Chloroform fumigation and the release of soil nitrogen: The effects of fumigation time and temperature

Fumigation with CHC1₃ (24 h, 25°C) increased the amount of NH₄-N and total N extracted by 0.5 M K₂SO₄ from two soils (one arable, one grassland). The amount of N released by CHC1₃ increased with the duration of fumigation up to 5 days, when it levelled off. Between about 10–34% of the total N released by CHC1₃ was in the form of NH₄-N, the proportion increasing with duration of exposure.When a grassland soil that had received a field application of ¹⁵N-labelled fertilizer 1 yr previously was fumigated, the N released by CHC1₃ was 4 times more heavily labelled than the soil N as a whole. Prolonging the exposure of this soil to CHC1₃ increased the amount of total N released, but hardly altered the proportion of labelled N in the CHC1₃-released N, suggesting that N is being released from a single soil fraction. The most likely soil fraction is the soil microbial biomass. It is suggested that CHC1₃ does not alter the K₂SO₄-extractability of soil-N fractions other than microbial N and that the extra N released by CHC1₃ and extracted by K₂SO₄ gives a direct measure of soil microbial biomass N.In contrast to fumigation done at lower temperatures, less total N was released by soil fumigated at 60°C, or above, than was released from unfumigated soil held at the same temperature. The greater release of N in the non-fumigated soils above 60°C could have been due to soil enzymic processes which were inhibited by CHC1₃ in the fumigated soil.