Trajectory of Topsoil Nitrogen Transformations Along a Thermo-Erosion Gully on the Tibetan Plateau

Permafrost thaw, especially thermokarst formation, that is, ground collapse due to thawing of ice-rich permafrost, is expected to alter soil gross nitrogen (N) transformations, which can regulate plant productivity and ecosystem carbon cycle. However, it remains unclear how thermokarst formation mod...

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Bibliographic Details
Published in:Journal of Geophysical Research: Biogeosciences
Main Authors: Mao, Chao, Kou, Dan, Wang, Guanqin, Peng, Yunfeng, Yang, Guibiao, Liu, Futing, Zhang, Jinbo, Yang, Yuanhe
Format: Article in Journal/Newspaper
Language:English
Published: AMER GEOPHYSICAL UNION 2019
Subjects:
carbon-nitrogen interactions
gross nitrogen mineralization
gross nitrification
microbial nitrogen limitation
permafrost thaw
thermokarst
Environmental Sciences
Geosciences
Multidisciplinary
AMMONIA-OXIDIZING BACTERIA
PERMAFROST CARBON
MICROBIAL COMMUNITY
EXTRACTION METHOD
SOIL
FOREST
DYNAMICS
AVAILABILITY
INCREASES
PRODUCTIVITY
Environmental Sciences & Ecology
Geology
Ice
permafrost
Thermokarst
Online Access:http://ir.ibcas.ac.cn/handle/2S10CLM1/19460
https://doi.org/10.1029/2018JG004805
Description
Summary:Permafrost thaw, especially thermokarst formation, that is, ground collapse due to thawing of ice-rich permafrost, is expected to alter soil gross nitrogen (N) transformations, which can regulate plant productivity and ecosystem carbon cycle. However, it remains unclear how thermokarst formation modifies soil N processes in permafrost ecosystems. Here N-15 pool dilution techniques were used to evaluate changes in topsoil gross N transformations during various thaw stages (early, middle, and late stages) along a thermo-erosion gully on the Tibetan Plateau. Structural equation modeling was then conducted to explore the relative importance of biotic and abiotic factors in affecting soil gross N transformations. The results showed that topsoil gross N mineralization (GNM) decreased at the three stages, reflecting declined inorganic N production after permafrost collapse. In contrast, topsoil gross nitrification increased only during the early stage. Additionally, the ratio of microbial N immobilization to GNM was enhanced during the middle and late stages, indicating a stronger microbial N limitation after thermokarst formation. The structural equation modeling analysis revealed that soil moisture played an important role in modulating gross N transformations. For GNM, decreased soil moisture had inhibiting effects via regulating the microbial biomass, microbial community, and enzyme activities along the thaw sequence. For gross nitrification, declined soil moisture exerted facilitating effects directly by improving oxygen availability and indirectly by modulating the abundances of ammonia-oxidizing archaea and bacteria during the early stage. Overall, these results demonstrated that thermokarst formation altered soil N processes, potentially triggering interactions between ecosystem N and carbon cycles after permafrost thaw.

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