Priming effects in soils after combined and repeated substrate additions

In incubation experiments with soil samples from two forest soil profiles (Dystric Cambisol, Haplic Podzol), we investigated the influence of the addition of easily available 14C-labelled organic substrates on the mineralisation of soil organic carbon (SOC). Substrates were added in combination (fructose, alanine) or repeatedly at different time intervals (fructose, alanine, oxalic acid, catechol). During the 1-4-month incubation, CO2-evolution was monitored hourly, while 14CO2 and microbial biomass 12C and 14C were determined at different time intervals. The combined addition of fructose and alanine to the Bs horizon of the Haplic Podzol induced a higher positive priming effect (+127%) than the single substrate additions (+91% and +85%, respectively). During the first 4 days, seven times more SOC was mineralised than in the control, while microbial biomass only increased by a factor of 1.4. These results and the C-balance at the end of the incubation indicate that the observed priming effects can not be solely attributed to co-metabolism or to the turnover of microbial biomass. When added repeatedly, most substrates induced higher positive priming effects, than single additions, thus indicating that the SOC pool affected by priming was not depleted. Since priming effects were not depressed after extended pre-incubation of a soil sample, it seems unlikely that only the labile SOC-pool is susceptible. With repeated substrate additions, substrate mineralisation increased with time, indicating changes in the microbial community structure. In the case of catechol, the increased substrate utilisation was accompanied by subsiding negative priming effects, which in one case even became positive. Although many factors controlling priming effects still remain unclear, the study shows that some of the priming mechanisms discussed by other authors such as co-metabolism and microbial biomass turnover are insufficient to explain the observed data.