Characteristics Of Organic Matter In Biogas Slurry From Livestock Manure And Its Effect On Soil Carbon Stocks In Wheat And Maize Rotation Field

Anaerobic fermentation technology is an important means of livestock manure treatment.The most common way to absorb a large amount of biogas slurry after fermentation is farmland utilization.Dissolved organic matter(DOM)and humic substances(HS)are two important types of organic matter in biogas slurry,which have a profound impact on farmland soil carbon stocks and turnover.In this study,the content and structure of DOM and HS in biogas slurry at different fermentation concentrations were analyzed;On this basis,the effects of dairy manure biogas slurry application on soil carbon stocks and its driving factors in wheat maize rotation farmland were explored.The purpose of this study is to provide a scientific reference for the safe use of biogas slurry in farmland and carbon sequestration and emission reduction.The main results are as follows:(1)The content and structure of DOM in pig and dairy manure biogas slurry were studied by batch anaerobic fermentation at moderate temperature(37±1℃).The results showed that the concentration and quality of DOM in biogas slurry decreased significantly(P<0.01)when the total solid content(TS)was 4% and 8%.The results of three-dimensional fluorescence(3DEEM)regional integration showed that the humification index(HIX)of DOM in pig and dairy manure after fermentation was significantly increased.Parallel factor analysis(PARAFAC)showed that DOM was mainly composed of tyrosine like(C1),fulvic acid like(C2),tryptophan like(C3)and humic acid like(C4)fluorescent components.The relative content of C1 and C3 decreased significantly(P<0.01),and the relative content of C4 increased significantly(P<0.01),but the relative content of C2 increased only when TS was 8%,and decreased when TS was 4%.This indicates that the soluble organic matter in biogas slurry tends to transform to more stable humic acid like acid after moderate temperature anaerobic fermentation of pig and dairy manure.(2)On the basis of batch anaerobic fermentation at moderate temperature,the content and structure of HS in pig and dairy manure were studied.The results showed that the contents of total HS and fulvic acid(FA)decreased significantly(P<0.01);However,the content of humic acid(HA)only decreased significantly(P<0.01)in pig manure biogas slurry.Fourier transform infrared spectroscopy(FTIR)analysis showed that the proportion of sugars in pig manure biogas slurry increased,while the proportion of aliphatic,phenolic and protein compounds decreased,while the proportion of polysaccharides in dairy manure biogas slurry decreased,and the proportion of aliphatic,lipid and phenolic compounds increased.Aromatic compounds in biogas slurry increased significantly,but decreased slightly in the biogas slurry with TS of8%.(3)A field plot experiment was conducted to study the effects of dairy manure biogas slurry on soil carbon stocks and its components in wheat maize rotation farmland.The results of two-year experiment showed that biogas slurry application significantly(P<0.05)improved the content of soil organic carbon(SOC)in surface layer(0-10 cm)and subsurface layer(10-20 cm),carbon storage and the stability of aggregates in surface soil.In the surface soil,the lability of soil carbon stocks increased significantly after the application of biogas slurry with150% chemical fertilizer and equal amount of nitrogen;In the subsurface soil,there was no significant difference in carbon stocks lability among the treatments after fertilization.Biogas slurry application had the greatest effect on the formation and stability of soil macroaggregates(>0.25 mm).The change rule of SOC content in aggregates from large to small is as follows:large aggregates > micro aggregates > silty clay,and the content of organic carbon with different lability in aggregates also follows this change feature.The increase of SOC in the surface layer is mainly due to the increase of large aggregates,while that in the subsurface layer is mainly due to the joint increase of large aggregates and micro aggregates.(4)EEM-PARAFAC analysis showed that the humification degree of topsoil after biogas slurry application was higher,which was conducive to the formation and stability of soil aggregates.Correlation analysis showed that mean weight diameter(MWD)and HS content had significant positive correlation with SOC and its active components in topsoil;In the surface soil,HS had a significant positive correlation with the high active organic carbon(HSOC)of the whole soil and different size fractions of soil(P<0.05).The results of redundancy analysis(RDA)showed that there was a correlation between soil carbon stocks and aggregate SOC and its active components,and H-SOC was an important factor driving the change of SOC.The contents of DOM,HS and HA in biogas slurry were the main influencing factors of surface soil carbon stocks and its lability and humification degree,while the contents of HS,FA and DOM in biogas slurry were the main influencing factors of subsurface soil carbon stocks and its lability and humification degree.