Comparison of two versions of the acetylene inhibition/soil core method for measuring denitrification loss from an irrigated wheat field

Two versions of the acetylene inhibition (AI)/soil core method were compare d for the measurement 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, t he denitrification rate was measured by analysing the headspace N2O, follow ed by estimation of N2O dissolved in the solution phase using Bunsen absorp tion coefficients. With AI/soil core method B, N2O entrapped in the soil wa s measured in addition to that released from soil cores into the head-space of incubation vessels. In addition, the two methods were also compared for measurement of the soil respiration rate. Of the total N2O produced, 6-77% (average 40%) remained entrapped in the soil, whereas for CO2, the corresp onding figures ranged from 12-65% (average 44%). The amount of the entrappe d N2O was significantly correlated with the water-filled pore space (WFPS) and with the N2O concentration in the headspace, whereas CO2 entrapment was dependent on the headspace CO2 concentration but not on the WFPS. Due to t he entrapment of N2O and CO2 in soil, the denitrification rate on several ( 18 of the 41) sampling dales, and soil respiration rate on almost all (27 o f the 30) sampling dates were significantly higher with method B compared t o method A. Averaged across sampling dates, the denitrification rate measur ed with method B (0.30 kg N ha(-1) day(-1)) was twice the rate measured wit h 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. Results of this study suggest that the N2O and CO2 entrapped in soil should also b e measured to ensure the recovery of the gaseous products of denitrificatio n by the soil core method.