Rhizosphere bacterial carbon turnover is higher in nucleic acids than membrane lipids: implications for understanding soil carbon cycling

Using a pulse-chase 1321 CO2 plant labeling experiment we compared the flow of plant 22 carbon into macromolecular fractions of root-associated soil microorganisms. Time dependent 1323 C dilution patterns in microbial cellular fractions were used to calculate their 24 turnover time. The turnover times of microbial biomolecules were found to vary: microbial 25 RNA (19 h) and DNA (30 h) turned over fastest followed by chloroform fumigation 26 extraction-derived soluble cell lysis products (14 d), while phospholipid fatty acids (PLFAs) had the slowest turnover (42 d). PLFA/NLFA 1327 C analyses suggest that both mutualistic 28 arbuscular mycorrhizal and saprophytic fungi are dominant in initial plant carbon uptake. In contrast, high initial 1329 C enrichment in RNA hints at bacterial importance in initial C uptake 30 due to the dominance of bacterial derived RNA in total extracts of soil RNA. To explain this 31 discrepancy, we observed low renewal rate of bacterial lipids, which may therefore bias lipid 32 fatty acid based interpretations of the role of bacteria in soil microbial food webs. Based on 33 our findings, we question current assumptions regarding plant-microbe carbon flux and 34 suggest that the rhizosphere bacterial contribution to plant assimilate uptake could be higher. 35 This highlights the need for more detailed quantitative investigations with nucleic acid 36 biomarkers to further validate these findings