Dissimilatory nitrate reduction processes along a forest hillslope
Authors: Du Jiajie , Zheng Jinsen , Shibata Makoto , Wang Zixiao , Watanabe Tetsuhiro , Funakawa Shinya 
Published: March 2021

&lt;p&gt;Dissimilatory nitrate (NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;&amp;#8722;&lt;/sup&gt;) reduction to ammonium (DNRA) and denitrification (DNF) are major dissimilatory NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;&amp;#8722;&lt;/sup&gt; reduction processes, competing for the available NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;&amp;#8722;&lt;/sup&gt; under anoxic conditions. The competition among these processes leads to different fates of NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;&amp;#8722;&lt;/sup&gt; in soil, i.e., loss of nitrogen (N) as benign N&lt;sub&gt;2&lt;/sub&gt; or potent greenhouse gas (nitrous oxide, N&lt;sub&gt;2&lt;/sub&gt;O), or retaining of N by converting NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;&amp;#8722;&lt;/sup&gt; to ammonium. Unfortunately, little is known about the soil-environmental factors controlling the NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;&amp;#8722;&lt;/sup&gt; partition. Here we report DNF and DNRA in soils from the top and bottom of the hillslope.&lt;/p&gt;&lt;p&gt;We sampled soils from a hillslope of forest to generate a soil-environmental gradient. The soil-environmental factors including soil pH, available carbon (potassium chloride-extractable organic carbon: EOC), NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;&amp;#8722;&lt;/sup&gt;, and microbial C and N (MBC and MBN) were determined. We incubated the soils under anoxic condition (i.e., helium atmosphere) and applied a &lt;sup&gt;15&lt;/sup&gt;N isotope pairing technique to quantify the potential rates of DNRA and DNF. Briefly, we incubated the soil under anoxic condition (i.e., helium atmosphere) to remove any N oxides and oxygen, then we added &lt;sup&gt;15&lt;/sup&gt;NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;&amp;#8722;&lt;/sup&gt; (99.9%) and measured the production rates of &lt;sup&gt;15&lt;/sup&gt;NH&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;+&lt;/sup&gt;, &lt;sup&gt;30&lt;/sup&gt;N&lt;sub&gt;2&lt;/sub&gt;, and &lt;sup&gt;46&lt;/sup&gt;N&lt;sub&gt;2&lt;/sub&gt;O.&lt;/p&gt;&lt;p&gt;The results showed that (1) a good gradient of the soil-environmental variables was observed along the hillslope from top to bottom, including pH (top&amp;#8211;bottom; 3.95&amp;#8211;4.78), EOC:NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;&amp;#8722;&lt;/sup&gt; (184&amp;#8211;18.7), and MBC: MBN (8.2&amp;#8211;6.3); (2) DNRA rate tended to be higher at the top of the hillslope with DNF being nearly inactive, resulting in a dominance of DNRA (59&amp;#8211;97%), while the trend was reversed at the bottom, with DNF rates being much higher and dominantly contributing to NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;&amp;#8722;&lt;/sup&gt; reduction (89&amp;#8211;97%); and (3) during DNF process, the magnitude of N&lt;sub&gt;2&lt;/sub&gt;O production rates was comparable or even higher than that of N&lt;sub&gt;2&lt;/sub&gt; in the soils from the bottom of the hillslope. The ratio of the N&lt;sub&gt;2&lt;/sub&gt;O to N&lt;sub&gt;2&lt;/sub&gt; production (N&lt;sub&gt;2&lt;/sub&gt;O:N&lt;sub&gt;2&lt;/sub&gt;) was much higher in the soils from the top despite the low DNF rates.&lt;/p&gt;&lt;p&gt;The remarkably different patterns of DNRA and DNF rates and relative contributions between the top and bottom of the hillslope are controlled by the EOC:NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;&amp;#8722;&lt;/sup&gt;: DNRA was preferred over DNF when NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;&amp;#8722;&lt;/sup&gt; was limited (i.e., high EOC:NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;&amp;#8722;&lt;/sup&gt;) because more free energy is liberated per unit of NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;&amp;#8722;&lt;/sup&gt; reduced for DNRA as compared to DNF. The substantial production of N&lt;sub&gt;2&lt;/sub&gt;O at the bottom of the hillslope indicates that previous studies that considered only &lt;sup&gt;30&lt;/sup&gt;N&lt;sub&gt;2&lt;/sub&gt; production rate could have highly underestimated the DNF rate. The high N&lt;sub&gt;2&lt;/sub&gt;O:N&lt;sub&gt;2&lt;/sub&gt; at the top is likely caused by the low pH as well as the dominance of fungi, of which the N&lt;sub&gt;2&lt;/sub&gt;O reductase is generally lacking, pointing to the key roles of soil pH and microbial community structure in regulating the product stoichiometry of N&lt;sub&gt;2&lt;/sub&gt;O and N&lt;sub&gt;2&lt;/sub&gt; in DNF.&lt;/p&gt;