Soil priming by sugar and leaf-litter substrates: A link to microbial groups

The impact of elevated CO2 on leaf-litter and root exudate production may alter soil carbon storage capacities for the future. In particular when so-called ‘priming effects’, the counterintuitive loss of soil carbon following input of organic carbon substrates, are taken into consideration. Here we investigate the dynamics of priming effects and ask whether the source of primed carbon is microbial biomass or soil organic matter and whether specific microbial groups, as identified by phospholipid fatty acid (PLFA) biomarkers, may be important in causing them. We measured δ13C within soil CO2 efflux and PLFA biomarkers following C3 soil priming effects caused by additions of C4 sugar-cane sucrose and maize (Zea mays L.) leaf-litter chopped and ground. All additions caused an initial pulse of priming effect CO2 and a later pulse of substrate-derived CO2, showing that priming effects can be induced rapidly following changes in substrate supply. Priming effects persisted over 32 days and led to a loss of soil carbon, with an increase in soil carbon decomposition of 169% following sucrose addition, 44% following chopped maize and 67% following ground maize additions. An increased concentration of soil-derived carbon within specific PLFA biomarkers provided evidence that a source of the primed carbon was soil organic matter. Certain Gram negative bacteria, identified by PLFA biomarkers (16:1ω5, 16:1ω7), showed increased uptake of soil carbon for both sucrose and maize treatments and may be directly linked to priming effects. Our study provides evidence that substrate carbon inputs to soil induce rapid changes in specific microbial groups, which in turn increase soil carbon metabolism.