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 belowground 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 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 13C while depleting available N, thus reducing the activity of microbial SOM decomposers. In the pine treatment, availab...