Study On Mechanism Of Fermentation And Screening Of Lactic Acid Bacteria Of Alfalfa Silage In Saline-alkali Soil

As an important base for alfalfa planting,saline-alkali land has been paid more and more attention by the government and society.However,due to its special habitat conditions,because of its high buffering capacity value,and low water-soluble carbohydrates content,which is not conducive to the processing of high-quality alfalfa silage.However,there are few reports on the fermentation mechanism of alfalfa silage in saline-alkali land.To ensure the fermentation quality and clarify the mechanism of alfalfa silage fermentation.This study aims to provide a theoretical basis for alfalfa silage in saline-alkali land.In this study,LS(salt content 1‰～ 2‰),MS(salt content 2‰～ 3‰),HS(salt content 3‰～ 4‰)salt stress treatment,and CK(salt content<1‰),Zhongmu No.3 alfalfa as test material grown in four treatments lands.The single-factor and two-factor experimental designs were adopted.First,we carried out a comprehensive evaluation of the nutritional quality,microbial community,and metabolites of raw alfalfa in saline-alkali land.Secondly,the nutritional quality,fermentation quality,microbial quantity,and community dynamics of alfalfa silage in different saline-alkali fields were analyzed.The lactic acid bacteria with the high acid-producing ability and fast growth rate were screened.Finally,the screened lactic acid bacteria were used as additives to make alfalfa silage,and further explain its silage fermentation effects.The main research conclusions are as follows:(1)Comprehensive evaluation of crude protein(CP),soluble carbohydrates(WSC),neutral detergent fiber(NDF),acid detergent fiber(ADF),and relative feed value(RFV)of alfalfa in saline-alkali land showed that the nutrient quality of alfalfa in MS saline-alkali land was the best,with CP content 23.43%,WSC content 7.47%,and RFV of 121.03.The expression levels of proline and L-serine were significantly up-regulated,and the total amino acids(14.65%)were significantly increased.The metabolic pathways of alfalfa were mainly concentrated in amino acid metabolism,carbohydrate metabolism,and energy metabolism.In addition,the bacterial communities of alfalfa in saline-alkali land belong to 173 bacterial genera in 4 phyla,and the dominant genera were Pantoea,Pseudomonas,and Sphingomonas.(2)After 60 days of fermentation,the nutrient and fermentation quality of alfalfa silage in MS salt-alkali land was the best with and without cellulase.Salt stress treatment,fermentation days,and the interaction between the two factors were the key factors affecting the nutrient quality,fermentation quality,and microbial quantity of alfalfa silage in different saline-alkali lands.The addition of cellulase to alfalfa silage significantly decreased p H value(P < 0.05),increased lactic acid content(P < 0.05),and promoted the fermentation of silage.The CP content was 21.5%,WSC content was 5.12%,p H value was 4.43,and lactic acid content was 5.85% in the MS group.The number of lactic acid bacteria increased significantly,while the number of aerobic bacteria,escherichia coli,and mold decreased significantly(P < 0.05).Sodium ions of alfalfa silage in saline-alkali land had a great influence on bacterial community structure,and the main fermentation genera were Enterococcus,Lactococcus,and Lactobacillus.(3)A total of 16 strains of lactic acid bacteria were isolated from raw alfalfa and silage in saline-alkali land and identified by physiological and biochemical tests and 16 S r RNA sequence homology analysis.Two strains belonged to Leuconococcus,YQM51 was L.mesenteroides and YQF18 was L.Citreum.Two strains belonged to Enterococcus,YQF25 was E.ureilyticus YJX03 was E.Hirae.Three strains belonged to Pediococcus,YQH72,YQM54,and YQL32 were P.pentosaceus.Nine strains belonged to Lactobacillus,YQM62 was L.Brevis,and YJX08 was L.farciminis,YQH75,YQM48,YQM42,YQH79,YQM66,YQH76,YQL31 were L.plantarum.A strain YQM48 L.plantarum with high acid production and fast growth was screened out.(4)We added the selected strain YQM48(L.Plantarum)to alfalfa silage in MS saline-alkali land that significantly improved the quality of alfalfa silage.After 60 days of silage fermentation,The CP content in L.Plantarum group was 21.9%,which was significantly higher than CK(P < 0.05).The p H value and lactic acid content of L.Plantarum group were 4.3 and 4.9% respectively,which were significantly higher than CK(P < 0.05).The addition of L.Plantarum changed the bacterial community diversity and structure of alfalfa silage in salt-alkali land.The dominant genera in the L.Plantarum group were Lactobacillus,Lactococcus,and Enterococcus in the control group.