Abstract: The effects of the interaction between biochar and arbuscular mycorrhizal fungi (AMF) under heavy metal stress on soil nutrients and growth of Cassia occidentalis were studied in a pot treatments. AMF had no effect on soil pH and nutrient content, whereas biochar significantly increased soil pH, organic matter, total nitrogen, total phosphorus, available phosphorus and available potassium, but had no effect on alkali hydrolysable nitrogen and total potassium. Biochar significantly increased the AMF infection rate, but iron modified biochar reduced AMF infection rate. AMF significantly increased a number of root morphological parameters. Under conventional biochar the total root length, root surface area, total projection area, root tip number and root fork number were increased, whereas the corresponding parameters decreased under the iron-modified biochar treatment; both biochar treatments reduced root volume and average root diameter. Compared with the control, AMF inoculation and conventional biochar treatments, all root morphological parameters were increased by the combination of AMF and biochar. AMF and biochar did not influence height, leaf length and leaf width of C. occidentalis in young plants but growth of C. occidentalis with inoculation was significantly higher than that of the non-inoculation treatment as growing period increased. Biochar treatments did not increase plant height, leaf length and leaf width, but biochar and AMF treatments did increase these traits. Shoot and root dry weights of C. occidentalis were associated with plant height. Overall, the interaction between biochar and AMF produced the greatest influence on soil pH, soil nutrients and root system, plant growth and dry matter accumulation.

Key words: biochar, arbuscular mycorrhizal fungi, Cassia occidentalis, root morphology, soil nutrients