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

Biological denitrification is affected by many environmental factors that control the amount of N₂ and N₂O entering the atmosphere. This study was conducted to measure the effect of water-filled pore space (WFPS), available C, and soil NO₃ concentration on total denitrification (N₂ + N₂O), using acetylene (C₂H₂) inhibition, and to ascertain if denitrification could be estimated from N₂O measurements in the field using an average N₂/N₂O 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 NO₃-N (0, 50, and 100 kg ha⁻¹). The cores were incubated at 25°C, with and without 100 mL C₂H₂ L⁻¹, for 5 d during which daily gas samples of the headspace were analyzed for N₂O and CO₂. 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 N₂/N₂O 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 NO₃ concentrations apparently inhibited the conversion of N₂O to N₂, resulting in lower N₂/N₂O ratios. Using an average N₂/N₂O ratio for estimation of denitrification from N₂O 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 N₂ and N₂O. Published as Paper No. 9865, Journal Series, Nebraska Agricultural Research Division, Lincoln.