A GAS FLOW-THROUGH SYSTEM FOR STUDYING DENITRIFICATION IN SOI

Abstract

A continuous gas ‘now-through’ system, incorporating a soil incubation cell and a gas sampling device, was developed to measure the nitrogen and carbon losses as gaseous N₂, N₂O, NH₃ and CO₂ from soil during denitrification under ‘steady state’ anaerobic conditions. Nitrogen was added as KNO₃ to the soil in amounts equivalent to 0, 30,100 and 300 kg N ha^-1. At 25 °C and one quarter of the water-holding capacity, N₂ and N₂O evolution accounted for most of the nitrogen lost. In the effluent gas stream, N₂O appeared about six hours after anaerobic conditions were established, and increased steadily. Initial N, production was small and reached a maximum between 90 and 100 h with a simultaneous decrease in N₂O evolution. Gaseous N losses were highest with the 100 kg N ha⁻¹ treatment and accounted for 59 per cent of the nitrate lost. A weight ratio, CO₂ -C/(N₂ O + N₂)-N of 1.0–1.3 was observed in the effluent gas once a ‘steady state’ was achieved, which conforms with that expected from nitrate respiration when N, and N₂O are produced simultaneously. When denitrification ceased, CO₂ production decreased to a steady rate. An increase in the duration, rather than the rate, of denitrification was observed when more N was added. A time lag before the onset of a maximum rate of denitrification was also observed, which increased with the amount of added N.