Quantifying nitrous oxide emissions in space and time using static chambers and eddy covariance from a temperate grassland
Authors: Murphy Rachael , Richards Karl , Krol Dominika , Gebremichael Amanuel , Lopez-Sangil Luis , Rambaud James , Cowan Nicholas , Lanigan Gary , Saunders Matt 
Published: March 2021

&lt;p&gt;Nitrous oxide (N&lt;sub&gt;2&lt;/sub&gt;O) is a potent greenhouse gas (GHG), with a global warming potential (GWP) of 265 relative to carbon dioxide (CO&lt;sub&gt;2&lt;/sub&gt;) and a lifespan of over 100 years. Where N input from fertilizers exceeds plant demands, hotspots of N&lt;sub&gt;2&lt;/sub&gt;O can be produced releasing short-lived pulses of N&lt;sub&gt;2&lt;/sub&gt;O from the soil that exhibit disproportionately high rates of emissions relative to longer periods of time, known as hot moments. Hotspots and hot moments of N&lt;sub&gt;2&lt;/sub&gt;O are sensitive to changes in agricultural management and the environment making it difficult to accurately quantify N&lt;sub&gt;2&lt;/sub&gt;O emissions with low uncertainties. This study investigates the methods used to quantify N&lt;sub&gt;2&lt;/sub&gt;O emissions in time and space, using both static chambers (SC) and eddy covariance (EC) techniques.N&lt;sub&gt;2&lt;/sub&gt;O fluxes were measured from both techniques from an intensively managed grassland site under four fertilizer applications of calcium ammonium nitrate (CAN) in 2019. EC measurements of N&lt;sub&gt;2&lt;/sub&gt;O were gap-filled by using a simple linear empirical model that incorporated environmental and management data. SC N&lt;sub&gt;2&lt;/sub&gt;O fluxes were calculated using the arithmetic method and Bayesian statistics via Markov Chain Monte-Carlo (MCMC) simulations to account for the log-normal distribution of fluxes measured. N&lt;sub&gt;2&lt;/sub&gt;O emissions were weakest in winter for both techniques (-3.27 &amp;#181;g N&lt;sub&gt;2&lt;/sub&gt;O-N m&lt;sup&gt;-2&lt;/sup&gt; hr&lt;sup&gt;-1&lt;/sup&gt; for SC and -3.9 &amp;#181;g N&lt;sub&gt;2&lt;/sub&gt;O-N m&lt;sup&gt;-2&lt;/sup&gt; hr&lt;sup&gt;-1&lt;/sup&gt; for EC). Following fertilizer application, daily averaged N&lt;sub&gt;2&lt;/sub&gt;O emissions peaked in March (538.89 &amp;#181;g m-&lt;sup&gt;2 &lt;/sup&gt;hr&lt;sup&gt;-1&lt;/sup&gt; for SC, 491.18 &amp;#181;g m-&lt;sup&gt;2 &lt;/sup&gt;hr&lt;sup&gt;-1 &lt;/sup&gt;for EC) and April (117.91 &amp;#181;g m-&lt;sup&gt;2 &lt;/sup&gt;hr&lt;sup&gt;-1&lt;/sup&gt;for SC and 306.90 &amp;#181;g m-&lt;sup&gt;2 &lt;/sup&gt;hr&lt;sup&gt;-1&lt;/sup&gt; for EC). Delayed peaks in N&lt;sub&gt;2&lt;/sub&gt;O emissions following fertilizer application occurred in June (101.03 &amp;#181;g m-&lt;sup&gt;2 &lt;/sup&gt;hr&lt;sup&gt;-1&lt;/sup&gt; for SC and 814.76 &amp;#181;g m-&lt;sup&gt;2 &lt;/sup&gt;hr&lt;sup&gt;-1&lt;/sup&gt; for EC) and October (417.14 &amp;#181;g m-&lt;sup&gt;2 &lt;/sup&gt;hr&lt;sup&gt;-1&lt;/sup&gt; for SC and 313.22 &amp;#181;g m-&lt;sup&gt;2 &lt;/sup&gt;hr&lt;sup&gt;-1&lt;/sup&gt; for EC) and these high emissions events coincided with dry periods followed by rainfall events. EC and SC measurements were most comparable when emissions were &gt; 115 &amp;#181;g m-&lt;sup&gt;2 &lt;/sup&gt;hr&lt;sup&gt;-1&lt;/sup&gt;, when the flux footprint of half-hourly EC flux measurements overlapped with the position and time of SC measurements and when the number of chamber replicates were &amp;#8805; 15 on a given sampling day. Where the chamber sample size was small (n &amp;#8804; 5), the Bayesian method produced large uncertainties (&gt; 25,000 &amp;#181;g m-&lt;sup&gt;2 &lt;/sup&gt;hr&lt;sup&gt;-1&lt;/sup&gt;) due to the inability to constrain an arithmetic mean from a log-normally distributed data set. Annual cumulative N&lt;sub&gt;2&lt;/sub&gt;O fluxes from EC and SC by the arithmetic and Bayesian method, were 3.35(&amp;#177; 0.5) kg N ha&lt;sup&gt;-1&lt;/sup&gt;, 2.98 (&amp;#177; 0.17) kg N ha&lt;sup&gt;-1&lt;/sup&gt;and 3.13 (&amp;#177; 0.24) kg N ha&lt;sup&gt;-1&lt;/sup&gt; respectively. Emission factors from EC and SC by the arithmetic and Bayesian method were higher than the Intergovernmental Panel on Climate Change (IPCC) default value of 1%, at 1.46%, 1.30% and 1.36%, respectively. Our study highlights that disparities exist between SC and EC in quantifying N&lt;sub&gt;2&lt;/sub&gt;O fluxes from a managed grassland and we recommend constraining disparities by utilizing a SC sample size &gt; 5 and by accounting for the log-normal distribution of N&lt;sub&gt;2&lt;/sub&gt;O flux data to accurately estimate N&lt;sub&gt;2&lt;/sub&gt;O flux uncertainty.&lt;/p&gt;