Dissimilatory nitrate reduction processes along a forest hillslope

Dissimilatory nitrate (NO₃⁻) reduction to ammonium (DNRA) and denitrification (DNF) are major dissimilatory NO₃⁻ reduction processes, competing for the available NO₃⁻ under anoxic conditions. The competition among these processes leads to different fates of NO₃⁻ 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⁻ to ammonium. Unfortunately, little is known about the soil-environmental factors controlling the NO₃⁻ partition. Here we report DNF and DNRA in soils from the top and bottom of the hillslope.

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⁻, 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⁻ (99.9%) and measured the production rates of ¹⁵NH₄⁺, ³⁰N₂, and ⁴⁶N₂O.

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–bottom; 3.95–4.78), EOC:NO₃⁻ (184–18.7), and MBC: MBN (8.2–6.3); (2) DNRA rate tended to be higher at the top of the hillslope with DNF being nearly inactive, resulting in a dominance of DNRA (59–97%), while the trend was reversed at the bottom, with DNF rates being much higher and dominantly contributing to NO₃⁻ reduction (89–97%); 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.

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⁻: DNRA was preferred over DNF when NO₃⁻ was limited (i.e., high EOC:NO₃⁻) because more free energy is liberated per unit of NO₃⁻ 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.