Study On Soil Greenhouse Gases Fluxes From Eucalyptus Urophylla × Eucalyptus Grandis Plantations

In recent years,the issue of global warming caused by the sharp increase of atmospheric greenhouse gases concentration,which has already become one of the focuses of the world.The key research aspects of greenhouse effect are mainly focus on three major greenhouse gases including carbon dioxide?CO₂?,methane?CH₄?,and nitrous oxide?N₂O?.The forest are the main body of terrestrial ecosystem and plays a highly important role in the global greenhouse gases exchange.Eucalyptus urophylla×Eucalyptus grandis is a main kind of fast growing and high-yield tree species and used as the short-rotation timber plantation in South China.In the process of extensive planting of E.urophylla×E.grandis in South China,the soil greenhouse gases emission/uptake have been concerned widely in the international community.In order to study the seasonal variation and the influence factors of emission fluxes of soil greenhouse gas?CO₂,N₂O and CH₄?from different stand age of E.urophylla×E.grandis plantation?1a,2a,4a,6a?,we continually conducted a one-year-observation,from August 2014 to July 2015,for the greenhouse gas fluxes from four plantations,using Static-Chamber and gas chromatograph.The results were given as follows:?1?The soil of four E.urophylla×E.grandis plantations behaved as CO₂ sources,and the annual means of CO₂ fluxes from soil for the plantation of E.urophylla×E.grandis planted in 2013?hereafter referred to as simply"1a Eucalyptus Plantation"?,the plantation of E.urophylla×E.grandis planted in 2012?hereafter referred to as simply"2a Eucalyptus Plantation"?,the plantation of E.urophylla×E.grandis planted in 2010 hereafter referred to as simply"4a Eucalyptus Plantation")and the plantation of E.urophylla×E.grandis planted in 2008?hereafter referred to as simply"6a Eucalyptus Plantation"?were 22.45t·hm^-2·a^-1,24.53 t·hm^-2·a^-1,27.02 t·hm^-2·a^-1 and 30.13 t·hm^-2·a^-1,respectively.The CO₂emission fluxes increased during stand age growth,and the CO₂ emission fluxes from 6a Eucalyptus Plantation soil were significantly higher?P<0.05?than 2a Eucalyptus Plantation and 1a Eucalyptus Plantation.The emission fluxes of CO₂ from different stand age of E.urophylla×E.grandis plantation were a single-peak curve variation,high in rainy season and lower in dry season?P<0.01?.The CO₂ fluxes from soil in the 1a Eucalyptus Plantation,2a Eucalyptus Plantation,4a Eucalyptus Plantation and 6a Eucalyptus Plantation during the rainy season accounted for 68.36%,74.31%,76.08%and 73.98%of the corresponding total CO₂ emission in the whole year,respectively,with the higher coefficient of variance in dry season than that in rainy season.According to the variance analysis,about 2.97%of the whole year CO₂ fluxes existed within stand age and 64.52%within the seasonal variation,in addition,the seasonal variation higher than that of stand age.CO₂ fluxes from four E.urophylla×E.grandis plantations soil had highly significant?P<0.01?exponential relation with soil temperature,which could explain 31.05%⁵8.40%of the variation of soil CO₂emission fluxes.Q₁₀ was estimated to be 2.32,3.06,3.08 and 3.43 respectively at 5 cm in the 1a Eucalyptus Plantation,2a Eucalyptus Plantation,4a Eucalyptus Plantation and 6a Eucalyptus Plantation.CO₂ fluxes from four E.urophylla×E.grandis plantations soil had highly significant?P<0.01?positive correlations with soil water content,which could explain 31.05%⁵8.40%of the variation of soil CO₂ emission fluxes.?2?The soil of E.urophylla×E.grandis plantation behaved as CH₄ sinks in dry season,but transformed to CH₄ sources someday in rainy season.Throughout the whole year,the soil of four plantations still behaved as CH₄ sinks.The annual means of CH₄ uptake fluxes from soil in 1a Eucalyptus Plantation,2a Eucalyptus Plantation,4a Eucalyptus Plantation and 6a Eucalyptus Plantation were 2.166 kg·hm^-2·a^-1,2.498 kg·hm^-2·a^-1,2.755 kg·hm^-2·a^-1and 2.902 kg·hm^-2·a^-1,respectively.The uptake fluxes of CH₄ from different stand age of E.urophylla×E.grandis plantation were a single-peak curve,with a higher CH₄ uptake fluxes in dry season than that in rainy season.The CH₄ uptake fluxes from soil in the 1a Eucalyptus Plantation,2a Eucalyptus Plantation,4a Eucalyptus Plantation and 6a Eucalyptus Plantation during the dry season accounted for 87.89%,86.87%,70.25%and65.49%of the corresponding total CH₄ uptake in the whole year,respectively,with the higher coefficient of variance in rainy season than that in dry season.According to the variance analysis,the whole year CH₄ uptake fluxes were driven mainly by seasonal variation,which had a strong effect than stand age.CH₄ uptake fluxes from four plantations soil had no significant?P>0.05?positive correlations with soil temperature at 5 cm,but had highly significant?P<0.01?or significant?P<0.05?positive linear correlations with soil water content,which could explain 10.15%²6.02%of the variation of soil CH₄ uptake fluxes.?3?The soil of E.urophylla×E.grandis plantation behaved as N₂O sources in rainy season,but transformed to N₂O sinks someday in dry season.Throughout the whole year,the soil of four plantations still behaved as N₂O sinks.The annual means of N₂O emission fluxes from soil for 1a Eucalyptus Plantation,2a Eucalyptus Plantation,4a Eucalyptus Plantation and 6a Eucalyptus Plantation were 0.63 kg·hm^-2·a^-1,0.78 kg·hm^-2·a^-1,0.91kg·hm^-2·a^-1 and 1.13 kg·hm^-2·a^-1,respectively.The N₂O emission fluxes increased during stand age,and the N₂O emission fluxes from 6a Eucalyptus Plantation soil were significantly higher?P<0.05?than that in 1a Eucalyptus Plantation.The emission fluxes of N₂O from different age stands of E.urophylla×E.grandis plantation were higher during rainy season than that in dry season,with two peaks in May and July.The N₂O fluxes from soil in the 1a Eucalyptus Plantation,2a Eucalyptus Plantation,4a Eucalyptus Plantation and6a Eucalyptus Plantation during the rainy season accounted for 89.48,87.75%,86.41%and79.33%of the corresponding total N₂O emission in the whole year,respectively,with the higher coefficient of variance in dry season than that in rainy season.According to the variance analysis,the whole year N₂O emission fluxes were driven mainly by seasonal variation,which had a strong effect than stand age.N₂O fluxes from four E.urophylla×E.grandis plantations soil had highly significant?P<0.01?positive linear correlations with soil temperature at 5 cm,which could explain 11.44%²8.40%of the variation of soil N₂O emission fluxes.N₂O fluxes from four E.urophylla×E.grandis plantations soil had highly significant?P<0.01?positive linear correlations with soil water content,which could explain 35.18%⁵5.78%of the variation of soil N₂O emission fluxes.?4?The Global Warming Potential?GWP?of E.urophylla×E.grandis plantation soil increased with stand age,with the average value of 26.23 t·hm^-2·a^-1,and mainly composed of CO₂?98.46%⁹9.15%?,whereas minor role of N₂O and CH₄.