We determined the gross rates of nitrification and nitrate immobilization in soil using a 15N-nitrate isotope dilution technique. We also measured the amounts of nitrate immobilization and denitrification simultaneously by combining the acetylene inhibition technique and mass balance of nitrate. The immobilization rate of nitrate was enhanced by about 2 orders of magnitude by the addition of straw, while it did not increase significantly by the addition of manure. The denitrification rate in soil under anaerobic conditions was remarkably enhanced in the presence of straw, and enhancement of denitrification was also observed to some extent by the addition of manure or bark manure. In the soils treated with straw, the immobilization rate of nitrate under aerobic conditions was 24% of the denitrification rate under anaerobic conditions. Effect of water addition on nitrate immobilization and denitrification in soil was investigated in the presence or absence of straw. The amount of nitrate immobilization in the soils with straw did not change drastically with the moisture content, while that of denitrification changed by 2 or 3 orders of magnitude with the moisture content between 54-84% of the maximum water-holding capacity of soil irrespective of straw addition.