Effect of acetylene on nitric oxide production in soil under denitrifying conditions

Acetylene inhibition of N2O reduction to N-2 is traditionally used to estimate denitrification. Although NO is also produced in the denitrification process, the effect of C2H2 On NO production in denitrification has not previously been examined, mainly because C2H2 interferes with chemiluminescent analysis of NO. We used a gas-bow system to study the effect of C2H2 on NO and N2O production in soil columns under denitrifying conditions and developed a method to avoid C2H2 interference of NO analysis. Nitric oxide, N2O, and C2H2 were measured in NO3- amended Brookston clay loam (fine-loamy, mixed, mesic Typic Argiaquoll) and Fox sandy loam (fine-loamy over sandy or sandy-skeletal, mixed, mesic Typic Hapludalf) with various moisture contents with and without added glucose during 50-h anaerobic incubations. In glucose-amended Brookston soil with approximate to 8% C2H2 added to the N-2 carrier gas, net NO production rates were increasingly inhibited after the first 24 h, and by 50 h were only 2 to 7% of the rates obtained in the absence of C2H2. Without glucose amendment and with C2H2, net NO production rates decreased by 24 to 62% in the Brookston soil. In the Fox soil inhibition occurred almost immediately on C2H2 addition and net NO production during 50 h was reduced by 36 to 62% with glucose amendment, and by 65 to 76% without glucose amendment. In most cases, net N2O production rates were decreased by C2H2 due to the decrease in production of its precursor, NO, and in some cases, increased due to inhibition of N2O reduction to N-2. Our data suggest that the C2H2 inhibition method may underestimate total denitrification rates under low water contents or high levels of C substrate.