Soil-Atmosphere Exchange of Nitrous Oxide,Methane and Carbon Dioxide of Different Forest Types at Different Elevations in Huoditang Forest Region of Qinling Mountains
Carbon dioxide(CO_2),methane(CH_4)and nitrous oxide(N_2O)are the three main greenhouse gases that have large impacts on the global warming.Temperate forest soils are considered as major sources of CO_2 and N_2O,and also are important sinks for CH_4.Although there are increasing number of studies on greenhouse gas fluxes between forest soils and the atmosphere,the majority of these studies focus on the temporal variations of these trace gases.The spatial patterns of these trace gases are still highly uncertain.From October 2014 to October 2015,static chamber and gas chromatography techniques were used to monitor CO_2,CH_4 and N_2O fluxes of the forest soils at 5 elevation sites(1 560,1 585,1 963,2 040,and 2 160 m,the corresponding forest types including deciduous broad-leaved,temperate coniferous,temperate coniferous,cold temperate coniferous and deciduous broad-leaved forests)in Huoditang Forest Region on the southern slope of the Qinling Mountains.The results showed that CO_2 emitted from the five elevation sites throughout the year.The seasonal CO_2 emissions varied widely.Generally,the annual cumulative CO_2 emissions decreased with the increase of the elevation.The annual cumulative CO_2 from low to high elevation sites(including three forest types)were as follows:19.12,12.53,11.78,16.95 and 14.87 t·hm~(-2).At most of the sampling times,CH_4 consumptions were monitored.However,several times of CH_4 emissions were also found.The seasonal CH_4 fluxes varied widely.In general,the annual cumulative CH_4 consumption increased with the increase of the elevation.The annual cumulative CH_4 consumption from low to high elevation sites were as follows:-2.57,-3.60,-5.94,-5.59 and-3.92 kg·hm~(-2).At the five elevation sites,N_2O emitted throughout the year.However,several times of N_2O consumption were also detected.The annual cumulative N_2O from low to high elevation sites were as follows:0.23,0.62,0.63,0.60,0.95 kg·hm~(-2).Soil temperature was identified as the key factor determining the temporal variability of CO_2 and N_2O fluxes.There was significant correlation between CO_2 flux and soil ammonium content(20-40 cm)(P<0.05)at all the five elevation sites.The high soil nitrate content had an inhibiting effect on CH_4 consumptions.During freezing and thawing period,rainfall had significant impact on the flux of N_2O.From low to high elevations,the global warming potential(GWP)of the five sites were as follows:119.13,12.65,11.85,17.02 and 15.07 t·hm~(-2).