Dissimilatory nitrate reduction processes along a forest hillslope

Abstract

<p>Dissimilatory nitrate (NO₃^&#8722;) reduction to ammonium (DNRA) and denitrification (DNF) are major dissimilatory NO₃^&#8722; reduction processes, competing for the available NO₃^&#8722; under anoxic conditions. The competition among these processes leads to different fates of NO₃^&#8722; in soil, i.e., loss of nitrogen (N) as benign N₂ or potent greenhouse gas (nitrous oxide, N₂O), or retaining of N by converting NO₃^&#8722; to ammonium. Unfortunately, little is known about the soil-environmental factors controlling the NO₃^&#8722; partition. Here we report DNF and DNRA in soils from the top and bottom of the hillslope.</p><p>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), NO₃^&#8722;, and microbial C and N (MBC and MBN) were determined. We incubated the soils under anoxic condition (i.e., helium atmosphere) and applied a ¹⁵N 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 ¹⁵NO₃^&#8722; (99.9%) and measured the production rates of ¹⁵NH₄⁺, ³⁰N₂, and ⁴⁶N₂O.</p><p>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&#8211;bottom; 3.95&#8211;4.78), EOC:NO₃^&#8722; (184&#8211;18.7), and MBC: MBN (8.2&#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&#8211;97%), while the trend was reversed at the bottom, with DNF rates being much higher and dominantly contributing to NO₃^&#8722; reduction (89&#8211;97%); and (3) during DNF process, the magnitude of N₂O production rates was comparable or even higher than that of N₂ in the soils from the bottom of the hillslope. The ratio of the N₂O to N₂ production (N₂O:N₂) was much higher in the soils from the top despite the low DNF rates.</p><p>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:NO₃^&#8722;: DNRA was preferred over DNF when NO₃^&#8722; was limited (i.e., high EOC:NO₃^&#8722;) because more free energy is liberated per unit of NO₃^&#8722; reduced for DNRA as compared to DNF. The substantial production of N₂O at the bottom of the hillslope indicates that previous studies that considered only ³⁰N₂ production rate could have highly underestimated the DNF rate. The high N₂O:N₂ at the top is likely caused by the low pH as well as the dominance of fungi, of which the N₂O reductase is generally lacking, pointing to the key roles of soil pH and microbial community structure in regulating the product stoichiometry of N₂O and N₂ in DNF.</p>