Evaluating the release and metabolism of ricinine from castor cake fertilizer in soils using a LC-QTOF/MS coupled with SIRIUS workflow.

ABSTRACT

Castor cake is a major by-product generated after castor oil extraction and has been widely used as an organic fertilizer. Once applied to soil, a toxic alkaloid ricinine in castor cake may be released into soils and subsequently taken up by crops, which poses a potential threat to food safety and human health. However, the environmental fate of castor cake derived ricinine in agroecosystems remains unclear. In this study, the release and metabolism of ricinine in soils were conducted using soil pot experiments with different castor cake application rates. The analytical methodology of ricinine quantification in soil pore water was first established using solid phase extraction (SPE) coupled with liquid chromatography quadrupole time-of-flight mass spectrometry (LC-QTOF/MS). A non-target screening workflow associated with LC-QTOF/MS and SIRIUS platform was further developed to identify ricinine metabolites in soil pore water. After castor cake application, the ricinine concentrations in soil pore water significantly increased to 297-7990 µg L-1 at 1 day and then gradually decreased to 62.1-3460 µg L-1 at 7 days and 1.70-279 µg L-1 at 14 days for the selected two tested soils with castor cake application rates of 2, 10, and 20 g castor cake/kg soil. In addition, two ricinine metabolites R-194 and R-180 were tentatively identified and one ricinine metabolite N-demethyl-ricinin was confirmed through authentic reference standard for the first time by the developed non-target screening workflow. This study highlights the release and metabolism of toxic alkaloid ricinine in soils once applied castor cake as an organic fertilizer. Ricinine could be released into soil pore water in a short-term after castor cake application and then undergo demethylation, hydroxylation, and hydroxylation followed by methylation metabolisms over time in agroecosystems.