soil metagenome Targeted loci environmental

Nitrogen (N) availability is a driver of soil microbial diversity and function, and is affected by prescribed burning (N removal through volatilization) and fertilization (N addition). Because soil microbes control critical feedbacks to ecosystem function, it is important to understand the dynamics and responses of microbial populations under conditions of contrasting N availability. This study took place at a long-term field manipulation in which native tallgrass prairie was annually burned or left unburned, and annually fertilized or left unfertilized, in a factorial design, since 1986. Composite surface soil samples (0-15 cm) were collected monthly (November 2014 - December 2015) from replicate plots to measure event-based (post-fire, post-fertilization), seasonal and 29-year turnover of soil microbial communities. Bacterial 16S rRNA gene and fungal ITS population sizes were estimated by qPCR, and bacterial community composition (BCC) was measured using Illumina MiSeq sequencing of 16S rRNA genes. We expected seasonal and event-based change in all parameters, and that total microbial population sizes and diversity would be lower in soils with higher N availability, due to greater competitive dominance of nitrophilic taxa. Bacterial and fungal population sizes varied significantly by sampling month, not with long-term treatment, in that bacterial populations were at least 10x greater in summer (June-August). In contrast, BCC did significantly vary by season, but was strongly impacted by both long-term fire and fertilization treatments. Additionally, we observed increases and decreases in "copiotrophic" and "oligotrophic" taxa in response to long-term N fertilization, which were significantly stronger and more predictable in unburned soils, but no changes in richness or evenness. These results reveal that while long-term grassland management changes BCC beyond typical levels of seasonal variability, total bacterial populations change coherently month-to-month, potentially due to plant activity supplying similar levels of labile carbon during summer. Furthermore, ambient soil N availability, controlled by fire in tallgrass prairie, can influence the sensitivity of bacterial communities to N fertilization.