Active microbial RNA turnover in a grassland soil estimated using a CO2 spike

Rhizosphere microbes are critical to the initial transfer and transformation of root carbon inputs to the soil but our understanding of the activity of these organisms remains constrained by their limited culturability. In this study we combined isotopic ¹³C tracer and molecular approaches to measure the incorporation of recently assimilated plant C into soil microbial RNA and DNA pools as a means to determine the turnover of the ‘active’ rhizosphere community. This required the development of a method for the extraction, purification and preparation of small-sample soil DNA and RNA (<5 μg C) for isotope analysis. Soil, plant and respired CO₂ samples were collected from a ¹³CO₂ pulse-chase experiment at intervals for 20 days post-labelling. The peak of ¹³C release in soil/root respired CO₂ came between 5 and 48 h after ¹³CO₂ pulse-labelling and was followed by a secondary peak of soil heterotroph ¹³C respiration after 136 h. Results showed that both soil DNA and RNA rapidly incorporated recent photosynthate with greatest ¹³C found in the ‘active’ microbial RNA fraction reflecting higher rates of microbial RNA turnover. The dilution rate of the pulse derived ¹³C in RNA-C was used to estimate a microbial RNA turnover of approximately 20% day⁻¹ with a 15-20 day residence time for photosynthate derived ¹³C in the RNA pool. The findings of this work confirm the rapid transfer of photosynthate C inputs through soil microorganisms to the atmosphere as CO₂ and the potential of the biomolecular-isotope tracer approach in soil C research.