Nitrate and water uptake, rather than rhizodeposition, control denitrification in the presence of growing plants
Authors: Rummel Pauline Sophie , Well Reinhard , Pfeiffer Birgit , Dittert Klaus , Floßmann Sebastian , Pausch Johanna 
Published: March 2021

&lt;p&gt;The main prerequisites for denitrification are availability of nitrate (NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;) and easily decomposable organic substances, and oxygen deficiency. Growing plants modify all these parameters and may thus play an important role in regulating denitrification. Previous studies investigating plant root effects on denitrification have found contradictive results. Both increased and decreased denitrification in the presence of plants have been reported and were associated with higher C&lt;sub&gt;org&lt;/sub&gt; or lower NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt; availability, respectively. Accordingly, it is still unclear whether growing plants stimulate denitrification through root exudation or restrict it through NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt; uptake. Furthermore, reliable measurements of N&lt;sub&gt;2&lt;/sub&gt; fluxes and N&lt;sub&gt;2&lt;/sub&gt;O/(N&lt;sub&gt;2&lt;/sub&gt;O+N&lt;sub&gt;2&lt;/sub&gt;) ratios in the presence of plants are scarce.&lt;/p&gt;&lt;p&gt;Therefore, we conducted a double labeling pot experiment with either maize (&lt;em&gt;Zea mays&lt;/em&gt; L.) or cup plant (&lt;em&gt;Silphium perfoliatum&lt;/em&gt; L.) of the same age but differing in size of their shoot and root systems. The &lt;sup&gt;15&lt;/sup&gt;N gas flux method was applied to directly quantify N&lt;sub&gt;2&lt;/sub&gt;O and N&lt;sub&gt;2&lt;/sub&gt; fluxes in situ. To link denitrification with available C in the rhizosphere, &lt;sup&gt;13&lt;/sup&gt;CO&lt;sub&gt;2&lt;/sub&gt; pulse labeling was used to trace C translocation from shoots to roots and its release by roots into the soil.&lt;/p&gt;&lt;p&gt;Plant water uptake was a main factor controlling soil moisture and, thus, daily N&lt;sub&gt;2&lt;/sub&gt;O+N&lt;sub&gt;2&lt;/sub&gt; fluxes, cumulative N emissions, and N&lt;sub&gt;2&lt;/sub&gt;O production pathways. However, N fluxes remained on a low level when NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt; availability was low due to rapid plant N uptake. Only when both N and water uptake were low, high NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt; availability and high soil moisture led to strongly increased denitrification-derived N losses.&lt;/p&gt;&lt;p&gt;Total CO&lt;sub&gt;2&lt;/sub&gt; efflux was positively correlated with root dry matter, but there was no indication of any relationship between recovered &lt;sup&gt;13&lt;/sup&gt;C from root exudation and cumulative N emissions. We anticipate that higher C&lt;sub&gt;org&lt;/sub&gt; availability in pots with large root systems did not lead to higher denitrification rates, as NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt; was limiting denitrification due to plant N uptake. Overall, we conclude that root-derived C stimulates denitrification only when soil NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt; is not limited and low O&lt;sub&gt;2&lt;/sub&gt; concentrations enable denitrification. Thus, root-derived C may stimulate denitrification under small plants, while N and water uptake become the controlling factors with increasing plant and root growth.&lt;/p&gt;