The Effects of Mercury and Lead on Microbial Biomass of Paddy Soil From Southwest of China

The effects of heavy metals of mercury and lead on microbial biomass carbon and microbial biomass nitrogen in soil were studied. The results indicate that mercury and lead all show their toxic effects on microbial biomass carbon and microbial biomass nitrogen, while the toxicity of mercury, is much larger than that of lead. As to Mercury, the critical content which resulted in significant reduction of microbial biomass carbon is 0.6 mg/kg soil, for microbial biomass nitrogen the content is 0.8 mg/kg•soil. As to lead, the critical content is 150 mg/kg•soil for microbial biomass carbon, and 100 mg/kg soil for microbial biomass nitrogen. For microbial biomass nitrogen, when the contents of mercury and lead were respectively between 0.6-1.2 mg/kg•soil and 150- 200 mg/kg•soil, the toxic effects had a sudden increasing, which is a worthwhile phenomenon to study in future.