Nitrite isotope characteristics in 15N-labelled and non-labelled agricultural soil
Authors: Lewicka-Szczebak Dominika , Well Reinhard 
Published: March 2020

&lt;p&gt;Nitrite (NO&lt;sub&gt;2&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;) is a crucial compound in the complex N soil cycle. As an intermediate of nearly all N transformations its isotopic signature may provide precious information on the active pathways and processes. NO&lt;sub&gt;2&lt;/sub&gt;&lt;sup&gt;- &lt;/sup&gt;analyses have been already applied in &lt;sup&gt;15&lt;/sup&gt;N tracing studies increasing their interpretation perspectives. Natural abundance NO&lt;sub&gt;2&lt;/sub&gt;&lt;sup&gt;- &lt;/sup&gt;isotope studies in soils were so far not applied and this study aims at testing if such analyses are useful in tracing the soil N cycle.&amp;#160;&amp;#160;&lt;/p&gt;&lt;p&gt;We conducted laboratory soil incubations with parallel natural abundance and &lt;sup&gt;15&lt;/sup&gt;N treatments accompanied by analyses of soil N compounds (NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;, NO&lt;sub&gt;2&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;, NH&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;+&lt;/sup&gt;) and released N gases (N&lt;sub&gt;2&lt;/sub&gt;O and N&lt;sub&gt;2&lt;/sub&gt;). Water content was varied during the experiment from 55 to 86% water-filled pore space. NO&lt;sub&gt;2&lt;/sub&gt;&lt;sup&gt;- &lt;/sup&gt;was immediately extracted and analysed with the denitrifier method for selective nitrite reduction with Stenotrophomonas nitritireducens.&lt;/p&gt;&lt;p&gt;NO&lt;sub&gt;2&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt; content varied in the wide range from 0.6 to 6.6 &amp;#956;mol kg&lt;sup&gt;-1&lt;/sup&gt; soil, whereas NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt; content was one order higher and quite stable from 1.3 to 1.7 mmol kg&lt;sup&gt;-1&lt;/sup&gt; soil. Similarly, the &amp;#948;&lt;sup&gt;15&lt;/sup&gt;N(NO&lt;sub&gt;2&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;) varied largely from -16.7 to +8.8&amp;#8240;, whereas &amp;#948;&lt;sup&gt;15&lt;/sup&gt;N(NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;) was very stable from 3.5 to 5.9&amp;#8240;. The &amp;#948;&lt;sup&gt;15&lt;/sup&gt;N(NO&lt;sub&gt;2&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;) was correlated with NO&lt;sub&gt;2&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt; content. Applying Keeling plot the isotopic signature for the NO&lt;sub&gt;2&lt;/sub&gt;&lt;sup&gt;- &lt;/sup&gt;input of -11.7&amp;#8240; was determined. When related to &amp;#948;&lt;sup&gt;15&lt;/sup&gt;N(NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;) this gives the &amp;#949;(NO&lt;sub&gt;2&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;/ NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;) of -16.2&amp;#8240;, which is within the literature data for NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt; to NO&lt;sub&gt;2&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt; reduction step of denitrification.&lt;/p&gt;&lt;p&gt;The parallel &lt;sup&gt;15&lt;/sup&gt;N treatment was used to provide interpretation for the natural abundance isotope nitrite dynamics. We observed a sudden drop in &lt;sup&gt;15&lt;/sup&gt;N abundance in NO&lt;sub&gt;2&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt; (a&lt;sup&gt;15&lt;/sup&gt;N(NO&lt;sub&gt;2&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;)) after water addition to the soil from 14.7 to 3.2 at%, whereas &lt;sup&gt;&amp;#160;15&lt;/sup&gt;N abundance in NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt; (a&lt;sup&gt;15&lt;/sup&gt;N_NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;) showed only slight decrease from 14.2 to 13.1 at%. This indicates an incorporation of another source of unlabelled NO&lt;sub&gt;2&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt; for the wet part of the experiment. In natural abundance isotopes this change was also reflected in higher &amp;#916;&lt;sup&gt;15&lt;/sup&gt;N(NO&lt;sub&gt;2&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;/ NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;); for the wet part of the experiment it was even positive with +1.6&amp;#8240;, whereas for the dry part it was lower with -5.9&amp;#8240;. This additional nitrite source is most probably oxidation of organic N, which will be clarified by further studies, including detailed analysis with the &lt;sup&gt;15&lt;/sup&gt;N Ntrace model.&lt;/p&gt;&lt;p&gt;The observed changes in nitrite isotope characteristics were not reflected in N&lt;sub&gt;2&lt;/sub&gt;O. Whereas a&lt;sup&gt;15&lt;/sup&gt;N(NO&lt;sub&gt;2&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;) droped to 3.2 at%, for a&lt;sup&gt;15&lt;/sup&gt;N(N&lt;sub&gt;2&lt;/sub&gt;O) still 13.6 at% were found. The pool derived N&lt;sub&gt;2&lt;/sub&gt;O fraction calculated with the &lt;sup&gt;15&lt;/sup&gt;N gas flux method showed that the entire N&lt;sub&gt;2&lt;/sub&gt;O originated from NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt; in the wet part of the experiment. This shows that NO&lt;sub&gt;2&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt; pools originating from different pathways must be isolated and not the entire NO&lt;sub&gt;2&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt; pool undergoes further reduction to N&lt;sub&gt;2&lt;/sub&gt;O.&lt;/p&gt;&lt;p&gt;Natural abundance nitrite isotope studies may provide a new important tool in constraining the N soil cycling.&lt;/p&gt;