Soybeans Grown with Carbonaceous Nanomaterials Maintain Nitrogen Stoichiometry by Assimilating Soil Nitrogen to Offset Impaired Dinitrogen Fixation

Abstract

Engineered nanomaterials (ENMs) can enter agroecosystems because of their widespread use and disposal. Within soil, ENMs may affect legumes and their dinitrogen (N₂) fixation, which are critical for food supply and N-cycling. Prior research focusing on end point treatment effects has reported that N₂-fixing symbioses in an important food legume, soybean, can be impaired by ENMs. Yet, it remains unknown how ENMs can influence the actual amounts of N₂ fixed and what plant total N contents are since plants can also acquire N from the soil. We determined the effects of one already widespread and two rapidly expanding carbonaceous nanomaterials (CNMs: carbon black, multiwalled carbon nanotubes, and graphene; each at three concentrations) on the N economy of soil-grown soybeans. Unlike previous studies, this research focused on processes and interactions within a plant-soil-microbial system. We found that total plant N accumulation was unaffected by CNMs. However, as shown by ¹⁵N isotope analyses, CNMs significantly diminished soybean N₂ fixation (by 31-78%). Plants maintained N stoichiometry by assimilating compensatory N from the soil, accompanied by increased net soil N mineralization. Our findings suggest that CNMs could undermine the role of legume N₂ fixation in supplying N to agroecosystems. Maintaining productivity in leguminous agriculture experiencing such effects would require more fossil-fuel-intensive N fertilizer and increase associated economic and environmental costs. This work highlights the value of a process-based analysis of a plant-soil-microbial system for assessing how ENMs in soil can affect legume N₂ fixation and N-cycling.

Keywords: 15N isotope; carbon black; carbon nanotubes; dinitrogen fixation; graphene; nanomaterials; soybeans.