Abstract: 【Objective】It was studied the immobilization of nitrogen by microbe and crystal cell in reddish clayey soil after amending urea and straw as well as the dynamics of soil microbial biomass nitrogen (SMBN) and fixed ammonium during the period of rice growth. 【Method】 A flooded pot experiment was conducted during the period of rice growth and composed of four treatments which were control group (CK), labeled urea group (15NU), the combined application of labeled urea and rice straw powder group (15NU-S) and the combined application of labeled rice straw power and urea group (15NS-U). 【Result】 During the period of rice growth, the percentage of SMBN and fixed ammonium to total nitrogen were 1.90% ~ 5.91% and 4.77% ~ 8.37%, respectively; SMBN was minimum at rice booting stage and fixed ammonium was minimum at rice tillering stage; For SMBN, there was no significant difference between fertilizers treatments at rice matured stage and the same to fixed ammonium. The immobilization of urea nitrogen by microbe and crystal cell were 4.01 ~ 15.14 mg·kg-1 and 0.47 ~ 4.82 mg·kg-1, respectively; and the percentage of labeled substrate SMBN and labeled substrate fixed ammonium to urea nitrogen were 1.76% ~ 8.83% and 0.28% ~ 2.11%, respectively. The immobilization of rice straw nitrogen by microbe and crystal cell were 0.97 ~ 2.85 mg·kg-1 and 0.09 ~ 0.50 mg·kg-1, respectively; and the percentage of labeled substrate SMBN and labeled substrate fixed ammonium to rice straw nitrogen were 1.69% ~ 4.98% and 0.15% ~ 0.87%, respectively. The combined application of urea and rice straw powder groups (CAURS, including 15NU-S and 15NS-U) increased the assimilation of microbe, decreased the immobilization of crystal cell. There was a significant negative relationship between SMBN and fixed ammonium. The maximum releasing rate of SMBN in 15NU and CAURS were 66.04% and 69.15%, respectively; those of fixed ammonium in 15NU and CAURS were 87.75% and 87.64%, respectively.【Conclusion】During the period of rice growth, the immobilization of labeled substrate nitrogen by microbe and crystal cell was a dynamic process. SMBN and fixed ammonium reached the minimum at the booting stage and the tillering stage, respectively. The CAURS increased the assimilation of chemical fertilizer by microbe and decreased the immobilization of chemical fertilizer by crystal cell.

Key words: Soil microbial biomass nitrogen, Fixed ammonium, Pot experiment, Reddish clayey soil