The effects of woodchip- and straw-derived biochars on the persistence of the herbicide 4-chloro-2-methylphenoxyacetic acid (MCPA) in soils

• The effect of biochar on soil biological parameters was dependent on soil type.
• MCPA degradation-retarding effect was the highest for straw biochar in sandy soil.
• Biochar influences the soil microbial activity values in the presence of MCPA.
• Sandy soil with MCPA and straw biochar strongly inhibited L. sativum after 100 days.

Sorption and degradation are the primary processes controlling the efficacy and runoff contamination risk of agrochemicals. This study assessed the influence of two biochars, made from woodchips and straw at a pyrolysis temperature of 725 °C and applied to a loamy sand and a sandy soil in the concentration of 5.3 g 100 g−1 sandy soil and 4.1 g 100 g−1 loamy sand soil, or 53 t ha−1 for both soil types, on degradation of the herbicide 4-chloro-2-methylphenoxyacetic acid (MCPA). Soils were spiked with 50 mg MCPA kg−1 soil. In the sandy soil, significantly more MCPA remained after 100 days if amended with straw-derived biochar in comparison to wood-derived biochar. Both biochars types significantly increased urease activity (p<0.05) after 37 days in the loamy sand soil, but these differences disappeared after 100 days. A root and shoot elongation test demonstrated that the soils containing straw-derived biochar and spiked with MCPA, showed the highest phytotoxicity. Both biochars were found to retard MCPA degradation in loamy sand and sandy soils. This effect could not be explained only by sorption processes due to comparatively low developed micro/mesoporous structure of both biochars shown by BET surface analysis. However, an enhanced MCPA persistence and soil toxicity in sandy soil amended with straw biochar was observed and further studies are needed to reveal the responsible mechanisms.