Effects Of Biochar And Nitrogen Fertilizer On Soil Aggregate Organic Carbon And Microbial Community In Wheat And Maize Multiple Cropping System

Soil depletion due to intensive crop cultivation methods reduces soil organic carbon（SOC）content and microbial diversity in agro-ecosystems.Fertilization application is an important factor influencing soil structure and SOC content,and research has shown that biochar addition is an important way to increase soil carbon sequestration.In order to explore the carbon sequestration effects and internal relationships of SOC fixation pathways under different application rates of biochar and nitrogen fertilizer.Based on a long-term field experiment,we set up a two-factor interaction experiment with four biochar application rates:0,5000,10000,and 20000 kg·hm^-2 were recorded as B0,B5,B10,and B20,respectively and two nitrogen fertilizer levels:240 kg N·hm^-2（NC）and 120 kg N·hm^-2（NH）in a multi-cropping system of wheat and maize in Guanzhong Region of China,which were composed of eight treatments.We examined the effects of biochar and nitrogen fertilizer on the distribution and stability of soil aggregates,different active carbon and nitrogen fractions,organic carbon chemical structure and microbial communities in different soil aggregates（>2 mm,0.25～2 mm and<0.25 mm）using dry sieves,infrared spectroscopy,and high-throughput sequencing technologies,and combined with other soil properties.The physical protection of aggregates and the association between carbon and nitrogen fractions and microbial communities were further analyzed.The main findings are as follows:（1）Biochar application significantly increased the stability of soil aggregate（P<0.05）.Compared with no biochar application,biochar application significantly increased the contents of mechanically stable aggregate with the size of>2 mm and water-stable aggregate with the size of>0.25 mm（P<0.05）,and this effect was enhanced with the increase of application rate.High biochar addition levels（B10 and B20）significantly increased the MWD of the soils.The MWD of NH treatment was significantly higher than that of NC,whether it was dry sieves or wet sieves.（2）Biochar application significantly increased the contents of soil organic carbon and total nitrogen（TN）in the bulk soil and aggregate fractions（P<0.05）,with the effect being significantly enhanced with the increases of application rate.Compared with no biochar application,the application of biochar increased SOC content by 9.32%～19.34%and TN content by 7.35%～10.02%.Under the same biochar application level,SOC and TN contents in the NC treatment were 4.41%and 1.53%higher than those in the NH treatment,respectively.The contents of readily oxidizable organic carbon（ROC）and dissolved organic carbon（DOC）increased,while the content of soil microbial biomass carbon（MBC）decreased after the application of biochar.On the contrary,the MBC and DOC contents were higher in NH treatment.The contents of SOC,ROC,DOC and TN in microaggregates were significantly higher than those in large aggregates,while the content of MBC was significantly lower than that in large aggregates.（3）Under the treatment of biochar and nitrogen fertilizer,the infrared spectral characteristic peaks of the bulk soil and aggregates with different sizes were mainly at 1030（polysaccharide C-O）,1433（aromatic C=C）,1635（amide C=O）,3421（Phenolic and alcoholic-OH）and 3620 cm^-1.The application of biochar and nitrogen fertilizer increased the relative intensity of polysaccharide C-O,aromatic amide C=O,Phenolic and alcoholic-OH and free-OH,but decreased the relative intensity of aromatic C=C.The relative intensity of aromatic amide C=O,Phenolic and alcoholic-OH and free-OH in 0.25～2 mm particle size aggregate was lower than that in other two aggregate sizes,and the relative intensity of aromatic C=C in microaggregates was lower than that in macroaggregates.The structural stability of organic carbon in bulk soil and different aggregate fractions decreased after applying biochar,and the structural stability of soil organic carbon under NH treatment was higher than that under NC.（4）Biochar,nitrogen fertilizer,and soil aggregate fractions significantly affected the diversity of soil microbial communities（P<0.05）.B10 treatment significantly increased Simpsoneven index of microbial community and Shannon index of fungal community.Compared with NC treatment,NH treatment significantly increased the richness of soil microbial community.The Chao and ACE indices of microaggregates were higher than those of macroaggregates.Biochar changed the composition of microbial community at the phylum level in all aggregates,significantly increasing the relative abundance of Firmicutes,Gemmatimonadota,and Mortierellomycota in all aggregates,and decreasing the relative abundance of Actinobacteria and Chytridiomycota in 0.25～2 mm aggregates,and decreasing the relative abundance of Ascomycota in<0.25 mm aggregates.Under the same biochar application level,the relative abundance of Bacteroidota in NH was significantly higher than that in NC,while the relative abundance of Chytridiomycota was significantly lower than that in NC.With the increases of aggregate size,the relative abundance of Actinobacteria,Proteobacteria,Bacteroidota,and Ascomycota gradually decreased,while that of Acidobacteriota,Chloroflexi,Planctomycetota and Mortierellomycota increased.B5NH and B20NH treatments enhanced connectivity and interaction strength among bacteria.Bacterial communities in macroaggregates are more complex,whereas fungal communities in microaggregates are more complex.The microbial community was significantly correlated with the contents of MBC,DOC,TN,and DON（P<0.01）.In conclusion,our results showed that the response of wheat-maize multiple cropping field in Guanzhong area to biochar and nitrogen fertilizer was mainly determined by the interaction of aggregates,the stability of carbon fractions and microorganisms.However,under different nitrogen application rates,the main regulatory factors were different:at high nitrogen application rates,biochar application changed SOC content mainly by affecting microbial community,while under low nitrogen application rate,biochar application changed SOC content mainly by affecting carbon fractions.