Community RNA-Seq: Multi-kingdom responses to living versus decaying root inputs in soil

Roots are the primary source of organic carbon inputs to most soils. Decomposition is a multi-trophic process involving multiple kingdoms of microbial life, but typically microbial ecology studies focus on one or two major lineages in isolation. We used Illumina shotgun RNA sequencing to conduct PCR-independent SSU rRNA community analysis ("community RNA-Seq") to simultaneously study the bacteria, archaea, fungi, and microfauna surrounding both living and decomposing roots of the annual grass, Avena fatua. Plants were grown in 13CO2-labeled microcosms amended with 15N-root litter. We identified rhizosphere substrate preferences for 13C-exudates versus 15N-litter using NanoSIMS microarray imaging (Chip-SIP). When litter was available, rhizosphere and bulk soil had significantly more Amoebozoa, which are potentially important yet often overlooked top-down drivers of detritusphere community dynamics and nutrient cycling. Bulk soil containing litter was depleted in Actinobacteria but had significantly more Bacteroidetes and Proteobacteria. While Actinobacteria were abundant in the rhizosphere, Chip-SIP showed Actinobacteria preferentially incorporated litter relative to root exudates, indicating this groups more prominent role in detritus elemental cycling in the rhizosphere. Our results emphasize that decomposition is a multi-trophic process involving cross-kingdom interactions, and the trajectory of carbon through this soil food web likely impacts the fate of carbon in soil.