Rhizosphere bacterial community composition responds to arbuscular mycorrhiza, but not to reductions in microbial activity induced by foliar cutting

Authors


  • Editor: Kornelia Smalla

Correspondence: Mette Vestergård, Section for Terrestrial Ecology, Department of Biology, University of Copenhagen, Ø. Farimagsgade 2D, DK-1353 Copenhagen Ø, Denmark. Tel.: +45 35324304; fax: +45 35322321; e-mail: mevmadsen@bi.ku.dk

Abstract

Differences in bacterial community composition (BCC) between bulk and rhizosphere soil and between rhizospheres of different plant species are assumed to be strongly governed by quantitative and qualitative rhizodeposit differences. However, data on the relationship between rhizodeposit amounts and BCC are lacking. Other soil microorganisms, e.g. arbuscular mycorrhizal fungi (AMF), may also influence BCC. We simulated foliar herbivory (cutting) to reduce belowground carbon allocation and rhizodeposition of pea plants grown either with or without AMF. This reduced soil respiration, rhizosphere microbial biomass and bacteriovorous protozoan abundance, whereas none of these were affected by AMF. After labelling plants with 13CO2, root and rhizosphere soil 13C enrichment of cut plants were reduced to a higher extent (24–46%) than shoot 13C enrichment (10–24%). AMF did not affect 13C enrichment. Despite these clear indications of reduced rhizosphere carbon-input, denaturing gradient gel electrophoresis (DGGE) of 16S rRNA genes PCR-amplified targeting DNA and RNA from rhizosphere soil did not reveal any effects of cutting on banding patterns. In contrast, AMF induced consistent differences in both DNA- and RNA-based DGGE profiles. These results show that a reduction in rhizosphere microbial activity is not necessarily accompanied by changes in BCC, whereas AMF presence inhibits proliferation of some bacterial taxa while stimulating others.

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