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.