单 位：
;Department of Agronomy, Agricultural College, Yangzhou University, Yangzhou 225009, China.
摘 要：
Two soils with relatively high (Soil 1) and low (Soil 2) ammonium fixation capacities were used to examine the effect of ammonium fixation on the determination of N mineralized from soil microbial biomass. Organism suspension was quantitatively introduced to Soil 1 at various rates. Both fumigation-incubation (FI) and fumigation-extraction (FE) methods were used to treat the soil. The amount of fixed NH4+-N increased with increasing rate of organism-N addition. A close correlation was found betweenthe amount of fixed ammonium and the rate of organism-N addition. The net increases of fixed NH4+-N were equivalent to 38 and 12% of the added organism-N for FI and FE treatments, respectively, in this specific soil. To provide isotopic evidence, 15N-labelled organism-N was added to Soils 1 and 2 at 121.4 mg N/kg. In FI treatment, 22 and 3 mg N/kg of labelled N were found in the fraction of fixed NH4+-N in Soils 1 and 2 respectively; while in FE treatment, 9 mg N/kg of labelled N was found in the fraction of fixed NH4+-N in Soil 1 only. There was no labelled N in the fraction of fixed NH4+-N in Soil 2. In all of the unfumigated (check) soils, there was little or no labelled N in the fixed fractions, probably because the organism-N added was easily mineralized and nitrified. A mean of 0.64 for KN value, the fraction of N mineralized in the killed microbial biomass, was obtained with inclusion of the net increase of fixed NH4+-N. The corresponding value calculated with exclusion of the net increase offixed NH4+-N was 0.46. It was concluded that ammonium fixation was a problem in determination of KN, particularly for soils with a high ammonium fixation capacity. Results also showed that microbial biomass N measurement by FE method was less affected byammonium process than that by FI method.
