Effect Of Cover Crop On Soil Greenhouse Gas Emission,Microbial Biomass And Community Structure

(1)Cover crops provide multiple agronomic and environmental benefits,such as enhanced soil carbon sequestration,aggregation,water infiltration,and reduced erosion and nutrient leaching compared with no cover crop.However,little is known regarding the effect of cover crop species,biomass quality and quantity,and method of residue placement on greenhouse gas?GHG?emissions.Using meta-analysis,this study examined the effect of cover crop species,quality and quantity of biomass,and residue management on response ratios?RRs?of cover crops to no cover crop on CO₂ and N₂O emissions following cash crops.All cover crop species increased CO₂ emissions,but reduced N₂O emissions compared with no cover crop,except legumes which increased N₂O emissions.Cover crop biomass explained 63%of variability in increased CO₂ emissions and 55%of variability in decreased N₂O emissions.Both CO₂ and N₂O emissions decreased polynomially with increased cover crop biomass C/N ratio,with greater rate of decline for legumes than nonlegumes.Cover crop residue incorporated into the soil increased CO₂ and N₂O emissions compared with the residue placed at surface or removed from the soil.Cover crops emitted lower CO₂ and N₂O emissions than no cover crops in silty loam and sandy clay loam soils,respectively,compared to clay loam and silty clay loam soils.Both soil organic carbon and total nitrogen increased with cover crop compared to no cover crop.Although CO₂ and N₂O emissions varied with cover crop species,using legume and nonlegume cover crop mixture to enhance residue C/N ratio compared to legumes and placing the residue at the surface instead of incorporating into the soil can reduce GHG emissions.Because of enhanced soil C and N storage and other known benefits,improvement in soil health and environmental quality due to cover crop may outweigh CO₂ emissions compared to no cover crop.?2?Cover crops have been increasingly grown for improving soil health and crop production and minimizing environmental impact compared to no cover crop.No systematic quantification of cover cropping on soil microbial communities,biomass,and activity,however,are documented in the literature.A meta-analysis of 81 currently available literature was conducted to elucidate the effect of cover crop on total phospholipid-derived fatty acids?PLFA?,microbial biomass C and N?MBC and MBN?,fungi and bacteria groups following cash crops compared to no cover crop.Compared to no cover crop,cover crop overall enhanced PLFA,MBC,MBN,total bacteria and fungi,gram-positive and-negative bacteria,actinomycete,arbuscular mycorrhizal fungi?AMF?and AMF root colonization and spore density.The range of increase ranged from 11.1%for gram-positive bacteria to 51.3%for MBN globally.Soil fungi were more affected by cover crop than bacteria as indicated by the greater fungi/bacteria ratio with cover crop.Legume cover crop had a lower actinomycete but a higher MBC/MBN ratio compared to nonlegume,while mixed of legume and nonlegume cover crops resulted in a lower PLFA,MBC and actinomycete compared to legume or nonlegume alone.Incorporation of cover crop residue had a greater PLFA,MBN,total bacteria,AMF root colonization and spore density than those being surface placed or removed out from the soils.Cover crop effects on soil microbial variables were weakly correlated to initial soil properties and annual mean air temperature and precipitation,indicating that soil microbial growth with cover crop may be more affected by the quality and quantity of organic materials from cover crop than soil and climatic conditions.The enhancements of cover crop on soil microbial in this study highlights the importance of cover crop on soil quality improvement in agricultural systems.?3?An incubation experiment was conducted to study the effects of cover crop biomass on soil greenhouse gas emissions?CO₂,N₂O,and CH₄?.Two kinds of cover crop residues as ryegrass and vetch with five levels as 0,1,2,4,6 Mg ha^-1 were included.Soils with cover crop residues were incubated at 25^oC for 68 days.Fluxes of soil CO₂,N₂O,and CH₄ were sampled dynamically and measured using a gas chromatography.Compared with the control?without cover crop residue?,cover crop residue addition increased CO₂ emission by 42.5%and N₂O emission by 10.5%,but decreased CH₄ uptake by 0.5%.Residues was more effective to stimulate soil microbes and extractable organic C after incorporation,and resulted in higher greenhouse gas emissions.Residues incorporation at the rate of 6 Mg ha^-1 decreased cumulative N₂O emissions by 0.2%and CH₄ uptake by 23%,while increased the CO₂emissions by 66.6%than control.Residues biomass rates had greater effects on soil greenhouse gas emissions,indicated higher respiration as soil CO₂ emissions.Compare to vetch,ryegrass increased cumulative CO₂ emissions by 11.7%and cumulative CH₄ uptake by12.4%,but decreased cumulative N₂O emissions by 6.8%.Increasing residue biomass linearly decreased soil NO₃^--N?R²=0.84 and R²=0.99?,NH₄⁺-N content?R²=0.88 and R²=0.91?and CH₄ uptake?R²=0.88 and R²=0.92?and polynomially decreased nitrous oxide?R²=0.96 and R²=0.99?and CO₂ fluxes?R²=0.97 and R²=0.92?for both ryegrass and vetch,respectively.The mixed cover crops?rye and vetch?incorporation reduced the greenhouse gas emission compared to vetch cover crop possibly due to lowering the residue C/N ratio.The improved soil C and N storage and other documented benefits such as soil health and environmental sustainability enhancement as a result of cover crop can outweigh CH₄ and CO₂ emissions as compared to no crop cover.?4?An incubation experiment was conducted to investigate the effect of residues C:N ratio on soil greenhouse gas emissions?CO₂,N₂O and CH₄?.Two kind of cover crop residues ac8so2)vr yewre egcrrreao spps)r aewnpdea rrseeo diy nbbceyu abnma,it iexnid n faigtv se2 o5cyo?bme abfnio nra a6nti3do nrd yaoeyfg s.dr iaTfsfhse eri enCn r/t aNtC i/roaN t oirfoa st1 i(0o10s 4a%.l5,o,7n35g1.%w4,i,5t h40 8a%.,3c,o 26n5t5r%.o1,l aa(nnndod0%residues from soybean with remaining from ryegrass.The residue was applied at the rate of 4 Mg ha^-1 for each combination.The greenhouse gas fluxes were analyzed by using gas chromatography.Cover crop incorporation increased cumulative CO₂ emission by 44.5%,N₂O emission by 119%and CH₄ uptake by 30%than control on average.Among the C:N ratios,residues with C:N ratio of 82 increased the cumulative CO₂ emissions by 57.6%and N₂O emissions by 50.6%,but decreased the cumulative uptake by 33.4%compared to control.The sole application of soybean residues?82 C:N ratio?increased cumulative CO₂emission by 19.7%and decreased cumulative N₂O emissions by 46.8%and cumulative CH₄uptake by 61.8%than sole application of ryegrass residues?14.5 C:N ratio?.The residues with low C:N ratios decomposed faster,which is the opportunity for producing higher DOC,hence resulting in higher N₂O emissions.Residues C:N ratio have greater influence on mineralization protentional,and ultimately on soil greenhouse gas emissions.The CO₂flux?R²=0.99?linearly increased with increasing residues C:N ratios,but linearly decreased N₂O and CH₄ fluxes?R²=0.98 and R²=0.98?,respectively.Polynomial decrease in the fluxes of CO₂?R²=73,P<0.001?,N₂O?R²=0 82,P<0.001?and CH₄ uptake?R²=0 58,P=0.001?were observed during 63 days of incubation.The findings in this dissertation provide an important contribution to the understanding of the mutual interrelations and to research in this field as well as to increase the use of cover crops introduction into the soil for better soil fertility,greenhouse gas mitigation and improving soil microbial community structure.