Diurnal Variation and Sampling Frequency Effects on Nitrous Oxide Emissions Following Nitrogen Fertilization and Spring‐Thaw Events

Abstract

Core Ideas High‐frequency N2O emission data was subsampled to assess sampling frequency errors. Mid‐morning sampling was adequate to derive daily average N2O emission during events. Twice‐weekly sampling gave an uncertainty of –6 to +12% during spring thaw. Twice weekly + sample after >10 mm rainfall had –1 to +19% error after fertilization. An infrequent sampling protocol can introduce bias into N2O studies. Different methods are used to measure nitrous oxide (N2O), a potent greenhouse gas emitted from agricultural soils. While some methods (e.g. micrometeorological methods) conduct near‐continuous measurements, manual chambers measure discontinuously. Estimates of N2O emissions based on discontinuous measurements can carry errors due to: (i) diurnal variation and (ii) integration of emissions over time. This study evaluated these two sources of uncertainties and identified the optimal sampling strategy for emission following spring‐thaw events (ST) and nitrogen fertilization (NF). Two times of day (mid‐morning [MM] and mid‐afternoon [MA]) and three sampling frequencies suggested in the literature on N2O emissions (bi‐weekly [BW], weekly [W], twice weekly [TW] for the ST, and W, TW, and TW plus extra sample after rainfall events >10 mm for the NF) were created by subsampling a high‐frequency reference dataset. We show that the mid‐morning sampling strategy effectively represented the daily N2O emission average, while the mid‐afternoon strategy overestimated fluxes by 14 and 32% for the ST and NF, respectively. For the ST, the weekly mid‐morning sampling protocol resulted in errors ranging from –3 to +33%, with lower uncertainties when sampling frequency increased to TW (–6 to +12%). The TW mid‐morning sampling also was adequate for the NF datasets (–5 to +19%), with narrower uncertainty levels when an additional sample was taken after >10 mm rainfall (–1 to +19%). These results provide increased confidence in selecting the sampling strategy for discontinuous measurements following N fertilization and STs in cropping systems.

Hot moments drive extreme nitrous oxide and methane emissions from agricultural peatlands

“…While continuous automated chamber or eddy covariance measurements are ideal to capture hot moments of emissions, long‐term continuous measurements are still cost prohibitive in many locations and ecosystems. If hot moments are predictable and well‐defined, daily flux measurements are likely effective in appropriately quantifying hot moments of N 2 O emissions (Ferrari Machado et al, 2019; Reeves et al, 2016). However, if the timing and controls on hot moments are unknown or sporadic, less frequent sampling may significantly underestimate N 2 O emissions (Grace et al, 2020).…”

“…Prolonged anaerobic conditions coupled with soil temperatures greater than 10º C appeared to drive hot moments of CH 4 fluxes in these systems. Short periods of elevated NH + 4 concentrations during flooded periods could have limited methanogenesis (Chen et al, 2008) or temporarily shifted the methanogenic pathway (Fotidis et al, 2013) (Ferrari Machado et al, 2019;Reeves et al, 2016). However, if the timing and controls on hot moments are unknown or sporadic, less frequent sampling may significantly underestimate N 2 O emissions (Grace et al, 2020).…”

Topography Controls N2O Emissions Differently during Early and Late Corn Growing Season

“…Daily N 2 O fluxes were calculated using Microsoft Office Excel 2016 (Microsoft, Inc., Washington, WA, USA). It was assumed that N 2 O emissions between 0900 and 1200 h on each sampling date were the average of the daily N 2 O flux [62,64]. Cumulative N 2 O emissions were calculated by multiplying the average fluxes of two successive determinations by the length of the period between samplings and adding that amount to the previous cumulative total [26,66] using following Equation (2).…”

“…For each measurement, four gas samples were taken from the chamber using a 20 mL non-sterile syringe at 10 min intervals (0, 10, 20, and 30 min after closure) [61]. To minimize any effect of diurnal variations in emissions, the samples were collected between 0900 and 1200 h on each sampling day [62]. The gas samples were transferred to 12 mL evacuated clear glass vials sealed with gas-tight neoprene septum (Labco).…”

Effect of slurry application techniques on nitrous oxide emission from temperate grassland under varying soil and climatic conditions

“…Gas sampling was conducted between 09:00 and 11:00 am. This sampling time was chosen because it was shown that mid‐morning sampling effectively represents the daily N 2 O emission average (Machado et al, 2019). During each sampling, four gas samples were periodically (0, 20, 40, and 60 min after chamber closure) taken with a syringe from the chambers' atmosphere and transferred into evacuated gas vials.…”