Background/Question/Methods Soil microorganisms drive biogeochemical cycles, such as the carbon (C) cycle, through their metabolic activities. Growth and metabolism are inherently connected, so studying microbial community growth dynamics in nature may help us better understand and model how microorganisms influence C cycling. Little is known about the growth responses of individual bacterial taxa in soil communities or how environmental conditions may affect them. We hypothesized that bacterial growth rates would span a wide spectrum of fast to slow growth, and that soil management and C availability would alter bacterial growth responses. We used an internal standard (modified 16S V4 dsDNA oligo of Aquifex aolicus) with 16S rRNA amplicon sequencing to estimate in-situ growth for individual bacterial taxa within soil communities. Growth was measured in cropped and successional soil microcosms incubated with a C-amendment (3.6 mg C/g dry soil) or water as a control. We wrote an algorithm paired with linear regression to estimate the start and end of the exponential phase of growth for individual taxa in the sequenced data. From that estimate we measured three aspects of growth: growth rate, start day (proxy for lag time), and change in internal-standard normalized abundance (proxy for biomass increase). Results/Conclusions We measured in-situ growth for 453 taxa in total after controlling for false positives. Generation times ranged from 0.7 to 64 days with a mean of 5 days. Taxa in C-amended soils started growth earlier than in water-control soils (ANOVA, p-value = 0.002). Likewise, the change in abundance was greater in C-amended compared to water-control soils (ANOVA, p-value = 0.001). These findings indicate that adding C lifted limitations on growth, as expected. However, we found no differences in overall growth rates between treatments (ANOVA, p-value > 0.05). We compared growth rate frequency distributions and found a significant difference between cropped and successional soils in the water-control but not in the C-amended treatment (Fisher’s exact test, p = 0.004 and p > 0.05) indicating that C addition made bacterial community growth responses more similar in differing habitats. Interestingly, these distributions appeared bimodal, suggesting two groups characterized by high or low growth rates. Overall, these findings support our hypotheses and show that 1) sequence data normalized by an internal standard can provide a wealth of growth data that provide new insights and 2) growth responses within soil communities follow complex patterns that may help delineate life history strategies relevant to C cycling.
