Effect Of Maize Straw Return And Nitrogen Application On The Stability Of Organic Carbon And Microbial Mechanism Of Brown Soil In Northeast China

Soil organic carbon is an important reference indicator for evaluating soil fertility and sustainable productivity of farmland,and has a direct impact on the soil physical and chemical and biological properties of farmland.However,due to intensive farming patterns and the extensive use of chemical fertilizers,the soil structure is severely damaged,organic carbon is strongly mineralized,soil acidification is severe and microbial diversity is lost,which seriously limit the green and sustainable development of agriculture and food security in Northeast China.At present,studies on straw return have mainly focused on exploring the effects of different processing modes on physicochemical properties,aggregates,greenhouse gas emissions and crop yields,however,no study has been reported on elucidating the mechanisms of how straw return promotes soil organic carbon sequestration,especially the functional potential of microbial species communities and carbon metabolism involved in soil carbon cycling.Therefore,this study was conducted by using the long-term straw return trial established in2015 in the brown soil region of Northeast China to investigate different straw return modes,rotary straw return（RTS）and plowed straw return（PTS）,and five N application rates N0(0 kg N hm^-2),N1(112 kg N hm^-2),N2(187 kg N hm^-2),N3(262 kg N hm^-2),N4(337 kg N hm^-2),from 2019-2021.To systematically study the stability of soil organic carbon pools and microbiological mechanisms,in order to provide a theoretical basis and technical support for the promotion of straw return technology to achieve the double-effective objective of nitrogen reduction and carbon increase.The main findings were as follows:1.The results of fitting maize yield as a function of N application were quadratic for both RTS and PTS during three consecutive years of experimentation.When maize yield was at its maximum,the average optimum N application during the three consecutive years of the experiment was 273 kg N hm^-2 for RTS and 302 kg N hm^-2 for PTS,with a significant 9.6%reduction in N fertilizer use for RTS compared to PTS.By calculating the total organic carbon storage in the soil layer（0-60 cm）of each test treatment after successive years of straw return,the results showed that the highest total organic carbon storage was 97.14 t hm^-2 at 187 kg N hm^-2 for RTS and 90.58 t hm^-2 at 262 kg N hm^-2 for PTS.Thus,RTS increased the total organic carbon storage to 7.2%by reducing the applied N by 28.6%compared to PTS.The linear fit results showed that increasing soil cellulase,amylase,sucrase andβ-glucosidase activities would significantly increase soil organic carbon,microbial biomass carbon,easily oxidizable organic carbon and particulate organic carbon content.RTS could significantly reduce the amount of N fertilization,and by combining with 187 kg N hm^-2,it could effectively improve the carbon cycle enzyme activity and simultaneously increase the content of soil organic carbon and labile carbon fraction,and finally increase the soil organic carbon storage,thereby promoting maize yield stability and growth.2.The distribution and stability of soil aggregates in the surface layer（0-20 cm）were significantly affected by straw return modes and N application.PTS significantly increased the mean weight diameter（MWD）,geometric mean diameter（GMD）and the proportion of large macroaggregates（R>0.25 mm）at a N application of 187 kg N hm^-2 in the 0-10 cm soil layer;In the 10-20 cm soil layer,RTS significantly increased MWD,GMD and R>0.25 mm%at a N application rate of 187 kg N hm^-2 and significantly improved the stability of aggregates in the10-30 cm soil layer compared to PTS.When RTS was combined with 187 kg N hm^-2 N fertilization,the organic carbon content of Humins carbon in 0-10 cm soil layer,the organic carbon and Humic acid carbon content of large macroaggregates in 0-20 cm soil layer,and the organic carbon and Humic acid carbon content of clay powder particles were significantly increased.Therefore,the selected