Objective  To understand the changes of microbial indexes related to nitrogen transformation in paddy field under “thin-shallow-wet-dry” and alternate drying and wetting irrigation methods.

Method  We used three irrigation methods (CIR: conventional irrigation, TIR: “thin-shallow-wet-dry” irrigation, DIR: alternate drying and wetting irrigation), and three nitrogen (N) treatments (FM1: all urea, FM2: 30% urea substituted by pig manure, FM3: 50% urea substituted by pig manure). The numbers of nitrite bacteria, nitrate bacteria, denitrifying bacteria and the activities of urease, hydroxylamine reductase, nitrite reductase and nitrate reductase were measured at tillering, booting, milky and ripening stages. The relationships among the microbial activity indices were analyzed.

Result  At booting stage, the numbers of nitrite bacteria, nitrate bacteria and denitrifying bacteria were relatively large, and the activities of urease, nitrite reductase and nitrate reductase were relatively high in soil under different treatments. Hydroxylamine reductase activity was relatively high by DIR method. Under FM3 treatment, compared to CIR method, DIR method increased the number of nitrite bacteria by 2.31 times at tillering stage, increased the number of nitrate bacteria and the activities of urease, hydroxylamine reductase and nitrite reductase by 2.07, 0.81, 554.72 and 1.78 times from tillering stage to milky stage, but reduced the average number of denitrifying bacteria and nitrate reductase activity by 31.34% and 43.82% respectively at four growth stages. The differences of microbial indexes related to nitrogen transformation in paddy soil between TIR and CIR methods varied with N treatments and growth stages. Using DIR method, compared to FM1, FM3 significantly increased the number of nitrite bacteria at milky stage, the number of nitrate bacteria at booting and ripening stages, the number of denitrifying bacteria at tillering and ripening stages, urease activity at milky and ripening stages, hydroxylamine reductase activity from tillering stage to milky stage, and activities of nitrite reductase and nitrate reductase at booting stage. Except the insignificant correlations among nitrate reductase activity, the number of nitrate bacteria and nitrite reductase activity, there were significant correlations among other microbial indexes.

Conclusion  The rice booting stage is the active stage of microbial activity related to nitrogen transformation in paddy field. DIR method can effectively enhance most microbial indexes related to nitrogen transformation in soil at tillering and booting stages. TIR method and FM3 treatment can significantly increase microbial activities related to nitrogen transformation in soil at milky and ripening stages.