Long-term fire management history affects N-fertilization sensitivity, but not seasonality, of grassland soil microbial communities

摘要

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 not burned, and annually fertilized or not fertilized, in a factorial design, since 1986. Composite surface soil samples (0-15 cm) were collected monthly between November 2014 and December 2015 from replicate plots to evaluate event-based (post-fire, post-fertilization), seasonal, and long-term responses of soil microbial communities to management and environmental changes. Bacterial 16S rRNA gene and fungal ITS population sizes were estimated using 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 or copiotrophic taxa. Bacterial and fungal population sizes varied significantly by sampling month, in that bacterial populations were approximately 10 x greater in summer (June-August), but did not change in response to management events or long-term treatments. In contrast, very few individual taxonomic groups displayed seasonal or event-based responses, and there was no significant whole-community turnover on weekly or monthly time-scales; instead, BCC was strongly impacted by both the long-term fire and fertilization treatments. Specifically, there were increases and decreases in putatively "copiotrophic" and "oligotrophic" prokaryotic Phyla in response to long-term N fertilization, which were significantly stronger and more predictable in soils following long-term fire suppression. These results reveal that while long-term grassland management changes BCC beyond the detected range of seasonal variability, total bacterial populations change coherently month-to-month, potentially due to significant plant inputs of labile carbon during the growing season. Furthermore, because prescribed burning reduces soil N availability, the interactive responses to fire suppression plus fertilization suggest that higher background levels of soil N availability may increase the magnitude of soil microbial sensitivity to N fertilization.