EGU21-10536
https://doi.org/10.5194/egusphere-egu21-10536
EGU General Assembly 2021
© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.

Dissimilatory nitrate reduction processes along a forest hillslope

Jiajie Du1, Jinsen Zheng2, Makoto Shibata3, Zixiao Wang4, Tetsuhiro Watanabe5,6, and Shinya Funakawa7,8
Jiajie Du et al.
  • 1Graduate School of Agriculture, Kyoto University, Kyoto City, Japan (du.jiajie.87m@st.kyoto-u.ac.jp)
  • 2Center for Ecological Research, Kyoto University, Otsu City, Japan (jinsenzheng@gmail.com)
  • 3Graduate School of Agriculture, Kyoto University, Kyoto City, Japan (shibata.makoto.7c@kyoto-u.ac.jp)
  • 4Graduate School of Agriculture, Kyoto University, Kyoto City, Japan (wang.zixiao.56z@st.kyoto-u.ac.jp)
  • 5Graduate School of Agriculture, Kyoto University, Kyoto City, Japan (watanabe.tetsuhiro.2m@kyoto-u.ac.jp)
  • 6Graduate School of Global Environmental Studies, Kyoto University, Kyoto City, Japan (watanabe.tetsuhiro.2m@kyoto-u.ac.jp)
  • 7Graduate School of Agriculture, Kyoto University, Kyoto City, Japan (funakawa.shinya.2w@kyoto-u.ac.jp)
  • 8Graduate School of Global Environmental Studies, Kyoto University, Kyoto City, Japan (funakawa.shinya.2w@kyoto-u.ac.jp)

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.

How to cite: Du, J., Zheng, J., Shibata, M., Wang, Z., Watanabe, T., and Funakawa, S.: Dissimilatory nitrate reduction processes along a forest hillslope, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10536, https://doi.org/10.5194/egusphere-egu21-10536, 2021.

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