Soil microbes are important drivers of nutrient availability, and are thus of paramount importance to ecosystem carbon (C) and nitrogen (N) cycles. In grasslands, fire maintains ecosystem N limitation, but increased N availability due to fertilization or fire suppression may change this status. In many studies, N fertilization caused a turnover of soil microbes to putatively “copiotrophic” populations, in other words, populations expected to have faster growth but lower growth efficiency. However, few measurements of bacterial traits to asses these tradeoffs exist. After 30 years of chronic N fertilization and fire cessation at a long-term tallgrass prairie field site (Konza Prairie Biological Station), we determined the growth rate and efficiency traits of diverse isolated microbes. We anticipated that bacteria from unburned and fertilized treatments would exhibit higher growth rates and lower growth efficiencies. To attain as diverse a collection of study strains as possible, bacteria were isolated using with a combination of eight different culturing approaches, resulting in 346 strains (between 44-70 from each field treatment) from 7 different bacterial Classes and 22 different Families. Growth rate and carbon respiration at exponential growth phase was estimated from culture turbidity (absorbance) curves and CO2 production (Picarro CO2 analyzer), respectively.
Results/Conclusions
Using a linear model in RStudio, ANOVA results from bacteria analyzed to date showed that neither fire history (burned and unburned treatments), chronic fertilization history, nor current fertilization status affected average bacterial isolate growth rates or efficiencies (P-values > 0.1). Instead, the growth rate was related to the taxonomic Class of the strain (F = 14.6, P < 0.0001). There was a very weak negative relationship between growth rate and efficiency (R = -0.14, P = 0.08), thus expected tradeoffs themselves were not detected. Data from our current pool of 186 isolates suggests that land management practices have no significant effect on bacterial growth rate or efficiency. However, growth rate is related to the phylogenetic affiliation of the strain. Overall, this may mean that the predicted copiotroph versus oligotroph tradeoff framework needs further evaluation, or that more detailed or in situ methods should be applied to evaluate the predictions. Continuing work will consider how these bacterial isolates might be used to learn more about the potential functions of the highly diverse field soil microbial community.