| Abstract: | Soil microbial C and N (Cmic, Nmic) estimation by the chloroform fumigation‐extraction method is erroneous in densely rooted soils due to CHCl3‐labile C and N compounds. The effect of a pre‐extraction with 50 mM K2SO4 and a pre‐incubation (conditioning at 25 °C for 7 days) on the flush in extractable, CHCl3‐labile C (C‐flush) and N (N‐flush) was tested with reference to rooting density (0.3—75 mg root dry matter g—1) in one arable and 3 grassland soils. In the arable soil and in the second horizon (10—20 cm) of a grassland soil, C‐flush values were not affected by the pre‐extraction. However, the pre‐extraction considerably reduced C‐flush values in the top soils of the grassland (above 10 cm). Only about 42 % was found in the pre‐extracted roots and the rest was lost during the pre‐extraction. The estimated concentrations of Nmic decreased due to pre‐extraction of soil samples with low root biomass. Clearly, the concentrations of Nmic were underestimated by introducing the pre‐extraction. Soil pre‐incubation reduced C‐flush values only slightly, whereas N‐flush values were not affected. It can be concluded that (1) CHCl3‐labile root C and N is partly extracted with K2SO4 after pre‐incubation and (2) CHCl3‐labile C and N removed with the roots during pre‐extraction is partly derived from microbial biomass. Soils with low rooting density (arable soils, grassland soils below approximately 10 cm depth) should therefore be fumigated and extracted without pre‐extraction. In densely rooted soils, fumigation extraction with and without pre‐extraction probably gives estimates for the minimum and maximum of Cmic and Nmic. |
