Impact of black carbon addition to soil on the determination of soil microbial biomass by fumigation extraction

The efficiency of the fumigation extraction method on the determination of soil microbial biomass carbon and ninhydrin-N was tested in three different soils (UK grassland, UK arable, Chinese arable) amended with black carbon (biochar or activated charcoal). Addition of activated charcoal to soil resulted in a significant decrease in K2SO4 extractable carbon and ninhydrin-N in all three soils, whereas the addition of biochar generally did not. A lower concentration of the extraction reagent (0.05 M vs. 0.5 M K2SO4) resulted in a significantly lower extraction efficiency in the grassland soil. The extraction efficiency of organic carbon was more affected by black carbon than that of ninhydrin-N, which resulted in a decreased biomass C/ninhydrin-N ratio. The impact of black carbon on the extraction efficiency of soil microbial biomass depended on the type of black carbon, on the concentration of the extraction medium and on soil type.