Plant and Soil 149: 292-296, 1993. © 1993 Kluwer Academic Publishers. Printed in the Netherlands. PLSO 9638 The effect of acetylene on N transformations in an acid oak-beech soil W. DE BOER, P.J.A. KLEIN GUNNEWIEK, R.A. KESTER, A. TIETEMA 1 and H.J. LAANBROEK 2 Department of Soil Biology, Centre for Terrestrial Ecology, Netherlands Institute of Ecology, P.O. Box 40, 6666 ZG, Heteren, The Netherlands. ~ Laboratory of Physical Geography and Soil Science, University of Amsterdam, Nieuwe Prinsengracht 130, 1018 VZ Amsterdam, The Netherlands. 2 Correspondmg author Received 5 June 1992. Accepted in revised form in 11 December 1992 Key words: acid forest soil, acetylene, nitrification, N mineralization Abstract The effectiveness of acetylene (C2H2) as inhibitor of nitrification was studied in relation to the decomposition of C2H 2. This was done by examining the effects of single and multiple additions of different C2H 2 concentrations (10, 100, 1000 Pa) on mineral N and NO~-N production in samples of the organic (FH) and upper mineral (Ah) layer of an acid oak-beech forest soil. The decomposition of C2H 2 was much faster in Ah samples than in FH samples. A single addition of 10 Pa C2H 2 was not sufficient for complete inhibition of nitrification in the Ah samples. Nitrification was blocked completely by all other C2H 2 treatments in both FH and Ah samples. Addition of C2H 2 decreased net mineral N production in Ah samples but not in FH samples. Addition of carboxymethyl-cellulose and chitin to Ah soil had no affect on the rate of decomposition of C2H 2. Chitin had a negative effect on net NO3-N production. Introduction Autotrophic ammonium oxidizers appear to be much more sensitive to C2H 2 than other mi- croorganisms (Hynes and Knowles, 1982; Schimel et al., 1984). Therefore, low concen- trations of C2H 2 have been used to inhibit autotrophic ammonium oxidation without affect- ing other processes. This has greatly improved our knowledge of N transformations in soils. Two major aspects of N dynamics that have been examined using CEH 2 are: (1) The source of NzO production i.e. either denitrification or nitrification (e.g. Klemedtsson et al., 1988). (2) The discrimination between heterotrophic and autotrophic nitrification (e.g. Adams, 1986). Furthermore, it has been proposed that C2H 2 may be used to improve the reliability of the assessment of soil nitrogen mineralized on aerobic incubation (index of nitrogen availabili- ty) because it prevents the loss of recently mineralized N via nitrifcation and subsequent denitrification (Warren, 1988). One of the major advantages of the use of C2H 2 over other nitrification inhibitors such as nitrapyrin and dicyandiamide is that it complete- ly permeates the soil matrix even if the samples have a high organic matter content (Sahrawat et al., 1987). This makes C2H 2 in contrast to the other compounds an effective inhibitor of nitrifi- cation in a wide range of soils. However, the effectiveness of inhibition may decrease during the incubation as microbial decomposition of C2H 2 in soils has been shown to occur (Terry and Duxbury, 1985). This may pose a problem in studies that aim to elucidate the nature of nitrification, as the C2H 2 concentrations that are