Effects And Mechanism Of Exogenous Organic Material Inputs On Greenhouse Gas Emissions And Carbon Sequestration In Salinized Cropland

Soil organic carbon（SOC）is not only an important indicator of soil fertility,but also related to food security and global climate change.How to increase soil carbon storage capacity and mitigate greenhouse gas（GHG）emissions on the basis of stable grain production is a hot issue in the field of agricultural science.A large number of studies have shown that excessive application of chemical fertilizers（especially nitrogen fertilizer）in intensive agricultural activities may reduce SOC content,promote greenhouse gas emissions and cause negative environmental effects.Adding exogenous organic materials into soil has been proved to be a relatively scientific fertilization method at present,which can improve the carbon sink function of farmland ecosystem,improve soil quality and gradually increase crop yield.However,due to the influence of many factors,such as climate,soil physical and chemical properties,land use and fertilization,the response mechanism of farmland GHG emission and carbon sequestration process to exogenous organic material input is still unclear.Therefore,it is of great practical significance to carry out relevant research to improve the potential of farmland carbon sequestration,improve the ecological environment problems caused by excessive application of chemical fertilizers,and achieve farmland“carbon sequestration and emission reduction”and national“carbon peaking and carbon neutrality goals”.The level of soil carbon reserves in northwest China is low,and most of them are affected by salinization,which has a great potential of carbon sink.Besides,farmland in northwest area is the main N₂O emission area in China.Therefore,this study selected Hetao irrigated area of Inner Mongolia as the test area.Combined with the situation of increasingly serious secondary salinization,excessive application of fertilizer,and decline of soil quality and carbon pool capacity,the mildly and moderately alkaline soils of cropland in this area were studied,and five treatments including conventional fertilization as control（CK）,conventional fertilization+biochar（BC）,conventional fertilization+farmyard manure（FM）,conventional fertilization+corn straw（CS）,and conventional fertilization+sheep manure（SM）were set,respectively.The dynamics of soil CO₂ and N₂O emissions,environmental factors such as 0-5 cm soil temperature and water content,air temperature and precipitation were continuously monitored under different treatments from 2019 to 2020through field experiments.Comparative analysis was conducted to evaluate the effects of different organic materials on greenhouse gas emissions from mildly and moderately alkaline soils,as well as the comprehensive greenhouse effect.The method of field experiment combined with laboratory analysis was used to evaluate the effects of different organic material addition on soil organic carbon pool and its stability systematically,and to investigate the response mechanism of carbon emissions and sequestration of salinized cropland to exogenous organic inputs from the perspective of labile organic carbon fractions,water stability aggregate distribution,chemical structure of SOC and microbial community structure and diversity.The main results are as follows:1.The organic material additions had no significant effect on N₂O emission in both mildly and moderately alkaline soils.Comprehensive analysis of soil greenhouse gas emission data in the two growing seasons showed that the cumulative CO₂ emission of the mildly alkaline soil under FM and CS treatment was increased by 36.7%and 44.9%,respectively,compared with CK,and the cumulative CO₂ emission under BC treatment was reduced by 16.1%compared with CK.The cumulative CO₂ emissions in the moderately alkaline soil treated with CS increased significantly by 27.4%compared with CK.Canonical and PCA analysis showed that the air temperature,0-5 cm soil temperature and water content were the main environmental factors affecting CO₂ and N₂O emissions.It is concluded that exogenous organic material inputs did not reduce CO₂ and N₂O emissions while maintaining or improving crop yields,among which that CS improved the CO₂ emission as well as increased the global warming potential（GWP）and greenhouse gas intensity（GHGI）.In comparison,BC,FM and SM were more potential in decreasing carbon emission in salinized cropland.2.The SOC content and stock（SOCs）in 0-30 cm depth under BC,FM and SM were significantly increased compared with CK in the mildly alkaline soil,while there were no obvious changes was detected among all the treatments in the moderately alkaline soil.The input of organic materials improved the contents of labile