The soil columns incubation experiments were to add enough water to three kinds of paddy soils,then dry those soils under nature environment in a greenhous. The experiments showed Qingzini and Huangbantian with high clay content produced cracks,but Xiaofentu with low clay content didn't appear cracks. The total length, maximal width and depth of cracks in same area had similar curves between Qingzini and Huangbantian on the course of incubation. The produce of cracks had important effects on the activities of nitrate reductase and hydroxylamine reductase,except nitrite reductase. From produce to stabilization of cracks, the activities of nitrate reductase,nitrite reductase and hydroxylamine reductase changed similiarly between Qingzini and Huangbantian.