Soil microbial biomass C, N and ninhydrin-N in aerobic and anaerobic soils measured by the fumigation-extraction method

The fumigation-extraction method was tested to see if it could be used to measure soil microbial biomass in waterlogged soil. Three Japanese paddy soils were incubated, aerobically or anaerobically (waterlogged), before being fumigated with CHCl 3 and extracted by 0.5 m K 2SO 4. CHCl 3-fumigation caused large increases in K 2SO 4-extractable C, N and ninhydrin-N in the aerobic soils, as observed previously. Fumigation caused smaller, but comparable, increases in waterlogged soils. The biomass, measured as K 2SO 4-extractablc C released by CHCl 3. declined only slowly, if at all, during an 80 day aerobic incubation. However, under waterlogged conditions K 2SO 4-extractable C released by CHCl 3 declined significantly: in 40 days by 11. 22 and 26% in the three soils. Aeration of waterlogged soils for up to 1 h before extraction did not change the amount of K 2SO 4-extractable C released by CHCl 3. This suggests that O 2 does not need to be rigorously excluded during the sampling or analysis of waterlogged soils. Microbial biomass C (B C) in aerobic soils can provisionally be calculated from the relationship B C = 2.64 E C. where E C is the organic C extracted by 0.5 m K 2SO 4 from fumigated soil, less than extracted from the non-fumigated control.