摘要Dietary nutrient manipulation(e.g.protein fractions)could lower the environmental footprints of ru-minants,especially reactive nitrogen(N).This study investigated the impacts of dietary soluble protein(SP)levels with decreased crude protein(CP)on intestinal N absorption,hindgut N metabolism,fecal microbiota and metabolites,and their linkage with N metabolism phenotype.Thirty-two male Hu sheep,with an age of six months and an initial BW of 40.37±1.18 kg,were randomly assigned to four dietary groups.The control diet(CON),aligning with NRC standards,maintained a CP content of 16.7％on a dry matter basis.Conversely,the experimental diets(LPA,LPB,and LPC)featured a 10％reduction in CP compared with CON,accompanied by SP adjustments to 21.2％,25.9％,and 29.4％of CP,respectively.Our results showed that low-protein diets led to significant reductions in the concentrations of plasma creatinine,ammonia,urea N,and fecal total short-chain fatty acids(SCFA)(P＜0.05).Notably,LPB and LPC exhibited increased total SCFA and propionate concentrations compared with LPA(P＜0.05).The enrichment of the Prevotella genus in fecal microbiota associated with energy metabolism and amino acid(AA)biosynthesis pathways was evident with SP levels in low-protein diets of approximately 25％to 30％.Moreover,LPB and LPC diets demonstrated a decrease in fecal NH4+-N and NO2--N contents as well as urease activity,compared with CON(P＜0.05).Concomitantly,reductions in fecal glutamic acid de-hydrogenase gene(gdh),nitrite reductase gene(nirS),and nitric oxide reductase gene(norB)abundances were observed(P＜0.05),pointing towards a potential reduction in reactive N production at the source.Of significance,the up-regulation of mRNA abundance of AA and peptide transporters in the small in-testine(duodenum,jejunum,and ileum)and the elevated concentration of plasma AA(e.g.arginine,methionine,aspartate,glutamate,etc.)underscored the enhancement of N absorption and N efficiency.In summary,a 10％reduction in CP,coupled with an SP level of approximately 25％to 30％,demonstrated the potential to curtail reactive N emissions through fecal Prevotella enrichment and improve intestinal energy and N utilization efficiency.