Estimation of microbial biomass nitrogen by chloroform fumigation extraction method in a forest soil

Abstract The amounts of K2SO4 extractable total N (Nt(K2SO4)), organic N (Norg(K2SO4)), and inorganic N (NH4 + – N(K2SO4), NO3 − – N(K2SO4)) were measured before and after CHCl3 of previously non fumigated soils at seven days intervals over a period of nine weeks. Soil samples were taken in plastic tubes down to a soil depth of 30 cm from a Scots pine forest soil (Eutric Cambisol) in NE-Saxony, Germany, in spring 2002 and incubated at 15°C and 75% of relative humidity. Each week, microbial N was measured in the samples of following soil depths: organic layer (Oea), 0 – 5 cm, 5 – 15 cm and 15 – 30 cm). During the experiment, microbial N was slightly reduced in the organic layer while in the mineral soil no clear trend was detected. In all soil depths and at any time of the investigation, the Nt(K2SO4) content as well as the Norg(K2SO4)/Nt(K2SO4) ratio were significantly higher (p ⩽ 0.001; p ⩽ 0.01) after the chloroform-fumigation than before. At any time of the investigation, the NH4 + – N(K2SO4) content in the organic layer was obviously higher after the fumigation (p ⩽ 0.001). The content of NO3 − – N(K2SO4) increased continuously over the course of time (p ⩽ 0.05). In few cases, the negative N-flux (EN) calculated for the Norg(K2SO4) was associated with a positive EN of the NH4 + – N(K2SO4) of the organic layer. Thus, an enormous percentage of the organic material seems to be ammonified during the 24-hour fumigation procedure. In consequence of the increasing NH4 + – N-pool in the organic layer the process of nitrification was enhanced. The results of this study show that the chloroform-fumigation-method should be discussed carefully concerning the determination of the microbial biomass N in organic layers.