Impact of biochar amendments on copper mobility, phytotoxicity, photosynthesis and mineral fluxes on (Zea mays L.) in contaminated soils

Abstract

Soil copper (Cu2+) contamination is a source of phytotoxicity and biochar incorporation is a possible mitigation strategy for improving plant performance and avoiding Cu2+ contamination. This study evaluated the effect of biochar (0, 1 and 2% BC) and/or copper (0 and 100 ppm) on growth, photosynthesis, Cu2+ accumulation, and mineral content of maize grown under controlled greenhouse conditions. Biochar application increased water holding capacity (WHC), while the addition of Cu2+ improved electric conductivity (EC) in soil samples. The sole copper treatment enhanced Cu-accumulation, mineral uptake (K+, Na+, Ca++ and Mg++), dark respiration and light dependent electron flux in PSII. However, it also led to a reduction of growth and net CO2 assimilation rate and caused an enhanced ETR/Pg, indicating a threat of ROS production. The sole addition of biochar to the soil had a positive impact on the total macronutrient content but a negative impact on growth, leaf CO2/H2O gasexchange. However, the combined application of BC and Cu2+ led to a reduced tissue Cu2+ levels, higher shoot growth CO2/H2O gasexchange (Pn, WUE, COE) and finally a reduced ETR/Pg ratio. Plants treated with biochar and higher Cu2+ levels decreased about 30% ETR/Pg which is associated with the optimum growth and photosynthetic efficiency of Photosystem II. Hence, it could be clearly shown that BC and Cu2+ achieved together, and contrary to their individual mode of action, an improvement of Pn, WUE, COE and Rd. We assume that BC application in the studied soil highly restricted the availability of added Cu promoting Cu-stable forms thus reducing the environmental risk and phytotoxicity at soils high in copper (2%BC, 100 ppm Cu2+). The study proposes that BC as a suitable amendment for restoring degraded soils with toxic trace elements but not for soils with limited supply of this micronutrient or copper deprivation