As the world’s potable water supply dwindles and our surplus of reclaimed wastewater grows, it is becoming critical that we understand the ecological impacts of the application of this water on natural ecosystems. Our study examined the responses of three plant species and their associated root symbionts to reclaimed wastewater. The effects of nutrients and other pollutants in reclaimed wastewater were uncoupled by comparing plant and symbiont responses to:  1) reclaimed wastewater (R); 2) a nutrient-matched control based on evaluation of the nutrients in the reclaimed wastewater (N); and 3) de-ionized water (D).  Seeds of three plant species where germinated and grown in natural forest soils in a greenhouse. The legume Oxytropis lambertii and the grass Bromus anomalus were native to the forest soils while the legume Medicago sativa was not native in these soils.  The following measurements were taken after 12 weeks of plant growth:  1) root, shoot, and total plant biomass; 2) photosynthetic capacity (SPAD); 3) legume nodule count and weight; and 4) arbuscular mycorrhizal (AM) colonization. 

Results/Conclusions:  

Our results show that: 1) total biomass and root:shoot ratio did not respond to the water treatments in any of the plant species; 2) in all three plant species, photosynthetic capacity was greatest in plants watered with the N solution; 3) the abundance and biomass of root nodules in the two legume species were not influenced by the treatments; however, in Oxytropis, the relative nodule mass per gram root was significantly greater in the D treatment compared to the R and N treatments; 4) root colonization by AM fungi was lower in the D treatment compared to the R and N treatments in Medicago. These findings suggest that nutrients in reclaimed wastewater may influence the formation of rhizobium and AM symbioses in some plant species. It is likely that nitrogen present in reclaimed wastewater inhibited nodule formation in Oxytropis and enhanced AM fungal colonization in Medicago. Other studies have shown that nitrogen enrichment may enhance AM colonization by making phosphorus relatively more limiting to plants. The fact that the three plant species differ in their responses to the water treatments suggests that reclaimed wastewater may impact the community composition of plants and their root associates by inhibiting some species and stimulating others.