Soils are a huge reservoir of carbon, exceeding phytomass and atmospheric carbon combined. Anthropogenic increases in CO2 are expected to augment primary productivity and thus enhance carbon transfer from the atmosphere to the soil potentially increasing soil carbon storage. However, the consequences of enhanced primary production on soil carbon storage remain unclear as microbial decomposition activities respond dynamically to fresh carbon substrates. Specifically, the decomposition of native soil organic C can be reduced or enhanced in response to fresh carbon inputs, a phenomenon known as the “priming effect”. We used quantitative stable isotope probing with 13C-labeled glucose and 18O-labled water to measure individual and community level activity in order to understand how microbial activity mediates priming in soil.
Results/Conclusions
Initially labile carbon addition decreased soil carbon mineralization (negative priming) but over time repeated additions increased the mineralization of soil carbon (positive priming). This shift in activity was associated with an increased relative abundance of Proteobacteria and TM7 and a decrease in the proportion of Acidobacteria and Actinobacteria. By comparing changes in 18O assimilation (growth) due to labile C addition with the amount of 13C assimilation from the added substrate, we assessed the changes in soil carbon utilization induced by fresh carbon inputs. Initially labile carbon was being consumed in lieu of soil organic matter, a phenomena often called preferential substrate utilization, causing the negative priming. After repeated carbon additions, labile carbon increased the growth of most prokaryotic taxa. This additional growth was achieved using a mixture of the added carbon and the soil organic matter resulting in enhanced native carbon mineralization explaining the positive priming. To understand how responses to labile carbon addition were distributed across bacterial taxa we categorized changes in activity and tested for phylogenetic clustering. Most bacterial taxa were involved in priming, and these organisms were not phylogenetically clustered. This suggests that increased growth and soil carbon utilization in response to fresh carbon inputs is routine among bacteria and does not require specialized physiological or ecological attributes. Consequently, priming may not be strongly constrained by bacterial biodiversity in soil.