Effects of biochar amendment on the fate of 17β-estradiol in agricultural soil: Insights from 14C-tracer studies on mineralization, transformation, and non-extractable residue formation.

While biochar (BC) is proposed to mitigate estrogen-associated ecological risks in soil, its effects on estrogen fate remain obscure. We studied mineralization, transformation, and non-extractable residue (NER) formation of 17β-estradiol (E2) in sterilized soil and active soil with 0 %, 0.1 %, and 1 % BC amendment under oxic conditions for 42 days. E2 dissipated rapidly in the active soil (half-life = 1.5 days), mainly forming NERs (65 ± 1 % at the end of experiments) or mineralized (11.1 ± 0.4 %), with generation of minor extractable residues containing estrone (2.0 ± 0.2 %) and one unknown biotransformation product TP (2.5 ± 0.2 %). E2-derived NERs were formed mainly via physico-chemical entrapment (59 ± 3 % of the total NERs) and were associated with humin fraction (66 ± 3 % of the total NERs). BC application had no significant effect on E2 mineralization, transformation into estrone and TP, and NERs quantity, but significantly promoted the formation of physico-chemical entrapped NERs and fulvic acids-bound NERs at 0.1 % level, while increasing BC level might increase the formation of humic acids-bound NERs. Our findings highlight a critical role of NERs in determining environmental risk of E2 and suggest that BC application may not be effective strategy for mitigating estrogen contamination in soil.