A novel injection technique: using a field-based quantum cascade laser for the analysis of gas samples derived from static chambers

<p>The development of fast-response analysers for the measurement of nitrous oxide (N<span class="inline-formula">₂</span>O) has resulted in exciting opportunities for new experimental techniques beyond commonly used static chambers and gas chromatography (GC) analysis. For example, quantum cascade laser (QCL) absorption spectrometers are now being used with eddy covariance (EC) or automated chambers. However, using a field-based QCL EC system to also quantify N<span class="inline-formula">₂</span>O concentrations in gas samples taken from static chambers has not yet been explored. Gas samples from static chambers are often analysed by GC, a method that requires labour and time-consuming procedures off-site. Here, we developed a novel field-based injection technique that allowed the use of a single QCL for (1) micrometeorological EC and (2) immediate manual injection of headspace samples taken from static chambers. To test this approach across a range of low to high N<span class="inline-formula">₂</span>O concentrations and fluxes, we applied ammonium nitrate (AN) at 0, 300, 600 and 900&thinsp;kg&thinsp;N&thinsp;ha<span class="inline-formula">⁻¹</span> (AN<span class="inline-formula">₀</span>, AN<span class="inline-formula">₃₀₀</span>, AN<span class="inline-formula">₆₀₀</span>, AN<span class="inline-formula">₉₀₀)</span> to plots on a pasture soil. After analysis, calculated N<span class="inline-formula">₂</span>O fluxes from QCL (<span class="inline-formula"><i>F</i>_{N2O_QCL})</span> were compared with fluxes determined by a standard method, i.e. laboratory-based GC (<span class="inline-formula"><i>F</i>_{N2O_GC})</span>. Subsequently, the comparability of QCL and GC data was tested using orthogonal regression, Bland–Altman and bioequivalence statistics. For AN-treated plots, mean cumulative N<span class="inline-formula">₂</span>O emissions across the 7&thinsp;d campaign were 0.97 (AN<span class="inline-formula">₃₀₀)</span>, 1.26 (AN<span class="inline-formula">₆₀₀)</span> and 2.00&thinsp;kg&thinsp;N<span class="inline-formula">₂</span>O-N&thinsp;ha<span class="inline-formula">⁻¹</span> (AN<span class="inline-formula">₉₀₀)</span> for <span class="inline-formula"><i>F</i>_{N2O_QCL}</span> and 0.99 (AN<span class="inline-formula">₃₀₀)</span>, 1.31 (AN<span class="inline-formula">₆₀₀)</span> and 2.03&thinsp;kg&thinsp;N<span class="inline-formula">₂</span>O-N&thinsp;ha<span class="inline-formula">⁻¹</span> (AN<span class="inline-formula">₉₀₀)</span> for <span class="inline-formula"><i>F</i>_{N2O_GC}</span>. These <span class="inline-formula"><i>F</i>_{N2O_QCL}</span> and <span class="inline-formula"><i>F</i>_{N2O_GC}</span> were highly correlated (<span class="inline-formula"><i>r</i>=0.996</span>, <span class="inline-formula"><i>n</i>=81</span>) based on orthogonal regression, in agreement following the Bland–Altman approach (i.e. within <span class="inline-formula">±1.96</span> standard deviation of the mean difference) and shown to be for all intents and purposes the same (i.e. equivalent). The <span class="inline-formula"><i>F</i>_{N2O_QCL}</span> and <span class="inline-formula"><i>F</i>_{N2O_GC}</span> derived under near-zero flux conditions (AN<span class="inline-formula">₀)</span> were weakly correlated (<span class="inline-formula"><i>r</i>=0.306</span>, <span class="inline-formula"><i>n</i>=27</span>) and not found to agree or to be equivalent. This was likely caused by the calculation of small, but apparent positive and negative, <span class="inline-formula"><i>F</i>_N2O</span> when in fact the actual flux was below the detection limit of static chambers. Our study demonstrated (1) that the capability of using one QCL to measure N<span class="inline-formula">₂</span>O at different scales, including manual injections, offers great potential to advance field measurements of N<span class="inline-formula">₂</span>O (and other greenhouse gases) in the future and (2) that suitable statistics have to be adopted when formally assessing the agreement and difference (not only the correlation) between two methods of measurement.</p>