Priming effects in Chernozem induced by glucose and N in relation to microbial growth strategies

Input of easily available C and N sources increases microbial activity in soil and may induce priming effects (PE)—short-term changes in SOM decomposition after substrate addition. The relationship between the origin of priming and growth characteristics of the microbial community is still unclear. We related real and apparent PEs induced by glucose and N addition with growth strategies of soil microorganisms. Two concentrations of uniformly labeled 14C glucose with and without N were added to Chernozem, and the released 14CO2 and CO2 efflux were monitored over a 300h period. The shift in strategies after glucose addition was monitored by microbial growth kinetics based on the estimation of maximal specific growth rate. The production of unlabelled extra CO2 induced by glucose was completed after 3 days and amounted to about 15–19% of the microbial biomass-C. The presence of real or apparent PE depended on the level of added C and N. An apparent positive PE was observed when the amount of applied glucose-C was 13 times lower than the amount of microbial biomass-C, i.e. under C-limiting conditions. Apparent PE was accompanied by a higher maximal microbial specific growth rate, i.e. by a shift towards r-strategy features. The absence of a priming effect was observed under N-limiting conditions at an eightfold excess of glucose-C versus microbial biomass-C. A large excess of glucose and N lowered maximal specific growth rates of soil microorganisms and had a negative priming effect. Accordingly, slow-growing microorganisms (K-strategists) switched from SOM mineralization to glucose uptake, probably due to preferential substrate utilization. Analysis of microbial growth kinetics was an efficient approach for evaluating short-term changes in the response of microorganisms to substrate addition; this approach is therefore suitable for assessing transitions between K and r strategies.