Flux intensity and diurnal variability of soil N ₂ O emissions in a highly fertilized cropping system

Manually sampling N 2 O soil fluxes is labor‐intensive and sampling frequencies are often insufficient to capture daily variation in N 2 O soil flux, compromising the accuracy of emission estimates. Knowledge of the diurnal fluctuations in N 2 O flux has been used to choose a sampling time that maximizes the accuracy of N 2 O flux estimates, thereby reducing the sampling frequency required, but results from previous studies are inconsistent. We analyzed N 2 O soil emissions measured quasi‐continuously over 3 yr in a highly fertilized (>200 kg N ha −1 ) corn ( Zea mays L.) system grown in southern Wisconsin. This is the first study of temporal variability in N 2 O flux that includes multiple difficult to measure peak emission events (“hot moments”) and estimates the relative contribution of hot moments to cumulative emissions. The relationship between the observed hourly average flux and the mean daily flux was assessed via linear regression of all measured data (≈22,000 fluxes) and data subsets grouped by magnitude. Diurnal variation in flux was closely associated with normalized flux size. During low‐emission periods, fluxes exhibited a diurnal pattern, where N 2 O flux measured at particular times of day, named “Preferred Measuring Times” (PMTs), were not significantly different from the mean daily flux. During high‐emission periods, N 2 O flux did not exhibit a diurnal pattern and there was no PMT. High‐emission periods included difficult to measure hot moments that did not exhibit a PMT and contributed up to 50% of the cumulative emissions; therefore, flux measurements with high temporal resolution were required to estimate cumulative emissions.