Host and nutrient availability interact to influence microbial communities’ effects on plant fitness

Background/Question/Methods

Mutualism theory suggests that resource mutualists should be under directional selection for increased partner quality. However, mutualists vary (often widely) in their quality. Two possible contributions to this variation in mutualism quality are: 1) variation in availability of traded resources and 2) variation in host plant availability. For example, in the legume-rhizobium system increased nitrogen availability and reduced host availability both are likely to result in increased rhizobium residence times in the soil environment (as opposed to interacting with the plant host). If rhizobium traits associated with survival in the soil environment are negatively correlated with rhizobium traits associated with mutualism quality, less cooperative rhizobium mutualists may be favored in these environments. Crop rotation and cover cropping are two agricultural practices that affect host and resource availability in the soy-rhizobium mutualism. Crop rotation means that rhizobia spend some growing seasons as free-living soil bacteria rather than associating with soy; this could create a tradeoff if selective pressures in the soil are at odds with selection for higher-quality mutualists. Cover cropping with red clover increases available soil nitrogen, but does not affect host availability since soy and clover partner with different genera of rhizobia. To examine the effects of temporal variation in host availability (crop rotation or no crop rotation) and increased nitrogen availability (clover cover crop increases soil nitrogen), I capitalized on a long-term (20 year) experiment at the Kellogg Biological Station LTER. Specifically, I inoculated soy plants in the greenhouse with soil collected from plots that had been under varying cropping regimens for 20 years and measured plant growth, nodule production, and also isolated rhizobia from each of these long-term treatments.

Results/Conclusions

Preliminary data shows that the effect of crop rotation on soil microbes’ effects on plant fitness depends on the presence of a clover cover crop. Soil microbes from long-term crop rotation treatments became significantly less helpful (or more detrimental) when a clover cover crop was present, but the opposite effect was seen in plots that had no crop rotation (soy every year) (aboveground biomass ~ years without soy * clover presence, p=0.017). These results suggest that host availability and soil nutrient availability interact to influence belowground microbial communities in ways that matter to plant growth. I am currently isolating rhizobia strains from each plot to examine the effects of each treatment on the rhizobia (rather than whole soil) communities.