Divergence in ecosystem carbon fluxes and soil nitrogen characteristics across alpine steppe, alpine meadow and alpine swamp ecosystems in a biome transition zone

Yongwen Liu; Xiaodong Geng; Tenzintarchen; Da Wei; Dongxue Dai; Xu-Ri

doi:10.1016/j.scitotenv.2020.142453

Divergence in ecosystem carbon fluxes and soil nitrogen characteristics across alpine steppe, alpine meadow and alpine swamp ecosystems in a biome transition zone

Sci Total Environ. 2020 Dec 15:748:142453. doi: 10.1016/j.scitotenv.2020.142453. Epub 2020 Sep 22.

Authors

Yongwen Liu¹, Xiaodong Geng², Tenzintarchen³, Da Wei⁴, Dongxue Dai⁵, Xu-Ri⁶

Affiliations

¹ Key Laboratory of Alpine Ecology, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China; CAS Center for Excellence in Tibetan Plateau Earth Sciences, Chinese Academy of Sciences, Beijing 100101, China. Electronic address: liuyongwen@itpcas.ac.cn.
² School of Resources and Environment, Anqing Normal University, Anqing 246133, China.
³ Institute of Pratacultural Science, Tibet Academy of Agricultural and Animal Husbandry Sciences, Lhasa 850000, China.
⁴ Key Laboratory of Mountain Surface Processes and Ecological Regulation, Institute of Mountain Hazards and Environment, Chinese Academy of Sciences, Chengdu 610041, China.
⁵ College of Biology and Food Science, Hebei Normal University for Nationalities, Chengde 067000, China.
⁶ Key Laboratory of Alpine Ecology, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China; CAS Center for Excellence in Tibetan Plateau Earth Sciences, Chinese Academy of Sciences, Beijing 100101, China. Electronic address: xu-ri@itpcas.ac.cn.

PMID: 33113693
DOI: 10.1016/j.scitotenv.2020.142453

Abstract

Alpine ecosystem carbon cycling is sensitive to climate change, particularly in the transition zones between biomes. Soil nitrogen conditions, including the ammonium to nitrate (NH₄⁺/NO₃^-) ratio, regulate ecosystem carbon uptake by coupling carbon‑nitrogen cycle. The largest alpine pasture on Earth is distributed on the Tibetan Plateau, where alpine biome transition zones are also widely distributed. However, it is largely unknown how the soil NH₄⁺/NO₃^- ratio and net ecosystem CO₂ exchange vary among vegetation types in the alpine biome transition zones due to a lack of in situ field observations. Here, we investigated soil NH₄⁺/NO₃^- ratio and ecosystem carbon fluxes across alpine steppe, alpine meadow and alpine swamp ecosystems in a biome transition zone on the central Tibetan Plateau. The results showed that soil NH₄⁺/NO₃^- ratio was lowest in the alpine steppe (driest environment), which had the highest soil pH, and highest in the alpine swamp (wettest environment), which had the lowest soil pH. We proposed a theoretical framework describing how soil moisture regulates soil NH₄⁺/NO₃^- ratio by altering both the denitrification process and soil pH. We further found that the growing season average net ecosystem CO₂ exchange for the alpine steppe, alpine meadow and alpine swamp was -1.46, -1.90 and -5.43 μmol m^-2 s^-1, respectively. This divergence in net ecosystem CO₂ exchange across the three grasslands is primarily explained by divergence in gross ecosystem photosynthesis, rather than ecosystem respiration. The air temperature sensitivity of ecosystem respiration (Q₁₀) for the alpine steppe, alpine meadow and alpine swamp was 1.73 ± 0.05, 1.44 ± 0.03 and 2.43 ± 0.45, respectively. Our study highlights large differences in both soil nutrient and ecosystem carbon uptake across different vegetation types in an alpine biome transition zone. More in situ investigations in various biome transition zones are urgently needed to quantitatively understand the spatial pattern of alpine ecosystem carbon‑nitrogen cycling processes.

Keywords: Carbon and nitrogen cycling; Climate change; Denitrification rate; Gross ecosystem photosynthesis; Net ecosystem carbon dioxide exchange; Tibetan Plateau.