Dairy slurry could be a significant source of nitrogen (N) for plants, but mismanagementcan lead to atmospheric ammonia losses or nitrate leaching into groundwater. To make the use ofdairy slurry efficient and reasonable, the loss of N pollution to the environment should be reduced.We used repacked lysimeters to comprehensively determine ammonia emission and N leachinglosses in an alfalfa–soil system. The application of dairy slurry had no significant effect on alfalfayield at the same rate of N application in comparison to chemical fertilizer, and adding humic acidssignificantly increased yield by about 12%. However, the application of dairy slurry increased theammonia emission rate significantly, leading to an increase in the cumulative amount of ammoniaemission, while the addition of humic acids reduced the ammonia emissions by 11%. Chemicalfertilizer and dairy slurry application significantly increased nitrate leaching compared to the controltreatment, while the addition of humic acids can significantly reduce ammonium N leaching. Dairyslurry was proven to be as effective as chemical N fertilizer in achieving the optimum biomass,and adding humic acids can significantly reduce N loss to the atmosphere and groundwater. Thisstudy showed the possibility of replacing chemical fertilizer with dairy slurry in alfalfa productionand the advantages of humic acids’ addition to alfalfa to maintain production yield and improveenvironmental friendliness.