Abstract
The study of community assembly patterns is a common theme in microbial ecology. Two major processes, namely stochastic (random changes in community) and deterministic (abiotic and biotic factors) are responsible for observed community shifts often seen through changes in abundance, composition and diversity. As such, community successions which represent timely changes that occur in communities represent interesting avenues for the study of community assemblies.Given the shear complexity of the ecosystems in which these communities originate from, trait-based approaches have been used to provide context to the changes that happen during successions. Such concepts provide a platform to link community changes and ecosystem functioning following successions.In pasture soils, community dynamics are often studied through secondary successions which occur as a result of urine deposition from livestock. Such disturbances trigger a shift in communities, potentially influencing N2O emissions but the mechanisms controlling these successions and the effect on emissions remain elusive. Here, I simulated a urine patch using soil microcosms to trigger a succession and test the role of competition (using biological inhibitors) as a mechanism mediating successions. Community changes were captured through 16S rRNA and 18S rRNA techniques. Subsequently, microbial contributions towards N2O emissions were determined using selective inhibition coupled with 15N tracer techniques. Results confirm prior findings related to a succession based on life history strategies, with urea enforcing a selective pressure on the communities and decreasing species richness. Inhibitors removed competition from groups actively growing under urea leading to recovery of lost species richness.Often times, studies linking both communities and community functional potential are lacking within urine patches. Using the same soils as above, metabolic potential of organism present within these systems were characterized through shotgun metagenomic sequencing. Genome reconstructions and gene annotations revealed a high level of functional redundancy within these systems. Identification of indicator organism for disturbance also provided a glimpse into selection mechanisms that come into play under disturbance events.Although succession triggered under lab settings provide a mechanistic understanding of successions, how accurately does it represent successions in the field?. Using the same urine patch model system, successions were triggered at a field scale with additional parameters involving pH manipulations to identify how liming practices would affect communities under a urine patch, the effect of community changes on N2O emissions and if pH would be a viable emission mitigation tool. Using 16S rRNA and gas measurement techniques, pH and urine had competing roles in shaping successions with successions being disrupted with pH increases. Further analysis, revealed that emissions were uncoupled to community changes with no links found between pH and emissions.
Through the usage of urine patches as model systems, I was able to investigate successions that occur during lab and field incubations and how such changes influence N2O emissions. Using both targeted sequencing and shotgun metagenomic sequencing, I provide context to community changes during successions by linking it to metabolic potential and resolve mechanisms underlying successions. I show that succession observed under controlled lab settings do no represent field successions in its entirety where confounding factors may easily reset these successions and pH may not be a viable mitigation tool of N2O emissions within the context of this study.