Quantifying apparent and real priming effects based on inverse labelling

Organic inputs to soils can accelerate soil organic matter (SOM) decomposition via so-called priming effects, but at the same time microbial biomass turnover can be accelerated - that will be termed as apparent priming effect. However, only a few studies have been set up to quantify the contribution of extra CO2 production from apparent priming. Here, we labeled the microbial biomass by C-14-glucose in pre-incubation and then added labile carbon (C; C-12 or C-14 glucose) and nitrogen (N; NH4+) in soil and incubated over 120 days to investigate the contribution of apparent priming to total SOM priming. After 120 days of pre-incubation, 48 % of added glucose was released as CO2, and 34 % of added glucose was recovered in microbial biomass. After glucose addition, microbial biomass and salt extractable organic C were similar between glucose and water addition. However, glucose addition increased the contribution of C-14-glucose to microbial biomass by 2.5-folds and to CO2 by 10-folds. This increased contribution of C-14-glucose indicated accelerated microbial biomass turnover by labile C. Furthermore, 10 and 47 mu g C g(-1) of previously added and incorporated into microbial biomass C-14 and C-12 were replaced by new C-14, which contributed to 10 % and 33 % of primed CO2 emissions, respectively. On the contrary, N addition reduced previously added C-14 in both microbial biomass pool and released CO2. This may reflect that the faster microbial turnover contributes to microbial necromass and further to stable SOM formation. Similar total apparent priming (similar to 1.5 mu g C g(-1) in 120 days, mainly in the first 20 days) was observed after glucose and N addition, which contributed around 1-4 % of total priming. Unlike after glucose and N additions, the C-14 released as CO2 (8 % of the remaining previously added C-14-glucose) after water addition was mainly derived from reutilization of microbial necromass. This was supported by the absence of changes in either the C-14 or total C in microbial biomass. Overall, the turnover of the microbial biomass pool - the apparent priming - should not be ignored, since microbial biomass acts not only as a major determinant of SOM turnover but also as a C pool.