| Conservation tillage has many advantages and can help achieve carbon neutrality sooner.Soil aggregate plays an important role in protecting soil organic carbon,improving soil quality and fertility,and conservation tillage is the key factor to control soil aggregates.Therefore,studying the effect of conservation tillage on soil aggregate is helpful to elucidate the important role of conservation tillage in agricultural production.In this study,a 9-year field positioning test: single conventional tillage without crop residue(SNT: rape with no-tillage,rice with conventional tillage)、double no-tillage without crop residue(DNT: rape with no-tillage,rice with no-tillage)、double conventional tillage without crop residue(PT: rape with conventional tillage,rice with conventional tillage)、double notillage plus crop residue(DNS: rape with no-tillage + rice residue mulch–rice with notillage + rape residue mulch).The study investigated the changes of soil aggregates,organic carbon,its components and microorganisms under long-term conservation tillage,and revealed the linkage between soil aggregates-soil microorganisms-soil organic carbon through structural equation modeling,aiming to provide a theoretical basis for the carbon sequestration mechanism of long-term conservation tillage and the promotion of conservation tillage.The results of the study are as follows:(1)Long-term conservation tillage significantly affected the distribution and stability of soil aggregates.Compared with the PT treatment,the DNS and DNT treatments increased the content of soil >250 μm aggregates by 19.0%-35.6% and 14.0%-28.0%,respectively.Therefore,average weight diameter(MWD)and geometric mean diameter(GMD)in DNS treatment increased by 8.8%-30.9% and 49.5%-98.1%,respectively.DNT treatment soil MWD and GMD increased by 13.2%-27.1% and 34.0%-74.2%,improving soil stability and structure.(2)Long-term conservation tillage significantly affected the content of organic carbon in soil aggregates.In aggregates >250 μm,compared with PT,DNS and DNT treatments significantly increase the light fraction organic carbon(LFOC)content by 10.0%-14.4%and 6.0%-12.3%,and the particulate organic carbon(POC)content by 12.3%-20.3% and8.0%-14.2%.In <250 μm aggregates,compared with PT treatment,DNS treatment significantly increased POC content by 11.1%-70.7%.Compared with PT treatment The dissolved organic carbon(DOC)and microbial biomass carbon(MBC)contents of DNS and DNT treatedment in >250 μm and <250 μm aggerates were significantly increased compared to the PT treatment.(3)Long-term conservation tillage had significant effects on soil enzyme activities.In >250 μm aggregates,compared with PT treatment,DNS treatment increased the activities of cellobiohydrolase,β-glucoside,xylanase and polyphenol oxidase by 1.19-1.43 times,1.17-1.25 times,1.21-1.62 times and 1.23-1.75 times;DNT treatment increased the activities of cellobiohydrolase,β-glucosidase,xylanase and polyphenol oxidase by 1.17-1.39 times,1.10-1.18 times,1.15-1.48 times and 1.15-1.56 times.In <250 μm aggregates,compared with PT treatment,DNS treatment increased the activities of cellobiohydrolase,β-glucoside enzyme,xylanase and polyphenol oxidase in DNS treatment were increased by1.20-1.72 times,1.16-1.37 times,1.41-1.78 times and 1.18-1.70 times;DNT treatment increased the activities of cellobiohydrolase,β-glucosidase,xylanase and polyphenol oxidase by 1.13-1.67 times,1.08-1.51 times,1.31-1.65 times and 1.14-1.34 times.(4)Long-term conservation tillage had significant effects on soil microbial community structure.In >250 μm aggregates,compared with PT treatment,the contents of total phospholipid fatty acids,Gram-positive bacteria,bacteria,fungi,anaerobic bacteria and actinomyces increased significantly by 14.6%-26.8%,16.0%-23.1%,6.0%-22.9%,24.7%-27.1%,30.8%-51.0% and 20.0%-36.6% in DNS treatment;and the contents of grampositive bacteria,bacteria,fungi,anaerobic bacteria and actinomyces are significantly increased by 8.7%-13.8%,2.2%-14.4,15.7%-19.9%,20.5%-34.4% and 14.1%-26.3% in DNT treatment.DNS and DNT treatments aerobic bacteria decreased by 15.1%-17.8% and11.3%-12.6%.In <250 μm aggregates,compared with PT treatment,the contents of total phospholipid fatty acids,Gram-positive bacteria,bacteria,fungi,anaerobic bacteria and actinomyces increased significantly by 34.5%-37.2%,51.2%-179.6%,33.4%-33.8%,24.6%-30.3%,38.6%-55.3% and 24.5%-77.3% in DNS treatment,and the contents of gram-positive bacteria,bacteria,fungi,anaerobic bacteria and actinomyces increase by8.7%-13.8%,2.2%-14.4%,15.7%-19.9%,20.5%-34.4% and 14.1%-26.3% in DNT treatment.DNS and DNT treated aerobic bacteria decreased by 19.9%-20.4% and 12.6%-13.1%.(5)Compared with PT treatment,the organic carbon content of DNS treatment in >250 μm and < 250 μm aggregates significantly increased by 11.7%-15.5% and 6.2%-29.9%,respectively.There was no significant difference in the organic carbon content of DNT,SNT and PT treatments,indicating that straw combined with no-tillage has a better effect.Yields of oilseed rape and rice in the DNS treatment increased by an average of 28.4% and8.9% compared to the PT treatment.(6)Path analysis showed that for >250 μm clusters,fungi/bacteria(F/B)and grampositive bacteria/Gram-negative bacteria(G+/G-)were significantly and positively correlated with soil enzyme activity,enzyme activity was significantly positively correlated with MBC and DOC content,MBC and DOC were also significantly positively correlated with LFOC,POC and mineral-associated organic carbon(MAOC),and LFOC,POC and MAOC were significantly and positively correlated with SOC.In <250 μm agglomerates,F/B was significantly positively correlated with soil enzyme activity,G+/G-was significantly negatively correlated with soil enzyme activity,enzyme activity was significantly positively correlated with MBC and DOC content,MBC and DOC were positively correlated with LFOC,POC and MAOC,and LFOC,POC and MAOC were significantly positively correlated with organic carbon.In conclusion,long-term double no-tillage without crop residue and double no-tillage plus crop residue facilitated the formation of soil macroaggregates,increasing enzyme activity and total phospholipid fatty acid content in >250 μm agglomerates and <250 μm agglomerates,which in turn increased the content of soil organic carbon fractions.Double no-till combined with straw rewetting promoted soil organic carbon accumulation by increasing the content of LFOC and POC in >250 μm aggregate,and POC in <250 μm aggregate. |
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