The microbial side of soil priming effects

Priming effects (PEs) are defined as short-term changes in the turnover of soil organic matter (SOM) caused by the addition of easily degradable organic compounds. The direction (positive / negative) and magnitude of PEs in response to organic carbon additions are not easy to predict. Yet, PEs are considered to be large enough to influence ecosystem carbon fluxes. The main aim of this thesis is to increase the understanding of the mechanisms involved in soil PEs, with a particular focus on the role of the quantity and quality of added organic substrates, the size of the soil microbial biomass, the soil microbial community structure and mineral nitrogen availability.

The major conclusions are:

1. The degree of resemblance of the chemical structure of the added organic compounds to SOM is an important factor in PEs.

2. PEs are more influenced by trigger substrate concentrations than by the size of the microbial biomass.

3. Triggering of PEs in soils by litter is not only influenced by litter quality but also by the ability of the soil microbial communities to decompose it (home field advantage for PEs).

4. High nitrogen availability can stimulate fungal biomass production but has little effect on PEs.

   The implications of these results for local and global C and N dynamics are discussed.