N application rate of 187 kg N hm^-2 will effectively improve the soil structure and aggregate stability,and facilitate straw decomposition and organic carbon sequestration.Structural equation modeling（SEM）showed that clay-powder grain humic acid carbon（HAclay）,clay-powder grain humins carbon（HMclay）,large macroaggregate organic carbon（SOClarge）,clay-powder grain organic carbon（SOCclay）,humic acid carbon（HAC）and humins carbon（HMC）were important carriers of soil and aggregate organic carbon transformation.It is clear that large macroaggregates and clay-powder particles are the main sites of carbon fixation,and humic acid carbon（HAC）and humins carbon（HMC）are the main components of soil carbon fixation.3.RTS significantly increased soil available phosphorus（AP）,available potassium（AK）,soil organic carbon（SOC）,easily oxidizable organic carbon（EOC）,microbial biomass carbon（MBC）and humins carbon（HMC）content when N application was 187 kg N hm^-2 compared to other treatments.Mantel test showed that soil p H,total nitrogen（TN）,bulk weight（BD）,total phosphorus（TP）,SOC,particulate organic carbon（POC）,EOC and HMC were significantly and positively correlated with metagenomic Alpha diversity.Redundancy analysis（RDA）showed significant correlations with dominant bacterial species in carbon fraction（POC,HAC,FAC,MBC,DOC）and in physicochemical properties（AK,AP,TN,TK and p H）,while significantly associated bacterial species were mainly from Actinobacteria and Proteobacteria,where the main bacterial species involved in nutrient cycling were Burkholderia＿cenocepacia and Arthrobacter＿dokdonellae.The dominant fungal species were significantly associated in the carbon fraction（DOC,POC,EOC and HMC）and in the physicochemical properties（TN,AK and AP）,and the fungal species were mainly from the Ascomycota phylum.The dominant archaebacterial species were significantly correlated in carbon fraction（FAC,DOC,POC and HAC）and in physicochemical properties（AK,AP,TN,SWC,TK and p H）,and the significantly correlated archaebacterial species were mainly from the genus Nitroso,all belonging to the ammonia-oxidizing archaea（AOA）.Virus dominant species were significantly correlated in carbon fractions（FAC,DOC and MBC）and in environmental factors（AK,AP,TP,TK and p H）,and soil viruses participate in soil carbon and nutrient cycling by infesting host microorganisms and affecting microbial food webs.4.When N application was 187 kg N hm^-2,RTS promoted significant enrichment of carbon cycle metabolic pathways,with the highest abundance of 37.01%in the citrate cycle（TCA cycle）pathway.The Mantel test showed that SOC,DOC,MBC,p H,AP and AK were the environmental factors that significantly affected the carbohydrate metabolic pathway,and the environmental factors that significantly affected the carbon fixation pathway were SOC,HAC,HMC and AP.Pearson correlation analysis showed that the carbon fixation functional genes K00029（malate dehydrogenase）,K01602（ribulose-diphosphate carboxylase）,K01623（fructose diphosphate esterase）,and K01807（ribose 5-phosphate isomerase A）significantly promoted soil HMC fixation.DOC and AP significantly affect the synthesis and oxidation of methane.The abundance of soil glycoside hydrolases（GHs）was significantly influenced by the method of straw return and the amount and interaction of nitrogen application.RTS was significantly enriched in the abundance of glycoside hydrolases（GHs）degrading straw cellulose,including GH3,GH4,GH5,GH9,GH13,GH16,GH20 and GH42,at a nitrogen application of 187 kg N hm^-2,thus promoting the conversion of straw-derived carbon to the soil organic carbon pool.Meanwhile,the Mantel test showed that soil p H,TN,SOC,POC,DOC and MBC were the main environmental factors that significantly affected carbohydrate-active enzymes（CAZymes）.In conclusion,rotary straw return combined with low amount of N fertilization(187 kg N hm^-2)promoted soil organic carbon sequestration and indirectly increased maize yield by enhancing aggregate stability,improving soil physicochemical properties,increasing microbial diversity and functional potential of microbial carbon metabolism,and promoting soil inert carbon accumulation.