Biochar Reduces Aminopyralid Residues and Phytotoxicity in Dairy Manure Compost

Aminopyralid (2-pyridine carboxylic acid, 4-amino-3, and 6-dichloro-2-pyridine carboxylic acid) is an auxin herbicide widely used to control broad leaf weeds in pasture and hay fields. Aminopyralid compound in forage material can pass through livestock into manure. Composts derived from aminopyralid-contaminated manure can cause phytotoxic effects in sensitive crop plants. Biochar has shown synergetic effects in composting and can immobilize organic pollutants that present in compost. This experiment examined the effects of incorporating 0%, 2%, 4%, and 10% (w/w) biochar for composting dairy manure containing 50 µg kg−1 aminopyralid (wet base) in 140 L plastic rotary drum reactors. Residual aminopyralid concentration after 2, 6, and 12 m composting periods, phytotoxicity effects of compost on tomato (Lycopersicon esculentum L.) plants, and the key chemical characteristics of composts after 6 and 12 m curing were assessed in two runs. After 12 months of curing, the aminopyralid concentration in the 10% biochar treatment decreased by more than 90% and eliminated the phytotoxicity of the compost. Improved adsorption and immobilization by biochar accounted for over 57% of the reduction in the 10% BC treatment. Biochar addition slightly increased the C/N ratio and total N content significantly but did not markedly impact the N transformation. The results indicate that biochar incorporation can be used as an effective practical tool to enhance the agronomic biosafety of bovine compost originated from persistent auxin herbicide aminopyralid-contaminated dairy manure.