Isotopic fractionation of N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O to quantify N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O reduction to N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; – validation with Helium incubation and &amp;lt;sup&amp;gt;15&amp;lt;/sup&amp;gt;N gas flux methods