Use of 14C-Labeled Alfalfa Saponins for Monitoring Their Fate in Soil

Alfalfa seedlings (cv. Cimmaron) were aseptically grown in a glass test tube containing a nutrient salts solution and 14C-labeled sodium acetate into which filtered air was pumped. After five days of exposure to fluorescent light at 22°C, the alfalfa seedlings were removed, washed with cold water, and their saponins extracted. Mean yield of 14C-labeled saponins was 50.2 mg/2.2 g of seed, and mean activity of the 14C-labeled saponins was 5.1 × 104 dpm/mg. Thin-layer chromatography (TLC) of 14C-saponins indicated soyasaponin I, medicagenic acid-3,28 glucoside, and medicagenic acid 3-O-glucoside, plus several unidentified spots, whereas TLC of hydrolyzed saponins (aglycones) showed medicagenic acid, hederagenin, and soyasapogenol B. After 150 hr at 22°C, 17.0% of the 14C-labeled saponins added to a sterile clay loam soil were converted to 14CO2, whereas 54.5% of the saponins were converted to 14CO2 in the nonsterile soil. 14CO2 evolution from each of four nonsterile soils that were amended with 14C-labeled saponins was 57.4–69.9% after 14 days of incubation, and 2.4–24.0% of the added 14C-labeled saponin was recovered in the humic acid fractions from the soils. Only 1.0–2.1% of the 14C label was associated with microbial biomass, as estimated following chloroform fumigation of the soils. Use of 14C-labeled saponins should facilitate a better understanding of the fate of these compounds in soil.