Effects And Regulation Mechanisms Of Leguminous And Gramineous Straw Returning On Soil Carbon Sequestration

Soil organic carbon is the pivotal indicator for fertility,and soil organic carbon sequestration is decisive for both soil fertility improvement and emission reduction.As an important dry farming area,the soil fertility in Weibei Highland area has generally declined in recent years,and its water and heat conditions only support one crop a year.Leguminous green manure is widely planted in summer fallow period.Therefore,the soil fertility cultivation model is quite special.Leguminous green manure and wheat stubble are rolled back together before wheat sowing.Now,the main purpose of planting green manure is its carbon sequestration effect,rather than to improve nitrogen fertility.Existing studies on the effects of exogenous organic materials on soil organic carbon sequestration are inconsistent,with zero accumulation or even loss,which may be related to the nature of the organic materials,but also to the nature of the soil.However,there is a gap in the research on the carbon sequestration of different soil properties by leguminous residues and gramineous straw.Whether this combination can promote soil organic carbon sequestration and related mechanisms are not clear.Most of the existing studies do not distinguish organic carbon from soil original organic carbon from exogenous crop residues,and the composition of microbial community and extracellular carbon in the process are not clear.There is a great lack of understanding of enzyme activity changes.Therefore,it is of great significance to study the effect of leguminous and gramineous straw combination on carbon sequestration in soil with different properties and its mechanism for ensuring food security,fertilizing farmland fertility and reducing carbon sequestration.In this study,a 127-day incubation experiment in laborary was conducted.Natural?¹³C and high-throughput sequencing techniques were used to study soil carbon sequestration,soil enzyme activity and microbial community.The factor is different organic carbon content,wheat and leguminous straw addition.The experiment was conducted as completely random design,8 treatments in all.Secondly,a 112-day incubation experiment was conducted.Natural?¹³C technology was still used.Three factors including wheat,leguminous straw and lime addition were selected,6 treatments in all:control;lime addition;wheat straw and leguminous straw combined addition;wheat straw addition and leguminous delayed addition;wheat straw and leguminous straw addition combined lime addition;wheat straw and lime combined addition and leguminous delayed addition.The effects of delayed addition of leguminous straw and wheat straw combined with lime return on soil carbon mineralization and carbon sequestration were studied.The main results were as follows:1.The effects of wheat and leguminous straw combined addition on soil carbon sequestration in soils with different SOC content were studied.The results showed that high SOC content and straw addition could lead to higher CO₂-C cumulative release.Compared with wheat straw,huai bean straw and two kinds of straw were added to low SOC content soil respectively,wheat straw,huai bean straw and two kinds of straw were added to the soil with high SOC content,resulted in the cumulative release of CO₂-C increased by 15.6%,13.4%and 13.9%respectively.MBC,MBN and DOC decreased gradually with time.High SOC content in soil and straw addition increased MBC,MBN and DOC.Adding straw effectively increased the organic carbon sequestration,while the soil with low SOC content increased greatly.Compared with the control treatment,the wheat straw,leguminous straw and two kinds of straw were add to soil with high SOC content respectively,resulted in SOC increased by 14.6%,7.8%and 20%respectively.Compared with the control treatment,the wheat straw,leguminous straw and two kinds of straws were add to soil with low SOC content respectively,resulted in SOC increased by 126.5%,10.2%and 22.0%respectively.Adding wheat straw and combining two kinds of straw not only has better carbon sequestration effect than adding huai bean straw,but also forms more new SOC.2.The soil microbial community was studied by high-throughput sequencing technology,and the effects of huai bean straw and wheat straw on microbial diversity and community composition were identified,as well as the coupling relationship with SOC.The activity of three hydrolytic enzymes decreased significantly with time.Adding straw could effectively improve their activity,and the corresponding hydrolytic enzymes with high SOC content were also high.The results of microbial diversity were similar to those.At the phylum level,main bacteria community is Proteobacteria?23.1%-30.0%?,Acidobacteria?7.9%-28.5%?,Actinobacteria?11.8%-27.8%?,Chloroflexi?13.4%-17.4%?,Bacillus?Getimonadetes,5.3%-12.0%?and Bacteroidetes?1.9%-5.9%?.The relative abundance of Acidobacteria in low SOC soil amended with huai bean straw and two kinds of straw was significantly lower,while the relative abundance of Actinobacteria was significantly higher.The relative abundance of Gemmatimonadetes and Cleroflexi decreased when huai bean straws were added.The relative abundance of RB41 and Sphingomonas decreased,while the relative abundance of Streptomyces and Bacillus increased.Carbon input is positively correlated with MBC,cumulative CO₂-C emissions and new SOC,and Actinobacteria,Firmicutes and Bacteroidete are also closely related to them.MBC,DOC,?-glucosidase activity,new SOC and accumulated CO₂-C release explained 21.0%,14.8%and 24.0%,6.9%and 4.5%of bacterial community variation,respectively.3.The effects of delayed addition of huai bean and wheat straw combined with lime addition on SOC mineralization and carbon sequestration was studied.It was found that the cumulative release of CO₂-C was significantly reduced by adding lime and delayed addition of huai bean straw.The cumulative release of CO₂-C decreased by 235.2 mg C kg^-1,609.3mg C kg^-1 and 273.9 mg C kg^-1,respectively,in the control,wheat straw addition and delayed addition of huai bean straw with lime;the proportions was 62.6%,22.6%and 18.8%,respectively.The cumulative release of CO₂-C decreased by 1248.0 mg C kg^-1 in the treatment of delayed addition of huai bean straw without adding lime,and the proportion was 85.8%;The cumulative release of CO₂-C decreased by 912.6 mg C kg^-1 in the treatment of delayed addition of lime and huai bean straw,with a proportion of 77.3%.The addition of lime can effectively improve DOC and AN in soil;reduce the activity of hydrolytic enzymes and MBC and MBN.The number of new SOC was 0.3 g kg^-1,0.5 g kg^-1,04 g kg^-1 and 0.8 g kg^-1in two kinds of straw combined addition,wheat straw addition and huai bean straw delayed addition,two kinds of straw combined lime addition and wheat straw combined lime addition and huai bean straw delayed addition treatment respectively,and the loss of native SOC was 0.6 g kg^-1,0.7 g kg^-1,1.3 g kg^-1 and 1.5 g kg^-1,respectively.In conclusion,the following main conclusions can be drawn:1)Under the condition of straw returning,the higher content of SOC resulted in higher release of CO₂-C.After adding straw,the proportion of SOC sequestration in soils with low SOC content was higher than that in soils with high SOC content,and the carbon sequestration effect of adding wheat straw and two kinds of straw was better;2)Adding straw and high SOC content will increase the activity of hydrolytic enzymes and the diversity of soil bacterial,and the combination of two kinds of straw will change the structure of soil bacterial community.At the same time,the native SOC content also has a certain effect on the structure change of soil bacterial community;3)The addition of lime and delayed huai bean straw can promote SOC mineralization,resulting in a decrease in carbon sequestration.