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Rhizosphere
Volume 17, 2021, 100317

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

Lijuan Suna,b, Mioko Atakab, Yuji Kominamib,1, Kenichi Yoshimurac,2, Kanehiro Kitayamab

Institute of Ecology, College of Urban and Environmental Sciences, Key Laboratory for Earth Surface Processes of the Ministry of Education, Peking University, Beijing, 100871, China.

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.

Keywords: Nitrogen cycle, Plant-soil interaction, Fungi:bacteria ratio, Microbial C/N ratio, N-acquisition strategy.

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