Environmental contextEngineered nanomaterials have the potential to accumulate in agricultural soils where they may influence crop plants. There is, however, little information about how adverse environmental conditions may interact with nanomaterial effects on plants and plant-microbe interactions. We report the comparative effects of three carbonaceous nanomaterials on the growth, nodulation and foliar health of a globally important legume crop, soybean, under the combined stresses of high temperature and insect pests. AbstractBecause carbonaceous nanomaterials (CNMs) are expected to enter soils, the exposure implications to crop plants and plant–microbe interactions should be understood. Most investigations have been under ideal growth conditions, yet crops commonly experience abiotic and biotic stresses. Little is known how co-exposure to these environmental stresses and CNMs would cause combined effects on plants. We investigated the effects of 1000mgkg−1 multiwalled carbon nanotubes (CNTs), graphene nanoplatelets (GNPs) and industrial carbon black (CB) on soybeans grown to the bean production stage in soil. Following seed sowing, plants became stressed by heat and infested with an insect (thrips). Consequently, all plants had similarly stunted growth, leaf damage, reduced final biomasses and fewer root nodules compared with healthy control soybeans previously grown without heat and thrips stresses. Thus, CNMs did not significantly influence the growth and yield of stressed soybeans, and the previously reported nodulation inhibition by CNMs was not specifically observed here. However, CNMs did significantly alter two leaf health indicators: the leaf chlorophyll a/b ratio, which was higher in the GNP treatment than in either the control (by 15%) or CB treatment (by 14%), and leaf lipid peroxidation, which was elevated in the CNT treatment compared with either the control (by 47%) or GNP treatment (by 66%). Overall, these results show that, while severe environmental stresses may impair plant production, CNMs (including CNTs and GNPs) in soil could additionally affect foliar health of an agriculturally important legume.