Priming effects in the rhizosphere and root detritusphere of two wet-grassland graminoids

The rhizosphere and root detritusphere are hotspots of microbial activity, where root-derived inputs induce intensive priming effects (PE) on soil organic carbon (SOC) decomposition. These conditions for induced PE differ between rhizosphere and detritusphere and are modified by plant traits. Continuous labelling with 13C-depleted CO2 allowed for the partitioning of plant and soil C sources of CO2 efflux and the investigation of the PE in the rhizosphere and detritusphere of slow-growing conservative Carex acuta and fast-growing acquisitive Glyceria maxima. Glyceria allocated more C into the soil, induced higher microbial activity and a larger portion of active microorganisms, and depleted mineral N stronger than Carex. Its rhizosphere PE was 2.5 times stronger than that of Carex. Root residues (detritusphere) induced negative PE at the early stage of decomposition (1–9 months). The depletion of available organic substances in the detritusphere of more easily decomposable Glyceria roots resulted in positive PE after 3 months. The PE in the detritusphere of N-poorer Carex roots was more intensive but started after 9 months. The rhizosphere PE was positive and stronger than the detritusphere PE, which switched from initially negative to positive PE after depletion of available substances during few months. More productive species with faster N-uptake and higher belowground C input (here Glyceria) induce larger rhizosphere PE than slower-growing species (here Carex). The N-rich Glyceria roots decompose faster than N-poor roots of Carex and, consequently, have a lower impact on SOC dynamics and induced a smaller positive detritusphere PE.