| Research on the variation charaters of forestry ecosystem methane and transition process of its source and sink has been considered to be the important and difficult topic in the Meteorology,climatology,hydrology,ecology and other related disciplines.The low hilly area of north China is located in the warm temperate climate zone.Due to its special geographical location and climate,it has always been one of the key areas of forestry ecological engineering construction in China.But so far,there is no detailed studies reported on the change of forest methane flux and its control mechanism.Then,it is of scientific importance to quantitatively evaluate the accumulated methane fluxes and global warming potenitial throngh studing on the characteristics and influencing mechanism of methane fluxes of planted forest in hilly area of north China.There are some uncertainties in the data processing and quality control by using close path eddy covariance method,the accuracy of some parameters is necessary to apply in measuring methane flux in forestry ecosystem.The methane flux in the cork oak,cedar-black and locust system was the object in the Hilly Northe of China.According to the characteristics of turbulence,the CPEC system's accuracy of sampling flow rate,delay time and average period were quantitatively analyzed,and combing with OPEC system,the methane flux was measured during July in 2016 to November in 2019.The variation characteristics and the main controlling factors at different periods were analyzed,and warming potential was discussed.The main results in this paper are as follows:(1)Atmospheric turbulence spectrum analysis showed that the quality of data measured by closed-path eddy covariance system(CPEC)meet the requirements of the flux calculation.Under(un)stable in sunny day,the spectrum of wind speed,temperature,CO2,H2O,CH4 in inertial slope meet-2/3 and-5/3,and the co-spectrum meet-4/3 slope.Then the instrument can be a very good response to the high frequency signal,and also can meet the basic requirement of flux observation.According to the standard of the OPEC system,the spectral analysis of the raw data(10Hz)observed by the CPEC system at different flow rates showed that the appropriate flow rate range was greater than 31.5l min-1,and the optimal flow rate was 35.5l min-1.The sampling frequency of the instrument was reasonable at 10Hz.Compared with the OPEC observation system,the average delay time of CPEC system is 8-9s.After the delay correction,the data quality of CPEC system is relatively good.In future flux observation experiments,the two systems can be used as complementary observation techniques for parallel observation.In the case of the underlying surface in this study area,the CH4 flux data with an average calculation cycle of 60min and 15min are suitable for the study of the CH4 flux variation characteristics at the scales of month and above,day and below,respectively.(2)By using the footprint model analysis on different wind flux distribution of the source area,the results show that the observation data from which can be well observed in any direction the wind up the flux source area,the observation data has good spatial representation:both in the growing season and in the growing season,the footprint of the unstable condition are less than the steady state,the growing season footprint is greater than the growing season.The diurnal variation distribution of the flux source region is non-uniform,and footprint is larger during the day than at night,and the area is the smallest at noon.(3)The CH4 flux in a mixed plantation ecosystem has obviously diurnal and seasonal variations.During the observation period,the monthly average diurnal variation of the CH4 flux in each month is a single-peak trend,and the CH4 flux value is positive and CH4 source in the daytime,while the CH4 flux value is negative at night,with no obvious change.The daily mean maximum value of CH4 flux in growing season was lower than that in non-growing season.The monthly average daily maximum CH4 pass value appeared in March,and the minimum value appeared in October.Different periods of typical sunny days and rainy days of daily variation of CH4 features found:the growing season and the growing season in the sunny day have obvious diurnal variation,daily variation trend of rainy day,rainy days the most,significantly lower than the value in the growing season sunny days,the growing season and the transition period of growing season,rainy day change is not obvious,and in the growing season in accordance with clear trends.The CH4 flux has obvious seasonal variation.In march,the volume reached the highest peak of the whole year.In June,it reached the first low peak of the whole year.In October,it dropped to the lowest peak of the whole year.The artificial mixed forest in the low hills and mountains of north China is an atmospheric CH4 source,and the annual CH4 flux emission is2017>2018>2019>2016,the main reason was that the precipitation in 2016 was the highest,about twice that of 2017.By studying the diurnal variation characteristics of CH4 flux before,during and after rainfall,it is found that rainfall has a hysteresis effect on CH4 flux,which is about 3 days in the hysteresis period.At the same time,rainfall also changes the transient conversion of CH4 daily flux source/sink.Precipitation intensity and frequency lead to different CH4 year flux results.(4)PCA and RDA analysis showed significant differences in the influence of different hydrothermal factors on CH4 flux.On the annual scale,all the hydrothermal factors were significantly correlated with the CH4 flux.Scale,but,summer,winter two season,atmospheric temperature,relative humidity,net radiation and photosynthetic active radiation and there was a significant correlation between CH4 fluxes,different depth of soil temperature and soil water content and CH4 fluxes have certain relevance,where 0 to 5 cm the correlation between soil moisture content and CH4 flux is highest,followed by 5-10 cm soil temperature.However,the influence of rainfall on CH4 flux is relatively complex,and it has significant influence on CH4flux on a daily scale,but there is no significant correlation.Analysis of natural continuous rainfall weather(October 19-28,2016;The relationship between various hydrothermal factors and CH4 flux in different periods shows that:at the early stage of rainfall,air temperature,net radiation and soil temperature in each layer have a significant positive correlation with CH4 flux,while soil moisture content in each layer has a significant negative correlation with CH4 flux.Relative humidity,net radiation,soil temperature in each layer and soil moisture content of 10-20cm had significant negative correlation with CH4flux,while soil moisture content of 5-10cm had significant positive correlation with CH4 flux.In the late precipitation period,CH4 flux has a significant positive correlation with air temperature,net radiation and 0-5cm soil temperature,and a significant negative correlation with relative humidity,10-15cm,15-20cm soil temperature and 5-10cm and 10-20cm soil water content.In the middle period of rainfall,the positive correlation between relative humidity and soil moisture content in each layer was extremely significant,while the air temperature and soil temperature in each layer had a dominant negative correlation with CH4 flux.At the end of rainfall,CH4 flux has a significant positive correlation with air temperature,net radiation and soil temperature of0-5cm,and a significant negative correlation with relative humidity and soil moisture content of10-20cm.Other influencing factors have no significant influence on CH4 flux.Through the path analysis,the comprehensive effect of the main hydrothermal factors and CH4 flux was analyzed.Precipitation,atmospheric temperature,net radiation,5-10cm soil temperature and relative humidity were the main factors affecting CH4 flux.The path coefficients for CH4 flux were 0.61,0.58,-0.49,0.24 and 0.11,respectively.Photosynthetic effective radiation and soil moisture content of 5-10cm had relatively small influence on CH4 flux path coefficients of 0.05 and 0.02,respectively.At the same time,net radiation and rainfall influence CH4 flux indirectly through atmospheric temperature and soil moisture content,respectively(5)the cumulative methane fluxes of this artificial mixed forest ecosystem in spring,summer,autumn and winter are 0.026,0.021,0.011?0.02 kg·hm-2,respectively,accounting for 33.37,26.82,4.53 and 25.28 for the whole year.Its relative greenhouse potential is 0.728,0.588,0.308and 0.56 kg·CO2·hm-2,respectively.Its annual cumulative flux is 0.078kg·hm-2·a-1,and its relative greenhouse potential is 2.184 kg·CO2·hm-2. |
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