Root influence on soil nitrogen availability and microbial community dynamics results in contrasting rhizosphere priming effects in pine and spruce soil

1. The rhizosphere priming effect (RPE) is increasingly considered an important regulator of below-ground C and N cycling, with implications for terrestrial ecosystem feedback to global change. Even so, there is a lack of knowledge about the mechanisms underlying RPEs. 2. We used novel stable isotope probing methods to investigate RPEs in an experiment with Scots pine and Norway spruce seedlings, two of the most common boreal forest species. We determined root exudation rates, RPEs on SOM decomposition and gross N mineralization, and the contribution of different microbial functional groups to the observed RPEs. 3. Pine induced positive and spruce induced negative RPEs on SOM decomposition, while no RPE on gross N mineralization was observed. Negative RPEs in the spruce treatment were attributed to an opportunistic subset of the fungal community that was growing on root-derived C-13 while depleting available N, thus reducing the activity of microbial SOM decomposers. In the pine treatment, available N was likely sufficient to support the root-induced decomposition activities of fungal decomposers, resulting in positive RPEs. 4. The findings suggest that RPEs, promoted by root stimulation of distinct subsets of the microbial community, can either contribute to conserving the soil C stock or to depleting it, depending on plant species and soil N availability.