Effects of graphene oxide on tomato growth and soil chemical properties

Graphene oxide (GO) nanomaterials have been widely applied in open environments at present, thus inevitably entering agricultural ecosystems. Current research on the effects of GO on crops is mostly limited to hydroponic conditions. It is often required to reflect real-world soil scenarios for agricultural safety. This study aims to investigate the effects of GO on tomato plants and soil chemical properties. The optimal concentration of GO was then determined for tomato cultivation. The soil samples with GO nanomaterials were selected from the potato onion germplasm nursery of Northeast Agricultural University, Heilongjiang Province, China. The variety of tomato, "Dongnong 722", was taken as test material. Different GO concentrations (50, 100, and 200 mg/kg) were added to the soil, with soil without GO (0 mg/kg) as the control. Tomato seedlings at the two-leaf-one-heart stage were transplanted into 9 cm × 9 cm seedling pots. Samples were collected before transplanting (day 0), 20, and 40 days after transplanting to measure relevant indicators of tomato growth, nutrient uptake, soil chemical properties, and enzyme activity. The results showed that the GO exhibited a hormonal effect on tomato growth: Low to moderate concentrations promoted tomato growth, while high concentrations had no significant promoting effect. The optimal growth of tomato was observed at a concentration of 100 mg/kg: plant height and stem diameter at 40 days after transplanting were significantly higher than those in the control and the 200 mg/kg treatment; Total plant dry weight and root dry weight significantly increased at both 20 and 40 days; Root volume and root surface area also increased significantly, especially at 40 days. In nutrient uptake, the nitrogen content at 40 days was significantly higher in the 100 mg/kg treatment than that in the rest, while the phosphorus content at both sampling time points was significantly higher in the 100 mg/kg treatment than that in the control and the 200 mg/kg treatment. Only the 100 mg/kg treatment significantly increased the potassium content of tomato plants. In soil properties, GO nanomaterials had no significant effect on pH values. Electrical conductivity increased with increasing GO concentration at 20 days, but there was the opposite trend at 40 days. At both 20 and 40 days, the ammonium nitrogen content in the 100 mg/kg treatment was significantly higher than that in the rest, while nitrate nitrogen content showed the opposite trend. At 20 days, the 100 mg/kg treatment shared the highest content of available phosphorus, while the highest content of available potassium was found at both 20 and 40 days. In enzyme activity, the highest urease activity was observed in the 100 mg/kg treatment, while acid phosphatase activity increased with increasing GO concentration. The catalase activity showed no significant difference among all treatments. In conclusion, the optimal GO concentration was 100 mg/kg for tomato growth in soil. Tomato growth was promoted by the activity of key soil enzymes. The ratio of ammonium nitrogen to nitrate nitrogen was optimized to reduce the phosphorus and potassium loss, thus improving root development and nutrient absorption. There was a positive regulatory effect of GO amounts on tomato growth. The findings can provide theoretical support and a safe concentration for the GO application in tomato cultivation.