Biol Fertil Soils (1999) 29 : 328–331 Q Springer-Verlag 1999 T. Mahmood (Y) 7 R. Ali 7 F. Azam Nuclear Institute for Agriculture and Biology, P.O. Box 128, Faisalabad 38000, Pakistan Fax: c92-041-654213 K.A. Malik National Institute for Biotechnology and Genetic Engineering, P.O. Box 577, Faisalabad 38000, Pakistan SHORT COMMUNICATION T. Mahmood, R. Ali, F. Azam, K.A. Malik Comparison of two versions of the acetylene inhibition/soil core method for measuring denitrification loss from an irrigated wheat field Received: 12 May 1998 Abstract Two versions of the acetylene inhibition (AI)/soil core method were compared for the measure- ment of denitrification loss from an irrigated wheat field receiving urea-N at a rate of 100 kg ha –1 . With AI/ soil core method A, the denitrification rate was mea- sured by analysing the headspace N 2 O, followed by es- timation of N 2 O dissolved in the solution phase using Bunsen absorption coefficients. With AI/soil core method B, N 2 O entrapped in the soil was measured in addition to that released from soil cores into the head- space of incubation vessels. In addition, the two meth- ods were also compared for measurement of the soil respiration rate. Of the total N 2 O produced, 6–77% (average 40%) remained entrapped in the soil, whereas for CO 2 , the corresponding figures ranged from 12–65% (average 44%). The amount of the entrapped N 2 O was significantly correlated with the water-filled pore space (WFPS) and with the N 2 O concentration in the headspace, whereas CO 2 entrapment was depend- ent on the headspace CO 2 concentration but not on the WFPS. Due to the entrapment of N 2 O and CO 2 in soil, the denitrification rate on several (18 of the 41) sam- pling dates, and soil respiration rate on almost all (27 of the 30) sampling dates were significantly higher with method B compared to method A. Averaged across sampling dates, the denitrification rate measured with method B (0.30 kg N ha –1 day –1 ) was twice the rate measured with method A, whereas the soil respiration rate measured with method B (34.9 kg C ha –1 day –1 ) was 1.6 times the rate measured with method A. Re- sults of this study suggest that the N 2 O and CO 2 entrap- ped in soil should also be measured to ensure the re- covery of the gaseous products of denitrification by the soil core method. Key words Acetylene inhibition 7 Soil core technique 7 Denitrification 7 Irrigation 7 Nitrous oxide entrap- ment Introduction Methods for the direct measurement of denitrification gaseous N flux are based on 15 N and acetylene inhibi- tion (AI), the latter being used more commonly be- cause of its lower cost and higher sensitivity (Ryden and Rolston 1983). Soil cover (Ryden et al. 1979) and soil core (Ryden et al. 1987) versions of the AI method have been widely used for the direct measurement of denitrification loss under field conditions, and the soil core version has generally been reported to yield high- er figures (Arah et al. 1991; Mahmood et al. 1998). However, the conventional AI/soil core method may still underestimate denitrification because substantial amounts of N 2 O may remain entrapped in soil (Aulakh and Doran 1990). Different procedures adopted to re- lease the N 2 O entrapped in soil cores include: vigorous shaking of soil cores (Aulakh and Doran 1990), break- ing up of soil cores followed by shaking (Ambus and Christensen 1993), and shaking of the broken up soil cores with excess water (Rice and Smith 1982). Howev- er, in most of the denitrification studies employing the AI/soil core method, N 2 O entrapment in soil was not considered. In a previous study with wheat (Mahmood et al. 1998), the denitrification loss measured by the AI/soil core method was 3.4 kg N ha –1 , in contrast to 33.1 kg N ha –1 of the total fertilizer-N loss recorded with the 15 N- balance method. A higher N loss with the 15 N-balance compared to the AI/soil core method was partly attri- buted to losses other than denitrification, and to the underestimation of denitrification, most probably due to the entrapment of N 2 O in soil cores. The present