A constant microbial C/N ratio mediates the microbial nitrogen mineralization induced by root exudation among four co-existing canopy species

Abstract Root exudates induce microbial nitrogen (N) mineralization and hence increase soil N availability to plants. To elucidate why the flux rates of root exudates explain microbial N mineralization quantitatively in woody plants, we assessed how the flux rates of root exudates affect the microbial C/N ratio and fungi-to-bacteria (F:B) ratio and therefore the N-degrading enzymes and net N mineralization. We investigated four co-existing canopy species in a warm temperate forest. The flux rates of root exudates were measured in situ using syringe-based cultivation. Microbial biomass carbon (C) and N, fungal and bacterial biomarkers, the activities of N-degrading enzymes, and net N-mineralization rate were measured in both rhizosphere and bulk soils. Microbial biomass C (N) and fungal and bacterial biomarkers were positively related to root exudation rates. Microbial C/N ratio remained unchanged, while the F:B ratio was positively correlated with exudation rates. Mediator analysis suggested that a ‘microbial biomass to N-degrading enzyme’ pathway mediated the microbial N mineralization induced by root exudation, probably because both the fungal and bacterial group contributed to the N-degrading enzyme NAG. In this forest, root exudation stimulated microbial N mineralization linearly due to a growing N demand to sustain a constant microbial C/N ratio. Therefore, the C investment of root exudation can be an efficient N-acquisition strategy for co-existing woody species.