PS 47-217 - Spatial and temporal dynamics of microbial community structure and network complexity in a California annual grassland soil

Abstract: Both prokaryotic and eukaryotic microbes are key components of the soil ecosystem; however, identifying microbial interactions and their drivers remain a challenge due to the dynamic and complex nature of these communities. Co-occurrence network analysis, in particular interdomain networks, can provide insights into microbial associations across spatial and temporal scales. Our study aimed to explore potential interactions among bacteria, fungi, and archaea in rhizosphere and rhizosphere-influenced (“bulk”) soils in a Mediterranean California annual grassland dominated by Avena spp. A total of 82 rhizosphere samples and 81 rhizosphere-influenced samples were collected across 5 timepoints from 2018 to 2020. Microbes were identified using high-throughput amplicon sequencing of the prokaryotic 16S rRNA gene and the fungal ITS rRNA gene to investigate the composition and potential interactions of soil microbiomes. We found that variations in the prokaryotic community were predominantly affected by sampling time (R²=0.21, p=0.001) and by compartment niches (R²=0.08, p=0.001). Variations in the fungal community were mainly explained by compartment niches (R²=0.25, p=0.001) followed by sampling time (R²=0.17, p=0.001). Random-matrix theory (RMT) based co-occurrence networks showed that rhizosphere networks shifted to a more complex structure over time, with higher interactivity between nodes evidenced by an increase in the average degree of connectivity from 2.67 to 8.58. In addition, the number of fungal nodes increased from 18 to 42 in rhizosphere networks. On the other hand, rhizosphere-influenced networks shifted to a more modular and fragmented structure where the number of modules increased from 20 to 91. The number of fungal nodes in rhizosphere-influenced soils decreased from 40 to 5. No specific OTUs were repeatedly observed as module hubs or connectors across the different networks. The number of archaeal links increased over time with most of the links being positive between ammonia-oxidizing archaea of the genus Nitrososphaeraceae and bacteria. In addition, most of the bacterial-fungal links were negative in rhizosphere (55.7%) and rhizosphere-influenced soils (55.2%). Although the drivers of these co-occurrences remain to be identified, this study suggests that microbial associations exhibit strong spatial and temporal dynamics that impact the features responsible for maintaining network stability.