Nitrogen (N) is an essential nutrient for plants. In boreal ecosystems (dominated by tundra and peatlands), low temperatures and soil pH result in slow organic matter decomposition and nitrogen turnover, leading to significantly lower levels of soil available N-particularly extractable inorganic N (EIN: including ammonium (NH₄⁺) and nitrate (NO₃⁻))-compared to low-latitude regions. Consequently, plant growth in boreal ecosystems has long been considered N-limited and primarily depends on extractable organic N (EON). Isotope labeling studies suggested that 63% of the total N in boreal plants derived from soil amino acids (Chapin et al. 1993 Nature; Näsholm et al. 1998 Nature). However, recent work by Prof. Liu Xueyan’s group revealed that the total N contents of boreal plants are comparable to those in low-latitude (Chen et al. 2021 Plant and Soil), and their capacity to absorb soil NO₃⁻ and soil NO₃⁻ contributions to boreal plants (averaged 1/3) are similar to or even higher than that of low-latitude plants (Liu et al. 2018 PNAS). These findings challenged traditional assumptions about boreal plant N use, raising critical questions: What are the contributions of different soil N forms to boreal plants and do these contributions respond to climate warming and increasing N pollution? Addressing this is fundamental for assessing plant N availability and its constraints on vegetation carbon sequestration in boreal ecosystems.
Recently, Prof. Liu Xueyan’s group measured N isotopes in plant communities and soils in northeastern China peatlands (mean annual temperature: -3.3°C; N deposition: 5.1 kg-N/ha/yr). Using a newly developed method (Hu et al. 2024 Nature Communications), they quantified the contributions of N sources to peatland plants and compared the results with prior observations of same genus (including 6 genera and 31 species) in Alaskan tundra (mean annual temperature: -6.8°C; N deposition: 0.4 kg-N/ha/yr; Liu et al. 2018 PNAS). This study found:
Soil EIN was three times less than EON, but it contributed more to total N in studied plants than EON (Figure 1), indicating that the importance of EIN has been substantially underestimated.
Compared to Alaskan tundra, warmer climate (by 3.5°C) and anthropogenic N pollution (12 times) in northeastern China peatlands caused more increments (by three times) in soil EON than EIN, but the relative contributions of EIN and EON to total N of plants remained unchanged (Figure 1).
Figure 1. Effects of environmental changes on soil N availability and plant N utilization in boreal ecosystems.
This study advances our understanding of N cycling processes and provides critical insights for evaluating how future climate change and anthropogenic disturbances may alter N cycling, vegetation dynamics, and carbon sequestration capacity in boreal ecosystems. The article was published in Earth’s Future in May 2025, with Dr. Chen Chongjuan in Tianjin University as the first author and Prof. Liu Xueyan as the corresponding author. Collaborators included Prof. Hu Chaochen in Institute of Geochemistry, CAS, Dr. Xu Shiqi in Institute of Soil Science, CAS, Prof. Wang Xianwei in Northeast Institute of Geography and Agroecology, CAS, Prof. Mao Rong in Jiangxi Agricultural University, and Prof. Keisuke Koba in Kyoto University. The research was supported by the National Natural Science Foundation of China (42125301, 42330505, and 42003061).
Article information:
Chong-Juan Chen, Xue-Yan Liu*, Chao-Chen Hu, Shi-Qi Xu, Xian-Wei Wang, Rong Mao, Keisuke Koba. (2025) Inorganic nitrogen nutrition in boreal plants is underestimated and insensitive to environmental changes. Earth’s Future, 13, e2024EF005723. https://doi.org/10.1029/2024EF005723
Chao-Chen Hu, Xue-Yan Liu*, Avery W. Driscoll, Yuan-Wen Kuang, E. N. Jack Brookshire, Xiao-Tao Lü, Chong-Juan Chen, Wei Song, Rong Mao, Cong-Qiang Liu, Benjamin Z. Houlton. (2024) Global distribution and drivers of relative contributions among soil nitrogen sources to terrestrial plants. Nature Communications, 15, 6407. https://doi.org/10.1038/s41467-024-50674-6
Chong-Juan Chen, Xue-Yan Liu*, Xian-Wei Wang, Chao-Chen Hu, Shi-Qi Xu, Rong Mao, Zhao-Jun Bu, Yun-Ting Fang, Keisuke Koba. (2021) Different leaf carbon, nitrogen, and phosphorus stoichiometry and carbon and nitrogen isotopes among peatland plants in northeastern China. Plant and Soil, 467, 345-357. https://doi.org/10.1007/s11104-021-05085-7
Xue-Yan Liu*, Keisuke Koba*, Lina A. Koyama, Sarah E. Hobbie, Marissa S. Weiss, Yoshiyuki Inagaki, Gaius R. Shaver, Anne E. Giblin, Satoru Hobara, Knute J. Nadelhoffer, Martin Sommerkorn, Edward B. Rastetter, George W. Kling, James A. Laundre, Yuriko Yano, Akiko Makabe, Midori Yano, Cong-Qiang Liu. (2018) Nitrate is an important nitrogen source for arctic tundra plants. PNAS, 115, 3398-3403. https://doi.org/10.1073/pnas.1715382115
