Midmorning Point Sampling May Not Accurately Represent Nitrous Oxide Emissions Following Fertilizer Applications

Core Ideas Midmorning N2O flux estimates were not consistent with near‐continual data for spring and fall applied urea. Local N2O sampling protocols must account for temporal changes in management and climatic conditions. Non‐alignment in soil temperature and N2O patterns in annual‐crop soils is consistent with Fick's Law. Non‐alignment between temperatuer and N2O emissions can occur when the soil is saturated with water. Near‐continuous automated data collection at anticipated max and min emissions may improve the accuracy of N2O estimates. A common approach for measuring N2O emissions is to collect midmorning or early evening gas samples from experiments utilizing the chamber‐based flux methodology. However, due to high spatial and temporal variability, N2O estimates based on midmorning or early evening sampling may not provide accurate estimates of total emissions. This study determined the impact of sampling collection timing on the precision and accuracy of N2O emissions estimates. Nitrous oxide emissions, air and soil temperatures, and soil moisture were measured for 21 d following the application of 224 kg urea‐N ha–1 on 20 Sept. 2017, 11 Oct. 2017, and 1 May 2018, at six time intervals (0130–0230, 0530–0630, 0930–1030, 1330–1430, 1730–1830, and 2130–2230 h) over a 24‐h period. Based on multiple daily measurements, point samples collected between 0930 and 1030 h (midmorning) were inconsistent in their ability to predict N2O emissions. However, samples collected between 2130 and 2230 h (early evening) were similar to average emissions. The number of randomly collected point samples to be within 20% of the mean 80% of the time over a 21‐d period ranged from 13 samples for fertilizer applied on 20 Sept. 2017 to 48 samples for fertilizer applied on 11 Oct. 2017. This research indicates that management and climatic variability affect N2O emissions, and that accurate sampling protocols vary across management and climates. To reduce uncertainty, N2O sampling protocol should be tested under conditions likely to occur and where possible, near‐continuous measurement systems should be adopted.