VOLATILIZATION OF AMMONIA DURING THE CHLOROFORM FUMIGATION OF SOIL FOR MEASURING MICROBIAL BIOMASS-N

Ammonia volatilizes from soil samples during their fumigation with CHCl3. Most of the NH3-N was evolved and lost during the application of vacuum, while some was again volatilized during the 24 h incubation period in the desiccator. The total N that could be extracted from soil by a 0.5 m K2SO4 solution did not vary appreciably after fumigation and appeared to be almost completely restored by other sources of N. namely "fixed" NH4+-N, i.e. NH4+ trapped by layer silicates. The ammonia lost during the exposure to CHCI3 may be captured by the same soil sample or by different soil samples present in the desiccator. In order to determine microbial biomass N, it is suggested that the fumigation method should be performed by placing samples of just one soil in each of the desiccators used, and comparing results for fumigated soil samples with those for unfumigated samples, placed under vacuum in similar conditions. © 1990.