Abstract: The mineralization of soil organic carbon(SOC) is one of the most important processes affecting soil carbon cycle and pool size.The freezing-thawing cycles is a common phenomenon in early spring and late autumn in the black soil areas in northern China,which has critical effects on SOC mineralization.However,its response to changes in soil moisture and nitrogen(N) availability under freezing-thawing cycles is unclear yet.Based on a laboratory incubation experiment,the study explored the effects of soil moisture,N addition,and their interactions on soil inorganic N content,microbial biomass,and SOC mineralization of the black soil under freezing-thawing cycles conditions(-10~10℃).Four treatments were set up,namely 60% water holding capacity(WHC) + no N addition,80%WHC + no N addition,60%WHC+N addition and 80%WHC + N addition.The results showed that,both water and N additions had no significant effects on soil NO₃^--N content after 19-day freezing-thawing cycles,while N addition greatly increased soil NH₄⁺-N content.In addition,water addition had no significant effects on soil microbial biomass carbon(MBC) and microbial biomass nitrogen(MBN) contents.Nitrogen addition increased soil MBC content in 60%WHC,while it increased soil MBN content in 80%WHC.The mineralized CO₂ in the freezing and thawing periods accounted for 54 and 46% of the total cumulative CO₂ emission,respectively.Water addition alone inhibited SOC mineralization in the thawing period,and the combined additions of water and N significantly promoted SOC mineralization in the freezing period.During the whole freezing-thawing cycles period,water and N additions interactively affected SOC mineralization.After the freezing-thawing cycles incubation,SOC mineralization was not significantly affected by water addition,regardless of N addition.Nitrogen addition significantly decreased SOC mineralization in 60%WHC soil,whereas it had no remarkable effects in 80%WHC soil.Therefore,soil moisture and N levels have important effects on organic carbon mineralization in black soil under freezing-thawing cycles.