Microbial catabolic diversity in and beyond the rhizosphere of plant species and plant genotypes

- Microbial substrate-induced respiration was studied in rhizosphere and bulk soils associated with ten plant species in field soils.
- Rhizosphere soils exhibited greater substrate-induced respiration than bulk soils for all species except three of the four grasses tested.
- Eight grassland species were observed to differentially influence microbial community level physiological profiles (CLPPs) in rhizosphere and bulk soils.
- Four genotypes of potato were observed to differentially affect microbial CLPPs in the rhizosphere but not bulk soil.
- No differences were observed among mustard genotypes in microbial CLPPs for either the rhizosphere or bulk soils.

Microorganisms in the rhizosphere drive important ecosystem processes such as nutrient cycling and organic matter decomposition. Microbial activity in the rhizosphere is a function of both rhizodeposition and the soil's inherent microbial community. In this study, we investigated plant species and genotype effects on microbial functioning in the rhizosphere and compared it to the corresponding bulk soil. We investigated the rhizospheres and bulk soils from eight natural grassland species (four grasses and four forbs) in a long-term biodiversity experiment and genotypes of two crop species (Solanum tuberosum and Brassica juncea) in a short term experiment. Soil microbial functioning was assessed by determining microbial catabolic diversity, which is the microbial response to addition of several carbon-rich substrates. Substrate-induced respiration was higher in the rhizosphere than in the bulk soil for all plant species and genotypes, except for the grasses Agrostis capillaris, Anthoxanthum odoratum and Holcus lanatus, which yielded similar microbial activities in the two soil zones. Microbial catabolic profiles in the rhizospheres of Rumex acetosa, Leucanthemum vulgare and Plantago lanceolata were most distinct from each other and from the other grassland species. The bulk soil's microbial community catabolic profile was also species dependent. For S. tuberosum we found a genotype effect on the microbial catabolic profile in the rhizsophere but not in the bulk soil. For Brassica juncea no such genotype effects were found in the rhizosphere or bulk soil. This is a first step to link microbial rhizosphere activities to soil functioning in natural and agricultural ecosystems.