Earth Science Frontiers ›› 2022, Vol. 29 ›› Issue (5): 35-46.DOI: 10.13745/j.esf.sf.2021.9.12

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Vertical distributions of CH4 and N2O in sediments of the Bohai and Yellow Seas in spring

LI Siqi1,2,3(), CHEN Ye1,2,3, YIN Xia2,3,4, ZANG Kunpeng5, ZHEN Yu2,3,4,*()   

  1. 1. College of Marine Life Science, Ocean University of China, Qingdao 266003, China
    2. Laboratory for Marine Ecology and Environmental Science, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266237, China
    3. Key Laboratory of Marine Environment and Ecology, Ministry of Education, Qingdao 266100, China
    4. College of Environmental Science and Engineering, Ocean University of China, Qingdao 266100, China
    5. National Marine Environmental Monitoring Center, Dalian 116023, China
  • Received:2020-07-10 Revised:2020-11-12 Online:2022-09-25 Published:2022-08-24
  • Contact: ZHEN Yu

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Abstract:

In this study, the head-space equilibrium and quantitative PCR methods were used respectively to obtain the concentrations of methane (CH4) and nitrous oxide (N2O) and the vertical abundances of methanogens and SRB in five representative sediment cores in the Bohai and Yellow Seas during two cruises in spring 2018. The relationships between these measurements and the related environmental parameters in pore water were also analyzed. The CH4 and N2O concentrations showed obvious spatial and vertical variations, due to the differences in the hydrological conditions above the sediment and the complexity of carbon/nitrogen biogeochemical processes in sediment cores. The average CH4 and N2O concentrations in sediments ranged between 0.23-0.92 nmol·kg-1 and between 18.90-104.96 nmol·kg-1, respectively, and they were higher overall in the Yellow Sea than in the Bohai Sea. The CH4 concentrations in sediments increased with increasing depth, whereas the $\text{SO}_{4}^{2-}$ concentrations decreased steadily with depth as in a mirror-image relationship with CH4 concentrations, while the abundances of methanogens and sulfate-reducing bacteria followed the same trend, indicating the CH4 production rate in sediments was controlled by the $\text{SO}_{4}^{2-}$ concentration in pore water. The average copy number of the mcrA gene was lower in the Bohai Sea than in the Yellow Sea and increased with depth at all stations except station 3500-7; whilst no significant correlation was detected at all stations between the mcrA gene abundance and CH4 concentration, and the same held true for $\text{SO}_{4}^{2-}$. The copy number of the dsrB gene was at least 2 orders of magnitude higher than that of the mcrA gene and, at all stations, increased at depths above 10 cm then gradually decreased till the bottom of sediments; and there was a weak mirror-image relationship between the dsrB gene copy number and CH4 profiles at all stations, although no significant negative correlation was found between the two. The above results indicate there are other processes that consume CH4 and $\text{SO}_{4}^{2-}$ in sediments. The N2O concentrations roughly decreased with increasing depth but gradually increased at depths below 30 cm; whilst the $\text{NO}_{3}^{-}$ and $\text{NO}_{2}^{-}$ concentrations gradually decreased with depth at all stations while the $\text{NH}_{4}^{+}$ profiles followed the opposite trend. The significant positive correlation between the N2O and $\text{NO}_{2}^{-}$ concentrations and slightly weaker positive correlation between N2O and $\text{NO}_{3}^{-}$ in sediments indicate that denitrification is the main process for the N2O production in marine sediments. These results provide a reference for further understanding the source and distribution of CH4 and N2O, as well as the carbon and nitrogen cycles in the sediments of the continental shelf.

Key words: methane, nitrous oxide, Bohai Sea, Yellow Sea

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