Effect of an increasing carbon: nitrate-N ratio on the reliability of acetylene in blocking the N2O-reductase activity of denitrifying bacteria in soil

Summary In model experiments with a silty loam soil the effect of different C : NO _inf3 ^sup- -N ratios on the reliability of C₂H₂ (1% v/v) in blocking N₂O-reductase activity was examined. The soil was carefully mixed with different amounts of powdered lime leaves (Tilia vulgaris) to obtain organic C contents of about 1.8, 2.3, and 2.8%, and of NO _inf3 ^sup- solution to give C : NO _inf3 ^sup- -N ratios of 84, 107, 130, 156, 200, and 243. The soil samples were incubated in specially modified anaerobic jars (22 days, 25°C, 80% water-holding capacity, He atmosphere) and the atmosphere was analysed for N₂, N₂O, CO₂, and C₂H₂ by gas chromatography at regular intervals. Destruction jars were used to analyse soil NO _inf3 ^sup- , NH ₄ ⁺ and C. The results clearly showed that N₂O-reductase activity was completely blocked by 1% (v/v) C₂H₂ only as long as NO _inf3 ^sup- was present. In the presence of C₂H₂, NO _inf3 ^sup- was apparently entirely converted into N₂O. The C₂H₂ blockage of N₂O-reductase activity ceased earlier in soils with a wide C : NO _inf3 ^sup- -N ratio (156, 200, and 243) than in those with closer C : NO _inf3 ^sup- -N ratios (84, 107, and 130). As soon as NO _inf3 ^sup- was exhausted, N₂O was reduced to N₂ in spite of C₂H₂. The wider the C : NO _inf3 ^sup- -N ratio, the earlier the production of N₂ and the less the reliability of the C₂H₂ blockage. In the untreated control complete inhibition of N₂O-reductase activity by C₂H₂ lasted for 7–12 days. In the field, estimates of total denitrification losses by the C₂H₂ inhibition technique should be considered reliable only as long as NO _inf3 ^sup- is present. Consequently, NO _inf3 ^sup- monitoring in the field is essential, particularly in soils supplied with easily decomposable organic matter.