With the rapid increase in global population that is anticipated over the coming decades, more food will need to be produced to meet the increasing demand. The current agricultural production system is unable to meet this challenge, and as such, crop cultivation will need to occur under more marginal conditions. Soil salinization and the consequent salt stress on crops will become increasingly problematic. In this study, we prepared two kinds of biochar nanoparticles (NPs) (particle size 253 and 259 nm) from rice and corn straw at 350 °C and investigated their use as a soil amendment to alleviate salt stress (2000 mg kg⁻¹ biochar NPs; 1000/2000 mg kg⁻¹ NaCl) on tomato seedlings. After four weeks of growth, the biomass of tomato seedlings was reduced by 48.7% under 2000 mg kg⁻¹ NaCl treatment. With rice straw biochar NP application, the biomass increased by 13.1% over the stress controls. In addition, chlorophyll (SPAD value) was increased by 46.7% with rice straw biochar NPs. The rice straw biochar NPs inhibited the transfer of Na from roots to shoots; the Na translocation factor (TF) was 0.46, which is significantly lower than the corresponding value (0.72) for the saline stress controls. The corn straw biochar NPs also contributed to salt stress alleviation in tomato, although the effects were lower than those with rice straw biochar NPs, and there was no reduction in Na transport. However, at lower levels of salt stress with 1000 mg kg⁻¹ NaCl treatment, there was little difference in the effects of the two biochar nanoparticles. These findings suggest that straw-derived biochar may be an effective soil amendment to increase the salt stress tolerance of crop species and can serve as an effective tool to facilitate food production under marginal soil conditions.

The exogenous application of nanoscale biochar has great potential in agriculture to improve crop resistance to pathogens, although the mechanisms underlying this physiological response are unclear. We investigated the potential of nanoscale and conventional biochar amendment to induce resistance in Nicotiana benthamiana upon infection with the pathogen Phytophthora nicotianae. Nanoscale biochar synthesized at 350 °C (400 mg L⁻¹) from corn straw with a particle size of ∼160 nm had a beneficial role in the active immunity of N. benthamiana through the ethylene pathway to provide resistance to P. nicotianae infection; in leaf assays, lesion diameters were reduced by 9.26% as compared to controls. Mechanistically, a 20.3% increase in reactive oxygen species (ROS) from biochar NP amendment activated the ethylene pathway, followed by SA-induced systemic acquired resistance (SAR) expression. The SA-induced immunity increased over 10-fold in the two biochar NP treatments. In addition, the biochar NPs induced PR1-a to a significantly greater extent than did regular biochar particles. In an experiment with the plant elicitor 5-methoxyindole, biochar NPs served as a unique delivery platform to improve plant growth, over 105.3% in the shoot length and 41.2% in the root length after seed treatment with biochar NPs and 5-methoxyindole compared to the untreated controls. This study demonstrates the successful use of exogenous nanoscale biochar amendment to stimulate plant defense responses and subsequent resistance to the important pathogen P. nicotianae. This approach holds great promise as a new plant protection strategy in sustainable nano-enabled agriculture.