Comparison of dicyandiamide and biochar for reducing nitrate leaching under winter forage grazing in Canterbury, New Zealand

Abstract

The presence of nitrate (NO₃⁻) in a water body can contribute to surface water eutrophication and can be harmful to the health of humans. The objective of this study was to determine suitable mitigation options for reducing NO₃⁻ leaching losses under dairy winter forage grazing conditions using dicyandiamide (DCD) and biochar in Canterbury, New Zealand. Total NO₃⁻-N leaching losses from urine-treated soil was decreased by 38% from 379 kg NO₃⁻-N ha⁻¹ to 237 kg NO₃⁻-N ha⁻¹ with the addition of DCD. The addition of DCD in combination with biochar to urine-treated soil reduced the total amount of NO₃⁻-N leached by 46% to 203 kg NO₃⁻-N ha⁻¹. However, the application of biochar to urine-treated soil did not significantly affect the total amount of NO₃⁻-N leached. In addition, the urine + DCD and urine + DCD + biochar treatments significantly reduced both the soil NO₃⁻-N concentration and ammonia oxidising bacteria (AOB) amoA gene abundance. However, the urine + biochar treatment had no effect on the abundance of AOB. The nitrification inhibitor DCD was found to be an effective mitigation tool for reducing soil nitrification rate and thus NO₃⁻ leaching losses. However, biochar was not effective in reducing NO₃⁻ leaching losses at the application rate used in this study.