Clarifying the relationships among bacteria, lipid-related enzymes, main polyunsaturated fatty acids and fat-soluble vitamins in alfalfa (Medicago sativa L.) silage using various sugar supplementations

•Sugar supplementations altered the bacterial community in alfalfa silage.•The Lactobacillus was increased by molasses to improve fermentation.•Specific bacteria raised lipoxygenase to degrade C18:3n3 and β-carotene. The objective of this study was to clarify the relationships among bacteria, lipid-related enzymes, α-linolenic acid, α-tocopherol and β-carotene in alfalfa (Medicago sativa L.) silages, using without sugars as the control, and with 4 types of sugars (10 g kg−1 fructose, 10 g kg−1 glucose, 10 g kg−1 sucrose, and 20 g kg−1 molasses on fresh matter basis). After ensiling for 70 d, results showed that big losses of α-linolenic acid, α-tocopherol, and β-carotene were observed in the control silage. Supplementations of fructose, glucose, and sucrose increased the contents of α-linolenic acid and polyunsaturated fatty acids (P < 0.05) compared with the fresh alfalfa material. Similar to the control, there was a big loss of α-linolenic acid in the molasses-treated silage due to the high lipoxygenase activity and low activity ratio of lipase to lipoxygenase. Compared with the control, fructose supplementation reduced the losses of α-tocopherol and β-carotene (P < 0.05), and supplementations of glucose and molasses reduced the β-carotene loss after ensiling (P < 0.05). Next-generation sequencing results showed that Pediococcus (25.2 %), undesired Citrobacter (20.6 %), Lactococcus (17.7 %), and Weissella (17.2 %) rather than Lactobacillus (5.12 %) were predominant in the control silage. Molasses-treated silage had 48.1 % lower the abundance of Citrobacter (P < 0.05), and 255 % higher the abundance of Lactobacillus than the control silage (P < 0.05). Consequently, the highest lactic acid concentration and the lowest pH were observed in molasses-treated silage, indicating good fermentation quality. Person correlation analysis showed that Citrobacter (r = 0.84), Pediococcus (r = 0.73), and Lactococcus (r = 0.91) correlated with lipoxygenase positively, and correlated with α-linolenic acid and β-carotene negatively (r < -0.45); Lactobacillus (r = -0.56) and Leuconostoc (r = -0.48) correlated with α-tocopherol negatively. Redundancy analysis exhibited that Citrobacter, Pediococcus, and Lactococcus increased lipoxygenase to degrade α-linolenic acid and β-carotene and Lactobacillus involved in degrading α-tocopherol. In conclusion, sugar supplementations altered the bacterial community in alfalfa silage to reveal the relationships of Lactobacillus, Citrobacter