Use of ¹⁴C-Labelled Model Exudates to Evaluate Plant Species Effects on Carbon Use Efficiency in Rhizosphere Soil

Rhizodeposition, defined as the flow of organic carbon from plant roots to soil, plays an important role in the soil carbon cycle: it represents a major flux of photosynthetic C from plant to soil and helps shape the structure of the rhizosphere microbial communities that are responsible for the rhizodeposit decomposition. Growing evidence suggests that below-ground plant inputs make a large contribution to the stable soil organic carbon (SOC) pool which is composed mainly of OC that has been processed by the microbial biomass rather than selectively preserved plant litter C. Therefore, it follows that the efficiency by which rhizosphere microbial communities process rhizodeposited carbon to microbial products (as opposed to CO₂; i.e. the carbon use efficiency, CUE) will be important for the long-term stabilisation potential of plant-derived C in the rhizosphere. Here we examine how CUE in rhizosphere soil varies with respect to plant species with a focus on both legume and non-legume grassland species. We determine CUE through tracking the fate of ¹⁴C-labelled model exudates (glucose, succinate, cysteine, salicylic acid) in sampled rhizosphere soil and examine the relationships with rhizosphere microbial community structure. Results show that C source significantly affects the CUE of the rhizosphere (p<0.001), however, no plant species effect was found (p=0.866). Phospholipid fatty acid analysis (PLFA) and 16S and ITS amplicon sequencing were used to evaluate linkages between the use efficiency of carbon and the phenotypic and genotypic characteristics of rhizosphere soil microbial communities of the different plant species.