EFFECT OF AN INCREASING CARBON - NITRATE-N RATIO ON THE RELIABILITY OF ACETYLENE IN BLOCKING THE N2O-REDUCTASE ACTIVITY OF DENITRIFYING BACTERIA IN SOIL

In model experiments with a silty loam soil the effect of different C:NO3--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 NO3- solution to give C:NO3--N ratios of 84, 107, 130, 156, 200, and 243. The soil samples were incubated in specially modified anaerobic jars (22 days, 25-degrees-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 NO3-, NH4+ and C. The results clearly showed that N2O-reductase activity was completely blocked by 1% (V/V) C2H2 only as long as NO3- was present. In the presence of C2H2, NO3- was apparently entirely converted into N2O. The C2H2 blockage of N2O-reductase activity ceased earlier in soils with a wide C:NO3--N ratio (156, 200, and 243) than in those with closer C:NO3--N ratios (84, 107, and 130). As soon as NO3- was exhausted, N2O was reduced to N2 in spite of C2H2. The wider the C:NO3--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 NO3- is present. Consequently, NO3-monitoring in the field is essential, particularly in soils supplied with easily decomposable organic matter.