Biochar increases arbuscular mycorrhizal plant growth enhancement and ameliorates salinity stress

• AM fungi and biochar had a moderately additive effect on plant yield.
• AM fungi and/or biochar treatment improved plant nutrition, especially P, K and Mn.
• Biochar alleviated salinity stress in plants, likely via salt sorption.

We examined combined effects of biochar, arbuscular mycorrhizal (AM) fungi and salinity on plant growth and physiology to test whether and how biochar influences AM fungi mediated growth and nutrition enhancements, and whether and how biochar provides amelioration in salt stressed soils. We carried out a full three-factorial greenhouse experiment with Lactuca sativa; and a second study with a wider range of biochar and salt additions to examine physicochemical effects on soil parameters. Biochar together with AM fungal inoculation resulted in an additional plant yield increase compared to each alone under non-saline conditions. In parallel with increased plant growth, we found increased uptake of P and Mn with AM fungi and biochar addition, but to a lesser extent than biochar-induced growth promotion. Both factors, but especially biochar alleviated salinity-caused growth depressions, and improved Na/K ratio in salinity stressed plants. Reduced Na uptake of plants and reduced conductivity in biochar-ameliorated soils suggest that a likely mechanism involves ion adsorption to biochar surfaces. Our results suggest that plants depend on symbiotic microorganisms to fully exploit biochar benefits in soils, suggesting avenues for joint management in agriculture. Biochar may be advantageous in saline soils, but long-term studies are required before recommendations should be given.