organic carbon（LOC）fractions and carbon pool management index（CPMI）.In the mildly alkaline soil,p H value decreased apparently in the organic material addition treatments compared with CK,and the exchangeable sodium percentage（ESP）decreased 36.9%and 29.3%in BC and FM treatments over CK,respectively.For the moderately alkaline soil,the electricity conductivity（EC）and ESP in organic material addition treatments significantly decreased by 32.8%-55.7%and 18.3%-49.5%,respectively.Comprehensive analysis suggested that FM and SM could significantly increase the organic carbon pool capacity and activity of salinized cropland,and BC was the most effective in soil improvement of salinized cropland.Salinization indexes such as p H,EC and ESP are the main limiting factors for the increase of carbon pool capacity in salinized cropland.3.The aggregate formation and distribution in the mildly alkaline soil reacted more sensitively to organic material inputs over the moderately alkaline.In the mildly alkaline soil,the proportion of macroaggregates（>0.25 mm）was significantly increased by 167.1%,106.9%and 287.0%in FM,SM and SM over CK,respectively.The aggregate stability was increased significantly as well.The carbon sequestration was enhanced by accelerating the accumulation of intra-aggregate organic matter（i POM）within macroaggregates.For the moderately alkaline soil,the proportion of macroaggregates was significantly increased by77.1%and 62.2%under CS and SM over CK,respectively.There were no obvious changes in aggregate stability,aggregate-associated carbon distribution and i POM.Biochar had no positive effect on the formation and stability of large aggregates,but it can promote SOC accumulation by enhancing the distribution of carbon in microaggregates（0.053-0.25 mm）.4.The infrared spectral peak position and spectral band distribution characteristics of soil under different organic material addition treatments in the mildly and moderately alkaline soils were basically the same,and the absorbance peaks were at 3420/3440 cm^-1,2924 cm^-1,1636 cm^-1,1085 cm^-1,1036 cm^-1and 789 cm^-1.There were no significant changes in the SOC chemical composition among different treatments in both mildly and moderately alkaline soils,whereas the relative content of each functional group changed.The content of carbohydrate compounds was significantly increased under organic material addition treatments compared with CK.For moderately alkaline soil,the SOC chemical structure stability was significantly enhanced under FM and SM compared with CK.In addition,the chemical properties of organic materials and soil properties are important factors affecting the structure and morphology of SOC.5.For mildly alkaline soil,the average mineralization rate under FM,CS and SM was significantly increased compared with CK,and the cumulative mineralization was ranged from 261.24 mg kg^-1 to 493.69 mg kg^-1,among which the cumulative mineralization under FM,CS and SM was significantly higher over CK,the priming effect of SOC under BC was negative.For moderately alkaline soil,the average mineralization rate and cumulative mineralization under organic material addition treatments were higher compared with CK,and the cumulative mineralization ranged from 347.46 mg kg^-1 to 437.33 mg kg^-1.The LOC and Available K contents might be the main factors affecting SOC mineralization.The relative higher mineralization loss of SOC in the moderately alkaline soil under organic material addition treatments might an important reason for the difficulty in increasing soil organic carbon pool capacity.6.The dominant microbial community in salinized cropland was bacteria,and the dominant communities were Proteobacteria,Gemmatimondetes,Bacteroidetes and Actinobacteria.The dominant fungi communities were Ascomycota,Basidiomycota and Zygomycota.Organic material inputs had no significant effect on soil microbial community composition,but had different effects on the diversity and richness.Among them,FM,CS and SM treatments increased Chao1 index and Shannon index of bacterial communities in the mildly alkaline soil in varying degrees,while the composition and diversity of bacterial communities in the moderately alkaline soil did not react sensitively to organic material inputs.Fungal community composition andα-diversity in both mildly and moderately alkaline soils did not react sensitively to organic material inputs.PCA analysis showed that the SOC content and its labile fractions and nutrient contents such as TN and Available K were the main factors affected soil microbial community diversity.Organic materials indirectly regulated microbial community composition and diversity by influencing soil physical and chemical properties,which further influenced the carbon sequestration process.