| In recently years, methane which is a greenhouse gas has become a research emphasis of global change due to the rise of methane levels caused by human activity. Forest ecosystem plays a very important role on the mitigation of global climate change. And many studies have shown that forest soil is a major source and sink of methane. In this study, we selected three typical forest types (Cunninghamia lanceolata pure forest, Cyclobalanopsis glaiica-Lthocarpus glaber mixed forest, Pinus massonian-Lthocarpus glaber mixed forest) in Changsha Dashanchong, used static chamber-gas chromatography to observe soil methane flux for a period of one year (between December 8,2013 to November 26,2014). And we studied the diurnal and seasonal variation of soil methane flux, analyzed the impact of soil moisture, soil temperature and other environmental factors on methane flux. The results are:(1) The diurnal variation of methane flux:Three forest types were presented as a source of atmospheric methane flux in january. The descending order was: Cunninghamia lanceolata pure forest (83.67±25.81 μg·m2·h-1)>Pinus massonian-Lthocarpus glaber mixed forest (20.25±28.2μg·m2·h-1)>Cyclobalanopsis glaiica-Lthocarpus glaber mixed forest (9.56 ±22.59 μg·m2·h-1). Different forest types had an influence on soil methane fluxes. Soil temperature of diurnal variation was not the main factor of methane flux change.(2) The monthly variation of methane flux:the source-sink status was different among different forest types. Cunninghamia lanceolata pure forest and Pinus massonian-Lthocarpus glaber mixed forest presented as sinks of atmospheric methane. Cyclobalanopsis glaiica-Lthocarpus glaber mixed forest presented as a source of atmospheric methane. Average soil methane flux during the year descending for Cyclobalanopsis glaiica-Lthocarpus glaber mixed forest (8.94±16.65 μg·m2· h-1)> Cunninghamia lanceolata pure forest (-8.39±6.84 μg·m2·h-1)> Pinus massonian-Lthocarpus glaber mixed forest (-38.01±13.9 μg·m2·h-1). Seasonal variation was significantly different in three forest types (P<0.05). Soil methane fluxes in three forests presented as sinks of atmospheric methane in growing season, and presented as sources of atmospheric methane in the non-growing season. The effect of slope on soil methane was not significant (P>0.05).(3) Effect of temperature on soil methane fluxes:soil methane flux (in underground 5 cm) in three forest types was significantly affected by temperature during the growing season (P<0.05). The results presented as that:Cunninghamia lanceolata pure forest (Y=-1.676X+15.877, R2= 0.3682), Cyclobalanopsis glaiica-Lthocarpus glaber mixed forest (Y=-6.8052X+112.72, R2=0.8435), Pinus massonian-Lthocarpus glaber mixed forest (Y=-6.8052X+112.72, R2= 0.8435).(4) The effect of moisture content on soil methane flux showed that:in the observed period, the soil moisture content was not the main factor led to the variation of methane flux. Soil moisture contents in three forest types were higher in spring, lower in other seasons. The difference of soil moisture among three forest types was significant (P<0.05). The annual average moisture content ranked in the order Cunninghamia lanceolata pure forest (31.88%±1.06%)> Cyclobalanopsis glaiica-Lthocarpus mixed forest (21.58%±1.12%)> Pinus massonian-Lthocarpus mixed forest (19.98%±1.1%). Differences of soil moisture among forest types were very significant (P<0.01). And the difference among different months was also very significant (P<0.01).(5) The effect of soil pH on soil methane flux was showed that:soils in Cunninghamia lanceolata pure forest, Cyclobalanopsis glaiica-Lthocarpus glaber mixed forest, Pinus massonian-Lthocarpus glaber mixed forest were acidic. The soil pH was 4.49,4.4 and 4.27. Soil pH may inhibit the production of methane, and promote the absorption of soil methane in research area.(6) The effect of soil texture on soil methane flux was showed that:soil in research area belongs to red soils. And moisture content was not high. They may be the reason led to methane flux change. The differences of soil bulk density in different forest types were significant (P<0.05). Soil bulk density ranked in the order Cunninghamia lanceolata pure forest (1.23±0.25 g/cm3)> Pinus massonian-Lthocarpus glaber mixed forest (1.21±0.12 g/cm3)> Cyclobalanopsis glaiica-Lthocarpus glaber mixed forest (1.19±0.18 g/cm3).(7) The effect of soil organic matter on soil methane flux was showed that:the effect of soil carbon content on methane flux was not significant (P>0.05). It indicated that soil carbon was not a major factor which led to soil methane flux change. The C:N ratio in Cunninghamia lanceolata pure forest was positively correlated with soil methane flux. |
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