Introduction
Biochar is a promising soil amendment that can enhance carbon sequestration, improve nutrient availability, and increase crop yield. To evaluate its potential in mitigating ammonium (NH₄⁺) toxicity, we conducted a glasshouse trial and a soil incubation study using different rates of oil mallee and wheat chaff biochars that were fully characterised prior to the experiments.

Methods
Two experiments were conducted: a glasshouse trial and a soil incubation study. In the glasshouse experiment, we examined the effects of biochar type (oil mallee, wheat chaff), application rate (0, 5, 10, 20 t/ha), and NH₄⁺ supply (15, 60, 120 mg N/kg soil) on canola growth and soil pH.

In the soil incubation experiment, we evaluated the effects of biochar type, application rate (0, 5, 20 t/ha), and NH₄⁺ concentration (60, 120 mg N/kg soil) on soil NH₄⁺, NO₃⁻, total N, pH, and CO₂ emissions. Nitrogen was applied as ammonium chloride (NH₄Cl) in all treatments. Different NH₄⁺ levels were used to match the objectives of each experiment; the greenhouse study required a broader toxicity range, whereas the incubation focused on soil N transformations.

Results
In the glasshouse experiment, both biochar types improved canola shoot and root growth. Oil mallee biochar at 10–20 t/ha most effectively alleviated NH₄⁺ toxicity symptoms, and at 20 t/ha, increased soil pH (CaCl₂) significantly from 5.8 to 6.9.

In the incubation experiment, 20 t/ha oil mallee biochar consistently decreased soil NH₄⁺ concentration and promoted a greater shoot accumulation of NO₃⁻ and total N compared with wheat chaff biochar. The effects on soil pH and CO₂ emissions varied depending on biochar type, application rate, and NH₄⁺ concentration.

Conclusion
These results demonstrate that biochar application can substantially improve canola growth and soil chemical properties. Oil mallee biochar is highly effective in mitigating NH₄⁺ toxicity in canola.