Ammonium immobilisation during chloroform fumigation

• Published in: Soil biology & biochemistry Vol. 35; no. 5; pp. 651 - 665
• Main Authors: Müller, Christoph, Stevens, R.J, Laughlin, R.J, Ottow, J.C.G, Jäger, H.-J
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01.05.2003; New York, NY Elsevier Science
• Subjects: 15N; Agronomy. Soil science and plant productions; Biological and medical sciences; Chloroform; Fumigation–extraction; Fundamental and applied biological sciences. Psychology; Immobilisation; Methods and techniques; Microbial biomass; Soil science; 15N; Chloroform; Microbial biomass; Immobilisation; Fumigation–extraction; Ammonium; Fumigation; Fumigation-extraction
• ISSN: 0038-0717; 1879-3428
• DOI: 10.1016/S0038-0717(03)00014-2

The assumption in using the chloroform fumigation technique for microbial biomass determination is that microbes are killed or at least inactivated by the treatment. Problems associated with transformations of the N released on or during fumigation have so far only been associated with the fumigation–incubation method. A laboratory and a field study were carried out to investigate the possible N transformations during biomass determination by the fumigation–extraction method. Labelled NH 4NO 3 (either the NO 3 −, NH 4 + or both pools were 15N enriched) was applied to the soil and biomass determinations made at intervals subsequently. The size and enrichment of the ammonium (NH 4 +), and nitrate (NO 3 −) pools were determined before and after chloroform fumigation. The 15N enrichment of the NH 4 + pool after fumigation could only be explained if immobilisation of ammonium occurred at some time during the 24 h fumigation period. The extent of this immobilisation was calculated. In addition, there was evidence that nitrification occurred during the fumigation procedure at the start of the laboratory study and throughout the field study. The laboratory and field study differed mainly in the dynamics related to NO 3 − uptake and release. There was evidence for uptake of NO 3 − by the microbial biomass with and without utilization. We conclude that the 15N enrichment in the microbial biomass cannot be accurately determined when N transformations and release of non-utilized N occurs during fumigation. The possible immobilisation of mineral N during fumigation will affect the magnitude of the factor used to convert measured microbial biomass N to actual microbial biomass N in soil.