STUDY ON THE METHODS FOR MEASURING MICROBIAL BIOMASS NITROGEN IN SOILS

Fumigaton-0.5mol/LK2SO4 extracting NH4+-N directly (named briefly as NH4+-N method), fumigation-waterlogged incubation and fumigation-aerobic incubation methods were used to measure the NH4+-N flush(FN) in 15 soils with a wide range of the organic matter, total nitrogen and C/N ratio. Results showed that there existed significant positive correlation and similar tendency in reflecting soil microbial biomass nitrogen among three methods. The nitrogen flush(FN) measured by 2 incubation methods was almost the same, and microbial biomass nitrogen calculated from their F N was also the same. To the manual loessial soils, the nitrogen flush measured by NH4+-N method was only one-tenth of that of 2 incubation methods. NH4+-N methods was almost equal to these measured by 2 incubation methods. The significant positive correlation existed between the soil microbial biomass nitrogen measured by three methods and soil organic carbon. The moisture condition was easily controlled and there was no ammonia volatilization loss in waterlogged incubatuon, and NH4+-N method had such advatages as simple and rapid, so both of the two methods were superior to the aerobic incubation method. Incubation experiment and results obtained by analysising soils collected from long-term experiments showed that nitrogen flush(FN) and microbial biomass nitrogen calculated from FN obtained from waterlogged incubatin decreased significantly when easily decomposable organic material being contained in the soil. However, the FN measured with NH4+-N method was not affected by easily decomposable organic material. By comparing with microorganism numbers, the results indicated that the waterlogged incubation method was more suitable for soils with little easily decomposable organic material or with small microbial biomass nitrogen amount as the measurement error was smaller and the results were more reliable; vice versa, the NH4+-N method was more reliable for soils with rich easily decomposable organic material. The ratios of nitrogen flush in the same soils determined by the two methods is relatively stable and the ratios of microbial biomass nitrogen measured by two methods was 0.98～1.01, and the average ratio was 1.0, and was not affected by fertilization.