Mutualistic soil microbes are important in their potential to shape ecosystems as they provide essential nutrients to many plant species. Plants in the legume family (Fabaceae) participate in a highly integrated symbiotic relationship with nitrogen-fixing rhizobia bacteria. The nutritional benefit derived by the host plant renders the association crucial for range expansion. Understanding the potential co-dependency of this species interaction can give insight into patterns of invasion by exotic legume species in North America. With the invasive legume Medicago polymorpha (Burr clover), we asked if seeds dispersed outside of the host patch lack symbionts and if certain plant genotypes are more resilient to symbiont limitation. We tested for the presence of rhizobia in soils collected at increasing distances from M. polymorpha populations to determine if there is a small-scale mutualist decline away from the host. Field soil samples from eight southern California host plant populations were planted with two invasive and two native plant genotypes in a greenhouse common garden, and half were inoculated with a lab strain rhizobia. All plants were fertilized with isotopic nitrogen, 15N, so that the %N(dfa) could be calculated from leaf tissue. The %N(dfa) is indicative of how much nitrogen in the plants is derived from symbiont fixed nitrogen.We measured plant biomass and counted root nodules to determine if plant fitness was symbiont limited outside focal populations.
Results/Conclusions
We found that plant nodule counts decreased rapidly to zero with distance outside of the focal population and shoot mass followed a similar functional form of decline. The number of nodules produced varied significantly by host plant genotype even when exposed to the same rhizobium density; however, plant genotype did not significantly affect shoot mass. Preliminary results show that the %N(dfa) decreases away from the host plant patch. These results suggest that the fitness of seeds dispersed outside of the host patch is symbiont limited, but there may be genetic variation upon which selection could act to reduce dependence on rhizobia or select for more generalist hosts.