Three-source-partitioning of microbial biomass and of CO2 efflux from soil to evaluate mechanisms of priming effects

We propose and successfully applied a new approach for 3-source-partitioning based on a combination of ¹⁴C labeling with ¹³C natural abundance. By adding ¹⁴C-labeled glucose to soil after C₃ - C₄ vegetation change, we partitioned three C sources in three compartments, namely CO₂, microbial biomass and dissolved organic C (DOC). This enabled us to estimate mechanisms and sources of priming effects (PE).Glucose application at low and high rate (GL: 100 and GH: 1000 μg C g⁻¹, respectively) caused positive PE both short-term (during 1-3 days) and long-term (3-55 days). Despite a 10-fold difference in the amount of substrate added, the PE observed was larger by a factor of only 1.6 at the high versus low rate of glucose. The real and apparent priming effects were distinguished by partitioning of microbial C for glucose-C and SOM-derived C. As the amount of primed CO₂ respired during short-term PE was 40% lower than microbial C, and the contribution of soil C in microbial biomass did not increase, we concluded that such short-term PE was apparent and was mainly caused by accelerated microbial turnover (at GL) and by pool substitution (at GH). Both the amount of primed CO₂-C, which was 1.3-2.1 times larger than microbial C, and the increased contribution of soil C in microbial biomass allowed us to consider the long-term PE as being real. The sole source of real PE (GL treatment) was the “recent” soil organic matter, which is younger than 12-year-old C. The real PE-induced by a glucose amount exceeding microbial biomass (GH) was due to the almost equal contribution of ‘recent’ (<12 years) and ‘old’ (>12 years) C. Thus, the decomposition of old recalcitrant SOM was induced only by an amount of primer exceeding microbial C. We conclude that combining ¹⁴C labeling with ¹³C natural abundance helped disentangle three C sources in CO₂, microbial biomass and DOC and evaluate mechanisms and sources of PE.