NO3- uptake and C exudation &#8211; do plant roots stimulate or inhibit denitrification?
Authors: Rummel Pauline Sophie , Well Reinhard , Pfeiffer Birgit , Dittert Klaus , Floßmann Sebastian , Pausch Johanna 
Published: March 2020

&lt;p&gt;Growing plants affect soil moisture, mineral N and organic C (C&lt;sub&gt;org&lt;/sub&gt;) availability in soil and may thus play an important role in regulating denitrification. The availability of the main substrates for denitrification (C&lt;sub&gt;org&lt;/sub&gt; and NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;) is controlled by root activity and higher denitrification activity in rhizosphere soils has been reported. We hypothesized that (I) plant N uptake governs NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt; availability for denitrification leading to increased N&lt;sub&gt;2&lt;/sub&gt;O and N&lt;sub&gt;2&lt;/sub&gt; emissions, when plant N uptake is low due to smaller root system or root senescence. (II) Denitrification is stimulated by higher C&lt;sub&gt;org&lt;/sub&gt; availability from root exudation or decaying roots increasing total gaseous N emissions while decreasing their 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.&lt;/p&gt;&lt;p&gt;We tested these assumptions in a double labeling pot experiment with maize (Zea mays L.) grown under three N fertilization levels S / M / L (no / moderate / high N fertilization) and with cup plant (Silphium perfoliatum L., moderate N fertilization). After 6 weeks, all plants were labeled with 0.1 g N kg&lt;sup&gt;-1&lt;/sup&gt; (Ca(&lt;sup&gt;15&lt;/sup&gt;NO&lt;sub&gt;3&lt;/sub&gt;)&lt;sub&gt;2&lt;/sub&gt;, 60 at%), and the &lt;sup&gt;15&lt;/sup&gt;N tracer method was applied to estimate plant N uptake, N&lt;sub&gt;2&lt;/sub&gt;O and N&lt;sub&gt;2&lt;/sub&gt; emissions. 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 (5 g Na&lt;sub&gt;2&lt;/sub&gt;&lt;sup&gt;13&lt;/sup&gt;CO&lt;sub&gt;3&lt;/sub&gt;, 99 at%) was used to trace C translocation from shoots to roots and its release by roots into the soil. CO&lt;sub&gt;2&lt;/sub&gt; evolving from soil was trapped in NaOH for &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C analyses, and gas samples were taken for analysis of N&lt;sub&gt;2&lt;/sub&gt;O and N&lt;sub&gt;2&lt;/sub&gt; from the headspace above the soil surface every 12 h.&lt;/p&gt;&lt;p&gt;Although pots were irrigated, changing soil moisture through differences in plant water uptake was the main factor controlling 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. In addition, total N&lt;sub&gt;2&lt;/sub&gt;O+N&lt;sub&gt;2&lt;/sub&gt; emissions were negatively correlated with plant N uptake and positively with soil N concentrations. Recently assimilated C released by roots (&lt;sup&gt;13&lt;/sup&gt;C) was positively correlated with root dry matter, but we could not detect any relationship with 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 limited due to plant uptake. In conclusion, plant growth controlled water and NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt; uptake and, subsequently, formation of anaerobic hotspots for denitrification.&lt;/p&gt;