Sorption, degradation and bioavailability of oxyfluorfen in biochar-amended soils

To investigate the effects of biochar amendment on the environmental fate of oxyfluorfen in soils, we evaluated the sorption, degradation and bioavailability of oxyfluorfen in various amended soils. The results showed that different biochars, i.e., derived from peanuts (BCP), chestnuts (BCC), bamboo (BCB), maize straw (BCM), or rice hull (BCR), exhibited different sorption capacities for oxyfluorfen in following the order: BCR > BCB > BCM > BCC > BCP. The sorption constant (K_f value) of the three soils (loamy clay, sandy loam, and clay loam) ranged from 62 to 116 (μg/g)/(mg/L)ⁿ and increased by 2.0- to 3.2-fold after introduction of 2% BCR. Furthermore, oxyfluorfen degraded faster in BCR-amended soil than in unamended soil, i.e., degradation increased by 1.0- to 1.4-fold with addition of 2% BCR. Introduction of BCR also decreased the oxyfluorfen uptake by soybean plants by 18-63%, and the capacity of soybeans to absorb oxyfluorfen decreased over time. Amendment with 6 months-aged BCR soil increased the uptake of oxyfluorfen by soybean plants by 2.3-fold compared with freshly prepared BCR soil. After the aging process, the observed increase in oxyfluorfen bioavailability was attributed to the reduced adsorption capacity of aged biochar. Nevertheless, BCR was effective for sequestrating oxyfluorfen as demonstrated by the fact that the adsorption capacity of biochar-amended soil after six months of aging was still 1.5- to 2.5-fold greater compared with that of unamended soil. These results demonstrate that amendment with BCR can be an effective method to modify soil and thereby decrease oxyfluorfen contamination in food crops.