Legume response to rhizobia varies with plant life history and land use

Background/Question/Methods

Grasslands provide a variety of vital ecosystem services, yet many are highly disturbed or degraded and require management to maintain and enhance biodiversity. Recent studies suggest that the soil microbial community may play a key role in the establishment of desirable species that provide key functions in managed grasslands, with the hypothesis that perennial, late-successional species are more dependent on mutualistic microbes found in high-quality remnant grasslands than early-successional or invasive species. We explored how different groups of grassland legumes (native late-successional, native early-successional, and invasive), which associate and nitrogen-fixing rhizobium bacteria, responded to rhizobium communities isolated from grasslands varying in land-use history. In a greenhouse study, we assessed growth and root-nodule production of 16 legume species inoculated with rhizobium communities isolated from remnant prairies, post-agricultural grasslands, and agricultural sites. In a complementary field study of three legume species, we examined growth and survival of inoculated legumes planted into a post-agricultural grassland.

Results/Conclusions

Our greenhouse study showed that native legumes (both late- and early-successional) are more responsive in general to rhizobia than invasive legumes, showing greater growth benefits when inoculated compared to sterilized controls. This effect did not depend on rhizobium origin – native legumes grew equally well and produced similar numbers of root nodules with all rhizobium types. In contrast, invasive legume response depended on rhizobia origin, with plants growing significantly better with rhizobia isolated from agricultural sites than with remnant or post-agricultural rhizobia, and producing significantly fewer root nodules with rhizobia from remnant prairies. In the field we found that rhizobium inoculation had no effect on plant survival and growth in the first growing season, but that inoculation with remnant prairie rhizobia increased survival of a late-successional legume in the second growing season compared to uninoculated plants. These results suggest that legume response to rhizobia varies both with plant life history and rhizobium origin, and that slow-growing, late-successional native legumes may benefit the most from associating with rhizobia from remnant prairies, but these benefits may only be realized over multiple growing seasons.