Scalar Drivers of N-Fixing Diazotroph Community Composition in Agricultural Systems and Their Network-Derived Sensitivity to Change.

Free-living, non-symbiotic nitrogen fixation occurs through the activity of a functional group of soil bacteria known as diazotrophs that can significantly contribute to N inputs in agricultural systems. Both the abundance and community composition of these diazotrophs have been correlated to measured N fixation rates. However, how the composition and abundance of these bacteria are influenced by environmental, edaphic and crop management factors remains largely unknown. Here, we will demonstrate the influences of climate, soil properties, plant composition and management on the composition and abundance of the nifH-harboring microbial community using a combination of targeted high throughput sequencing and quantification of the nifH gene harboring microbial community across five diverse agricultural sites in Australia. These data reveal that, at the continental scale, both soil type and climatic factors influence community composition. However, the primary drivers change along scales, at the regional scale it is field management, at the field scale it is plant composition and at the plot scale it is the interplay between carbon and nitrogen availability that influence the composition and abundance of the nifH-harboring microbial community. In addition, we utilized molecular ecological network analysis to visualize co-occurrence networks of this functional group in a sandy soil versus a high clay, vertisol soil. Analysis of the underlying network architecture revealed that the vertisol soils resembled a “small-world” view. In addition, the higher diversity of vertisol network nodes in combination with lower connectivity suggests a diazotroph community less fragile to forcing pressures, such as management decisions that impact the survival of the community, or specific populations within that community that may ultimately impact rates of N fixation.