Molecular mechanisms of water table lowering and nitrogen deposition in affecting greenhouse gas emissions from a Tibetan alpine wetland

Abstract

Rapid climate change and intensified human activities have resulted in water table lowering (WTL) and enhanced nitrogen (N) deposition in Tibetan alpine wetlands. These changes may alter the magnitude and direction of greenhouse gas (GHG) emissions, affecting the climate impact of these fragile ecosystems. We conducted a mesocosm experiment combined with a metagenomics approach (GeoChip 5.0) to elucidate the effects of WTL (-20 cm relative to control) and N deposition (30 kg N ha yr ) on carbon dioxide (CO ), methane (CH ) and nitrous oxide (N O) fluxes as well as the underlying mechanisms. Our results showed that WTL reduced CH emissions by 57.4% averaged over three growing seasons compared with no-WTL plots, but had no significant effect on net CO uptake or N O flux. N deposition increased net CO uptake by 25.2% in comparison with no-N deposition plots and turned the mesocosms from N O sinks to N O sources, but had little influence on CH emissions. The interactions between WTL and N deposition were not detected in all GHG emissions. As a result, WTL and N deposition both reduced the global warming potential (GWP) of growing season GHG budgets on a 100-year time horizon, but via different mechanisms. WTL reduced GWP from 337.3 to -480.1 g CO -eq m mostly because of decreased CH emissions, while N deposition reduced GWP from 21.0 to -163.8 g CO -eq m , mainly owing to increased net CO uptake. GeoChip analysis revealed that decreased CH production potential, rather than increased CH oxidation potential, may lead to the reduction in net CH emissions, and decreased nitrification potential and increased denitrification potential affected N O fluxes under WTL conditions. Our study highlights the importance of microbial mechanisms in regulating ecosystem-scale GHG responses to environmental changes.

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