Denitrification and the Dinitrogen/Nitrous Oxide Ratio as Affected by Soil Water, Available Carbon, and Nitrate

Abstract Biological denitrification is affected by many environmental factors that control the amount of N 2 and N 2 O entering the atmosphere. This study was conducted to measure the effect of water‐filled pore space (WFPS), available C, and soil NO 3 concentration on total denitrification (N 2 + N 2 O), using acetylene (C 2 H 2 ) inhibition, and to ascertain if denitrification could be estimated from N 2 O measurements in the field using an average N 2 /N 2 O ratio. Repacked cores of four benchmark soils were brought to 60, 75, and 90% WFPS by applying treatments of glucose‐C (0, 180, and 360 kg ha −1 ) and NO 3 ‐N (0, 50, and 100 kg ha −1 ). The cores were incubated at 25°C, with and without 100 mL C 2 H 2 L −1 , for 5 d during which daily gas samples of the headspace were analyzed for N 2 O and CO 2 . Total N loss due to denitrification generally increased as soil texture became finer and WFPS increased. The only exception to this was the C‐amended sand, where N losses up to 26 and 66% were recorded at 60 and 75% WFPS, respectively. Denitrification rates at high N concentrations were quite small in the absence of an available C source but increased with increasing available C (glucose). The N 2 /N 2 O ratio generally increased with time of incubation after the initial treatment application. The largest ratios (up to 549) were found at the highest available C rate and generally at the highest soil water content. The presence of high NO 3 concentrations apparently inhibited the conversion of N 2 O to N 2 , resulting in lower N 2 /N 2 O ratios. Using an average N 2 /N 2 O ratio for estimation of denitrification from N 2 O field measurements cannot be recommended because of the variation in this ratio due to the many environmental factors altered by field management that influence denitrification and the relative production of N 2 and N 2 O.