Soils are a huge reservoir of carbon, yet our ability to predict how this important carbon pool will respond to future climate remains poor. Understanding how soil microbial community composition influences ecosystem biogeochemistry could help resolve this knowledge gap. Using a new tool, quantitative stable isotope probing (qSIP), we found taxonomic patterns in the activity of microorganisms; for instance, some bacterial families grew quickly (e.g. Micrococcaceae), while others grow slowly (e.g. Planococcacea). However, it is not clear whether these pattern in activity are robust, or sensitive, to environmental variation. This uncertainty limits our ability to model the influence of microbial biodiversity on ecosystem function. Here we aimed to test the hypothesis that taxonomic identity can explain a significant amount of the variation in prokaryotic growth and carbon assimilation across ecosystems. To this end, we used qSIP with 18O-water and 13C-glucose to measure microbial growth and carbon assimilation in four ecosystems along a climatic gradient of temperature and precipitation in Northern Arizona. These sites consisted of a mixed conifer forest, a ponderosa pine forest, a pinyon-juniper woodland, and a desert grassland.
Results/Conclusions
Although bacterial community composition differed significantly from ecosystem to ecosystem, many microbial taxa were common to more than one study site, allowing an inter-site comparison of activity. Taxon-specific activity was correlated in pairwise comparisons between ecosystems that included all co-occurring bacterial taxa. The strongest relationships were observed for 13C assimilation, where correlations (Pearson’s r) between ecosystem ranged from r=0.55 to r=0.80. Consistent patterns in bacterial activity were also apparent for taxonomic groups. Specifically, family membership explained between 10% and 57% of the variation in bacterial activity across all four ecosystems, supporting our hypothesis. By way of example, members of Paenibacillaceae exhibited above average C assimilation, while members of Solibacteraceae showed very little C assimilation in all four ecosystems. Overall, our results suggest an impact of both taxonomic identity and ecosystem type on the activity of bacterial populations in soil. This may indicate limited phenotypic plasticity in the activity of bacterial taxa, with some ability to respond to environmental variation within consistent ecological strategies. Characterizing the activity of microbial taxa could result in a paradigm shift that enables the incorporation of microbial information into biogeochemical models, leading to dramatic improvements in our ability to predict soil carbon cycling.