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-degrees-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.