results indicate that newly fixed C moved into soil within days and was associated with the soil mineral fraction within weeks. While most of the annual plant C input in these grasslands cycles rapidly (<2-year timescale), a sizable proportion (about 23% of the ¹³C present at day 0) persisted in the soil for longer than 2 years. 

To identify the mechanisms responsible for the conversion of root C into stabilized SOC, we characterized the roles played by arbuscular mycorrhizal fungi (AMF) associated with grass roots.  While we know that AMF are responsible for a major input of C to soil, we know little about the dynamics and mechanisms involved in the fate of this C input.  We used ¹³C stable isotope tracing to measure the transfer of C from the host plant Avena barbata, a widespread annual grass, into the soil via the AMF Rhizophagus intraradices. We used a two-chamber microcosm design to distinguish the fluxes of C by AMF from those of roots and to identify the physical fate and chemical forms of the C in soil.  To explore the influence of AMF hyphae on soil microbial communities, we quantified the movement of ¹³C from the AMF hyphae into surrounding organisms using SIP-enabled metagenomics analyses.