High nitrogen isotope fractionation of nitrate during denitrification in four forest soils and its implications for denitrification rate estimates

Ang Wang; Yunting Fang; Dexiang Chen; Oliver Phillips; Keisuke Koba; Weixing Zhu; Jiaojun Zhu

doi:10.1016/j.scitotenv.2018.03.261

High nitrogen isotope fractionation of nitrate during denitrification in four forest soils and its implications for denitrification rate estimates

Sci Total Environ. 2018 Aug 15:633:1078-1088. doi: 10.1016/j.scitotenv.2018.03.261. Epub 2018 Mar 28.

Authors

Ang Wang¹, Yunting Fang², Dexiang Chen³, Oliver Phillips⁴, Keisuke Koba⁵, Weixing Zhu⁶, Jiaojun Zhu⁷

Affiliations

¹ CAS Key Laboratory of Forest Ecology and Management, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110164, China; Research Institute of Tropical Forestry, Chinese Academy of Forestry, Guangzhou 510520, China; Qingyuan Forest CERN, Chinese Academy of Sciences, Shenyang 110016, China; College of Resources and Environment, University of Chinese Academy of Sciences, Beijing 100049, China.
² CAS Key Laboratory of Forest Ecology and Management, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110164, China; Qingyuan Forest CERN, Chinese Academy of Sciences, Shenyang 110016, China. Electronic address: fangyt@iae.ac.cn.
³ Research Institute of Tropical Forestry, Chinese Academy of Forestry, Guangzhou 510520, China. Electronic address: dexiangchen@ritf.ac.cn.
⁴ School of Geography, University of Leeds, Leeds, UK.
⁵ Center for Ecological Research, Kyoto University, Shiga 520-2113, Japan.
⁶ CAS Key Laboratory of Forest Ecology and Management, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110164, China; Department of Biological Sciences, Binghamton University, The State University of New York, Binghamton, NY 13902, USA.
⁷ CAS Key Laboratory of Forest Ecology and Management, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110164, China; Qingyuan Forest CERN, Chinese Academy of Sciences, Shenyang 110016, China.

PMID: 29758860
DOI: 10.1016/j.scitotenv.2018.03.261

Abstract

Denitrification is a major process contributing to the removal of nitrogen (N) from ecosystems, but its rate is difficult to quantify. The natural abundance of isotopes can be used to identify the occurrence of denitrification and has recently been used to quantify denitrification rates at the ecosystem level. However, the technique requires an understanding of the isotopic enrichment factor associated with denitrification, which few studies have investigated in forest soils. Here, soils collected from two tropical and two temperate forests in China were incubated under anaerobic or aerobic laboratory conditions for two weeks to determine the N and oxygen (O) isotope enrichment factors during denitrification. We found that at room temperature (20°C), NO₃^- was reduced at a rate of 0.17 to 0.35μgNg^-1h^-1, accompanied by the isotope fractionation of N (¹⁵ε) and O (¹⁸ε) of 31‰ to 65‰ (48.3±2.0‰ on average) and 11‰ to 39‰ (18.9±1.7‰ on average), respectively. The N isotope effects were, unexpectedly, much higher than reported in the literature for heterotrophic denitrification (typically ranging from 5‰ to 30‰) and in other environmental settings (e.g., groundwater, marine sediments and agricultural soils). In addition, the ratios of Δδ¹⁸O:Δδ¹⁵N ranged from 0.28 to 0.60 (0.38±0.02 on average), which were lower than the canonical ratios of 0.5 to 1 for denitrification reported in other terrestrial and freshwater systems. We suggest that the isotope effects of denitrification for soils may vary greatly among regions and soil types and that gaseous N losses may have been overestimated for terrestrial ecosystems in previous studies in which lower fractionation factors were applied.

Keywords: Denitrification; Forest soils; Gaseous N losses; N-isotope fractionation; O-isotope fractionation; Δδ(18)O:Δδ(15)N.