Use of acetylene as a nitrification inhibitor to reduce biases in gross N transformation rates in a soil showing rapid disappearance of added ammonium

Violation of the assumption of equilibrium and identical behaviour of added and native N inherent in the 15N isotope dilution technique may have a large impact on estimated gross N transformation rates. To allow the added 15N to equilibrate, initial sampling has been recommended to be delayed until 24 h after addition of 15N. However, in soils with high nitrification potential this may not be feasible as little extractable NH4+-N may remain after 24 h. To circumvent this problem the use of nitrification inhibitors such as acetylene (C2H2) has been suggested. Two laboratory experiments were designed to test the effects of C2H2 on C mineralization and gross N transformation rates using one soil treatment (Straw+Ca(NO3)2) from the Ultuna Long-Term Soil Organic Matter Experiment. The assumption of identical behaviour of added and native N was assessed and processes that may have violated the assumption were identified using a dynamic compartmental model. Brief exposure to low atmospheric C2H2 concentrations in the headspace (0.1–1.0 kPa) prior to 15N addition did not significantly affect C mineralization, gross N mineralization or immobilization rates when nitrification was completely inhibited. Results from the dynamic compartmental model suggested that, in the absence of complete inhibition, native NH4+-N was preferentially nitrified resulting in a 24–75% underestimation of gross N mineralization rates. We confirm the appropriateness of using 24 h for the initial sampling as this allows for complete nitrification inhibition, avoids the effects of soil disturbance caused by 15N addition and minimizes differences in the nitrification rate of added and native NH4+-N which is most pronounced during the 0.5–24 h period. The application of 1.0 kPa C2H2 prior to 15N addition provided complete nitrification inhibition so that gross N transformation rates can be estimated beyond 24 h in soils with high nitrification potential. Larger additions of 15N-labelled NH4+-N were not an alternative to inhibition of nitrification as estimated gross N mineralization rates were significantly different when 20 μg N g−1 soil was added without C2H2 compared with 5 μg N g−1 soil prior C2H2 exposure. In addition, incorrect description of process kinetics led to large errors in gross nitrification rate estimates when 20 μg N g−1 soil was added. In soils with high nitrification potential, we therefore recommend the use of 1.0 kPa C2H2 prior to estimation of gross N transformation rates.