| Methane (CH4) has raised a lot of concern in recent years. This study aimed to investigate the magnitude and seasonal variations of soil CH4flux beneath four subtropical forests in southern China, and to identify the factors controlling the variations. The four subtropical forests are evergreen coniferous (Cunninghamia lanceolata) plantation (CL), evergreen coniferous and broadleaved (Pinus massonia-Lthocarpus glaber) forest (PM), deciduous (Choerospondias axillaries) forest (CA), and evergreen broadleaved (Cyclobalanopsis glauca-Lthocarpus glaber) forest (CG). The in situ static chamber combined with gas chromatograph method were applied to measure soil CH4efflux rate in the four forests in Dashanchong Forest Park for one year.The results indicated that soils in the four forests performed either source or sink over the observe period, that is PM and CA turned out to be the atmospheric CH4sink, simultaneously CG and CL performed as the atmospheric CH4source. The rank from low to high was PM<CA<CG<CL, and annual CH4flux in the four forest types soils are-57.86±45.65(ug CH4-m·2ug CH4·m-2·h-1),-3.15±23.57(ug CH4·m-2·h-1),39.98±28.89(ug CH4·m-2·h-1) and90.65±62.35(ugCH4·m-2·h-1) respectively. This indicated that soil methane flux would change dramatically when tree spices were changed. Soil CH4flux showed a significant seasonal variation, and all the four forest sand performed a same trend, that is lower or take up atmospheric CH4in summer and autumn whereas higher and emit CH4in winter and spring. Further analysis found a negative correlation between soil CH4flux and5cm depth soil temperature, soil will consume atmospheric CH4when soil temperature went above20℃. There was a relationship between soil water content and soil CH4flux in some degree, but the correlation coefficient was low (the correlation equation of Pinus massonian-Lthocarpus glaber mixed forest (PM), Cunninghamia lanceolata pure forest (CL), Choerospondias axillaris forest (CA) and Cyclobalanopsis glauca-Lthocarpus glaber mixed forest (CG) was0.541,0.505,0.243and0.495respectively), because soil water content was affected by slop degree, precipitation, atmospheric temperature and plant simultaneously, the effect was complex, linear regression was not efficient to analyze it, so more research will be needed. Linear regression analysis shows a relation between soil temperature at5cm depth, soil water content and water filled porespace (WFPS). Soil temperature at5cm depth affects soil CH4flux most. Our study implies the importance for sampling and scaling strategies for estimating methane flux in subtropical forest soils. Soil methane flux performed a direct correlation with soil WFPS, though correlation coefficient was low (r2=0.08), but regression equation turned out to be extremely significant (p<0.01); indicated water will fill gas exchange tract, in other words pore space, when soil WFPS went up, reduce soil-atmosphere gas exchange property made soil oxidizing methane capacity went down, but soil produce methane capacity went up. Soil pH in research aera was lower than the optimum pH value, constrained methane oxidizing bacteria physiological activity, and A13+ion exchange from soil particulate by H+ion would depress methane oxidizing bacteria activity. Analysis of variance result showed a significant variance between the four forest type soil bulk density (p<0.05), result of multiple compassion found significant variation only existed between Cunninghamia lanceolata pure forest and Pinus massonian-Lthocarpus glaber mixed forest, Choerospondias axillaris forest, Cyclobalanopsis glauca-Lthocarpus glaber mixed forest, this partly explained why there was variation between different forest types. |
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