Rhizosphere nitrogen cycling: Intra- and inter-specific variation among agronomically important summer annuals
The rhizosphere is a hotspot of soil nitrogen cycling with increased rates of nitrogen mineralization and subsequent cycling commonly observed. Evidence that plant genotypes vary in their effects on these processes has lead to growing interest in selection for rhizosphere traits to support agroecosystems with greater reliance on soil organic nitrogen (SON) pools rather than large pools of inorganic N. However, there is little understanding of the extent of variation among plants in rhizosphere N cycling or the significance of such variation for plant N acquisition and productivity. To address this knowledge gap, a common garden experiment was established to evaluate intra- and inter-specific variation in growth characteristics and plant rhizosphere effects among agronomically important summer annuals. Twenty six genotypes, including maize inbred lines, C4 grasses, and mycorrhizal and non-mycorrhizal broad-leaved plants were grown in monoculture with and without supplemental nitrogen (0 – 95kg N ha-1) in a randomized split-plot design. At flowering, plants were sampled for characterization of plant traits, including above ground biomass, tissue carbon: nitrogen ratio, specific leaf area and leaf N content. Rhizosphere soils were analyzed to determine the potential activity of enzymes mediating carbon (BX and CB) and N (LAP and NAG) acquisition, and to characterize microbial community structure through miSeq sequencing of tagged SSU rRNA amplicons.
Several results highlight the importance of plant variation in access to SON. Average daily N uptake in unfertilized plots varied over three-fold, while N uptake response from added fertilizer ranged from 0 to 225%. Potential activity of C and N accessing extracellular enzymes was higher in rhizosphere samples compared to the bare soil controls and the magnitude of this effect varied considerably between genotypes. Across species, plant C:N ratio at anthesis was positively correlated with the activity of the N accessing enzyme LAP. In turn, LAP activity was positively correlated with rates of N uptake across species. However, these relationships were not observed for the maize inbred lines. These results support the hypothesis that plant growth characteristics are linked with rhizosphere function and raises questions about controls on rhizosphere function among the maize in-bred lines. Plant variation in rhizosphere microbial community will be discussed in relation to extracellular enzyme activity and plant N acquisition. By linking rhizosphere function with plant growth characteristics, this work can inform future plant breeding efforts and contribute to a broader understanding of plant controls on rhizosphere N cycling.