| Methane（CH4）is a kind of greenhouse gas that contributed the most to anthropogenic global climate change after CO2,and its global warming potential is 25times that of CO2 on the centennial scale.CH4 flux in natural ecosystems is an important factor affecting atmospheric CH4 concentration,which is mainly regulated by water status.For example,the wetland is the largest natural source of global CH4 emissions,while grassland often acted as a CH4 sink.The Qinghai-Tibetan Plateau is the third pole of the world and a"water tower"in Asia.It has a complex terrain and has developed many types of ecosystems with unique water characteristics.Over the past 50 years,the Qinghai-Tibetan Plateau has experienced a warming rate twice that of the global average,significantly altering the water profile of the alpine ecosystem.However,little is known about how much water status changes affect CH4 emissions in the alpine ecosystem.It is crucial to clarify the regulatory mechanism of water status on the temporal and spatial patterns of CH4 flux in the alpine ecosystem.Therefore,this study was conducted in 2021,relying on Haibei Experimental Station,combined the vorticity covariance method and the box method to study the seasonal characteristics of CH4 flux in the alpine wetland and meadow,spatial heterogeneity of CH4 flux in the alpine ecosystems,and the regulation of soil moisture and temperature on CH4 flux under natural water table level gradients.The main results are as follows:（1）Alpine meadow was a CH4 sink with an annual uptake of 1.93 g CH4 m-2,while alpine wetland was a strong source of CH4 with an annual emission of 46.17g CH4 m-2in 2021.The CH4 flux in the alpine meadow showed a unimodal seasonal dynamic.The CH4 flux in the non-growing season was consistently low,while the CH4 uptake in the growing season was higher and more variable.The alpine wetland showed a"double peak"seasonal pattern,with a short CH4 emission peak in the spring thawing season in addition to the mid-growing season.Further analysis showed that the seasonal dynamics of CH4 flux in the alpine meadow were mainly affected by solar radiation,temperature and soil water content,while the seasonal pattern of CH4 emission in alpine wetland was related to solar radiation,temperature and GPP.（2）CH4 flux behaved differently between different micro-topography in flooded wetland and seasonally flooded wetland.Among them,the CH4 emission rate of tussock in flooded wetland is about 2.5 times that of flat wetland,and the CH4 emission rate of the puddles in the wet meadow is about 2 times that of the wet meadow.In contrast,there was no significant difference in CH4 uptake rate between the two micro-topographies in mesic meadow.In addition,micro-topography with wetter soil moisture exhibited greater spatial heterogeneity.Specifically,the spatial heterogeneity of CH4flux in the alpine wetland tussock and the wet meadow puddles were 1.32 and 0.59,respectivel.However,the heterogeneity of non-degraded alpine meadow and degraded pika burrow patches were 0.50 and 0.45,respectively.Correlation analysis showed that the spatial variation of CH4 flux was mainly related to factors such as soil moisture,plant biomass,and soil carbon and nitrogen content.（3）On the gradient of water level（from dry to wet）,the source-sink characteristics of CH4 flux changed,gradually changing from a weak sink(-0.04 mg m-2 h-1)to a strong source(0.46 mg m-2 h-1).Moreover,when the annual soil water content was between38-50 v/v%,the CH4 flux on this water table gradient changed from negative to positive.With the increasing of the water table level,the control of soil water on CH4 flux gradually weakened,while the effect of soil temperature on CH4 flux gradually increased.In summary,water status is a key factor regulating the source-sink characteristics,seasonal dynamics and spatial heterogeneity of CH4 flux in alpine ecosystems.Moreover,with the improvement of soil water conditions,the control of CH4 flux by temperature was gradually enhanced.Our results also emphasized that systematic studies based on micro-topography will help to further improve the accurate assessment of CH4 emissions from the Qinghai-Tibetan Plateau. |
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