Source-process partitioning of soil N2O and CO2 production: nitrogen and simulated exudate additions
Authors: Daly Erin , Hernandez Ramirez Guillermo 
Published: March 2020

&lt;p&gt;Understanding the source partitioning of carbon dioxide (CO&lt;sub&gt;2&lt;/sub&gt;) and nitrous oxide (N&lt;sub&gt;2&lt;/sub&gt;O) fluxes from soil is integral for the characterization of total fluxes and the quantification of potential soil organic matter priming effects. Additionally, we utilized &lt;sup&gt;15&lt;/sup&gt;N-N&lt;sub&gt;2&lt;/sub&gt;O site preference data to analyze the process priming of microbial nitrification and denitrification on subsequent N&lt;sub&gt;2&lt;/sub&gt;O fluxes. A 32-day laboratory incubation was designed to examine the effects of artificial exudate, nitrogen fertilizer and their potential interactive effects on CO&lt;sub&gt;2&lt;/sub&gt; and N&lt;sub&gt;2&lt;/sub&gt;O fluxes, soil organic matter source-priming and N&lt;sub&gt;2&lt;/sub&gt;O process-priming. Artificial root exudate (ARE) consisting of a mixture of 99 atom% &lt;sup&gt;13&lt;/sup&gt;C labelled compounds at three addition rates (0, 6.2, 12.5 mg C kg&lt;sup&gt;-1&lt;/sup&gt; soil day&lt;sup&gt;-1&lt;/sup&gt;) was applied daily for 21 days to microcosms with or without urea fertilizer, a subset of which was labelled with 5 atom % &lt;sup&gt;15&lt;/sup&gt;N. Measurements of CO&lt;sub&gt;2&lt;/sub&gt; and N&lt;sub&gt;2&lt;/sub&gt;O fluxes, isotopic composition and N&lt;sub&gt;2&lt;/sub&gt;O site preference were frequent throughout the duration of the experiment. Source partitioning of CO&lt;sub&gt;2&lt;/sub&gt; fluxes showed that soil organic carbon (SOM-C) positive priming was significantly altered by additions of artificial exudate and urea (p &lt; 0.001 and 0.001, respectively). When applied concurrently, urea addition had an antagonistic interactive effect on SOM-C sourced CO&lt;sub&gt;2&lt;/sub&gt; fluxes (p &lt; 0.001).&amp;#160; Source partitioning of N&lt;sub&gt;2&lt;/sub&gt;O flux data revealed that soil organic matter nitrogen (SOM-N) was positively primed for N&lt;sub&gt;2&lt;/sub&gt;O flux by the addition of urea fertilizer (p &lt; 0.001), but positive SOM-N priming was reduced by an antagonistic interaction with artificial exudate application (p &lt; 0.001). Further, examination of &lt;sup&gt;15&lt;/sup&gt;N-N&lt;sub&gt;2&lt;/sub&gt;O site preference found that the main processes by which N&lt;sub&gt;2&lt;/sub&gt;O is formed (nitrification and denitrification) were differentially process-primed by the addition or absence of ARE. Cumulative denitrification and nitrification contributions to total N&lt;sub&gt;2&lt;/sub&gt;O flux were both positively primed in the soils receiving both ARE and urea inputs relative to a control (50.0 &amp;#177; 10.1 and 28.2&amp;#177; 8.0 &amp;#956;g N&lt;sub&gt;2&lt;/sub&gt;O-N kg&lt;sup&gt;-1&lt;/sup&gt;, respectively). In soils receiving only ARE application, denitrification-derived N&lt;sub&gt;2&lt;/sub&gt;O was negatively primed relative to a control and thus contributed less to overall N&lt;sub&gt;2&lt;/sub&gt;O flux (-9.5 &amp;#177; 12.4 &amp;#956;g N&lt;sub&gt;2&lt;/sub&gt;O-N kg&lt;sup&gt;-1&lt;/sup&gt;) but nitrification-derived N&lt;sub&gt;2&lt;/sub&gt;O was positively primed (17.2 &amp;#177; 9.0 &amp;#956;g N&lt;sub&gt;2&lt;/sub&gt;O-N kg&lt;sup&gt;-1&lt;/sup&gt;).&lt;/p&gt;