Soil Greenhouse Gas Emissions From Multiple Land-use Types And The Environmental Influencing Factors

Greenhouse gases（GHGs）are one of the main drivers leading to global climate change.The rate of GHGs emissions（such as CO₂,CH₄,N₂O）have been directly or indirectly accelerated accompanied by the anthropogenic activities,such as changes of land-use,reclamation,and fertilization.How to reduce and assess GHGs under this changing global climate have become the research hotspot for the 21st century.However,there is no definite explanation on the influence of land-use and environmental factors on GHGs fluxes at present.This research results will provide data for GHGs emission and carbon sink estimation.Meanwihle,it will help the government to achieve the goal of‘carbon neutrality’and formulate reduction policies of carbon or nitrogen emission.Thus,this will provide theoretical basis on solving global climate change problems.In this research,we collected soil samples from seven different land-use sites（Estuarine paddy,Estuarine forest,Estuarine salt marsh,Red-soil forest,Red-soil upland,Red-soil orangery,Red-soil paddy）.We used one of the most advanced GHGs analyzer Picarro-G2508 combined with closed chamber method to measure the soil GHGs（CO₂,CH₄,N₂O）simultaneously.Serials of laboratory control experiments were performed on soil by incubating soil at chosen temperature,soil moisture,salinity and nitrogen fertilization levels in growth chamber,aiming to explore the differences of soil GHGs fluxes in different land-use types and their response mechanisms to key environmental impact factors.The main results are as follows:（1）Changes in temperature,soil moisture,salinity,and nitrogen fertilization all significantly affect the soil CO₂ emissions（through soil respiration）,but at different sensitivity levels.There is a significant positive exponential correlation between soil respiration and temperature（R²>0.81,p<0.01）;the response to soil moisture and salinity both increased first and then decreased,reaching the peak values at 20%GWC（which is the soil saturated moisture）and 5‰salinity.While nitrogen application causes inhibitation to the soil respiration from paddy field of red soil.In addition,the calculation results of the soil respiration Q₁₀ model for estuarine forest,estuarine salt marsh,estuarine paddy,red-soil paddy,red-soil orangery,red soil forest and red-soil upland 1.51,1.47,1.39,3.17,3.12,2.61 and 2.51,respectively,which suggests the higher temperature sensitivity of red soil generally.Since the redox potential decreases with increasing soil moisture,the consumption of CH₄（oxidation）will reduce,which promotes the CH₄ emission of paddy soil（R²=0.89）.While CH₄ emission from upland,orangery,and forest red soil was not correlated with soil moisture.Besides,the inhibition of methanogens activities by nitrogen fertilization and salt water intrusion led to the suppression of CH₄ emissions（a negative correspondence was found between CH₄ emission of salt marsh and salinity（R²=0.81,p<0.0001））.The response of soil N₂O emission to different environmental factors is more complicated.Firstly,only red soil paddy N₂O emissions are positively correlated with temperature.Secondly,N₂O emissions have significant positive responses to soil moisture,with peak values at25%GWC soil moisture.Moreover,nitrogen fertilization significantly promotes the emission of N₂O from red paddy soils,which maybe caused by the accumulation of NO₃^﹣and NH₄⁺substrates.In comparison,the promotion degree was NO3^﹣-N>NH4⁺-N>H2NCONH2-N.In addition,soil N₂O emission peaked on the second day after nitrogen being applied and decreased over time,which returned to the initial level on the 15th day,while was due to the decrease of nitrification and denitrification substrates.（2）Through in-situ observations of GHGs in the phragmate salt marsh wetlands with different salinities along the Yangtze River estuary,the gas fluxes differ significantly with various salinities,and shares similar trends with indoor experiments.The salinity value of 5‰was a key point that affects the trend of soil GHGs emission.Field and indoor experiments had consistently shown that salt marsh soil respiration reaches a peak at 5‰salinity,while soil CH₄ flux decreased slowly at 5‰salinity,and stabilized in a lower range,dropping to about one-sixtieth of the 0‰salinity.（3）There were significant differences in soil greenhouse gas emissions between different land-use types in the two ecosystems.The global warming potential（GWP）is expressed as following:Estuarine paddy>Estuarine forest>Estuarine salt marsh,Red-soil paddy>Red-soil orangery>Red-soil forest>Red-soil upland.This result suggests that soil carbon emissions under the influence of antrophogenic activities are much higher than the natural salt marsh soils.In addition,the two composite space utilization models in this study shows the regional carbon accouting results:estuarine soil 0.68T·hm^-2·yr^-1,red soil 1.55 T·hm^-2·yr^-1;The soil densities of organic carbon were 12.67T·hm^-2 in the estuary and 23.58 T·hm^-2 in the red soil.Thus,a higher degree of spatial intensification makes southern red soils have higher carbon emissions and carbon storage.（4）Redundant analysis of the effects of all influencing factors on GHGs emission show that the effects of all environmental factors are rated as:temperature>salinity（conductivity）>soil moisture>nitrogen fertilization.Among all the influencing factors,soil carbon and nitrogen content had the greatest influence on GHGs emissions,followed by p H.Besides,the resolution of RDA results for CO₂,CH₄,and N₂O reached63.20%,67.48%and 52.03%,respectively.In summary,the physical and chemical properties of soil carbon and nitrogen content were the main reasons for the significant differences in soil GHGs emissions of multipal land-use types.In this study,paddy soils have the largest global warming potential,with salt marsh soil has the smallest.Soil CO₂,CH₄,N₂O emissions had a certain regular response to temperature,soil moisture,salinity,and nitrogen fertilization.Their impact on GHGs emissions is:temperature>salinity（conductivity）>soil moisture>nitrogen application.The increase in NO₃^﹣and NH₄⁺substrate concentrations brought by nitrogen was the main reason for the increase of N₂O emissions,and the increased SO₄^2-caused by salt water intrusion was the main reason for suppressing CH₄ emissions.