Effects of biosolids from a wastewater treatment plant receiving manufactured nanomaterials on Medicago truncatula and associated soil microbial communities at low nanomaterial concentrations

Concern has grown regarding engineered nanomaterials (ENMs) entering agricultural soils through the application of biosolids and their possible effects on agroecosystems, even though the ENMs are extensively transformed. The effects of exposure to biosolids containing transformation products of these ENMs at low concentrations remain largely unexplored. We examined the responses of Medicago truncatula and its symbiotic rhizobia Sinorhizobium meliloti exposed to soil amended with biosolids from WWTP containing low added concentrations of ENMs (ENM Low), bulk/dissolved metals (bulk/dissolved Low), or no metal additions (control). We targeted adding approximately 5 mg/kg of Ag and 50 mg/kg of Zn, and Ti. Measured endpoints included M. truncatula growth, nodulation, changes in the expression of stress response genes, uptake of metals (Ag, Zn and Ti) into shoots, and quantification of S. meliloti populations and soil microbial communities. After 30 days exposure, no effects on root or shoot biomass were observed in ENM Low and bulk/dissolved Low treatments, whereas both treatments had a larger average number of nodules (5.7 and 5.57, respectively) compared to controls (0.33). There were no significant differences in either total accumulated metal or metal concentrations in shoots among the treatments. Expression of five stress-related genes (metal tolerance protein (MTP), metal transporter (MTR), peroxidase (PEROX), NADPH oxidase (NADPH) and 1-aminocyclopropane-1-carboxylate oxidase-like protein (ACC_Oxidase)) was significantly down-regulated in both bulk/dissolved Low and ENM Low treatments. However, a change in soil microbial community composition and a significant increase in total microbial biomass were observed in ENM Low relative to control. The ENM Low treatment had increased abundance of Gram-negative and anaerobic bacteria and reduced abundance of eukaryotes compared to control. The study demonstrated that although there were some subtle shifts in microbial community composition, plant health was minimally impacted by ENMs within the time frame and at the low exposure concentrations used in this study.