Positive relationship between substrate-induced respiration rate and translationally active bacteria count in soil

Soil microorganisms carry out many biological functions that are fundamental to soil ecosystems; however, less than 2% of soil bacterial cells are commonly estimated to be active at any point in time. Substrate-induced respiration (SIR) measures community-level microbial activity as the bulk soil response to carbon substrate addition. We hypothesized that this bulk respiration rate is dependent on changes in the number of active cells. We incubated subsamples of an agricultural soil with 2 carbon substrate treatments and a third water-only treatment, and we periodically monitored respiration and active cell counts for 24 h. To enumerate active bacterial cells, we applied the novel Bioorthogonal Non-canonical Amino Acid Tagging (BONCAT) method. BONCAT provides a labeled amino acid for active cells to incorporate into newly synthesized proteins, and then a fluorescent dye enables enumeration of labeled cells by flow cytometry.Both respiration rates and active cell counts increased over time and were positively correlated with each other after 6 h of incubation. After 24 h, however, increases in active cells were more than proportional to increases in respiration.Additionally, carbon-amended soils had higher respiration rates than water-only soils with similar active cell counts, suggesting differences in carbon use efficiency. Our results indicate that number of translationally active bacteria is an important but not singular contributor to observed bulk soil respiration. ### Competing Interest Statement The authors have declared no competing interest.