Castor cake, the primary by-product of castor oil production, has become globally popular organic fertilizer due to its high nitrogen content and rapid mineralization rate, which could fulfill nitrogen sources for organic crop production. However, castor cake also contains alkaloid ricinine, a toxin could stimulate the human central nervous system and cause seizures. Significant amounts of toxic alkaloid ricinine may be released into soils after large scale land application of castor cake, but this potential impact on the soil ecosystem has been overlooked. Better understanding the fate of released toxic alkaloid ricinine in soil and its influence on the functional diversity of bacteria after castor cake application is needed. Therefore, this study used optimized solid-phase extraction (SPE) method coupled with a liquid chromatography quadrupole time-of-flight mass spectrometry (LC-QTOF/MS) to analysis ricinine concentration in soil pore water collected from soil pot experiments with different castor cake application rates. The metabolic potential of bacteria was determined through community level physiological profiling. The release of ricinine was observed in soil pore water with the concentrations decreased with time for all the castor cake application rates. Additionally, the increase in the application rates resulted in the increase of not only the ricinine concentration but also the metabolic potential of fast-growing bacteria. However, the application rate of 20 g castor cake per kilogram soil resulted in the reduce of the functional diversity index of fast-growing bacteria. The utilization of several carbon sources such as β-methyl-D-glucoside, D-galactonic acid γ-lactone, D-galacturonic acid, L-asparagine, D-mannitol, N-acetyl-D-glucosamine, D-cellobiose, and Putrescine was positively related to castor cake application rates. The decrease of ricinine concentration could be the result of bacterial activities, and therefore in-depth studies on determining bacterial community structure is needed to further clarify the relation of released ricinine and bacteria in soils.

Potential for release of toxic alkaloid ricinine and for influence of bacterial functional diversity from castor cake after soil application

ACS Spring 2022

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