| As a typical farmland ecosystem,paddy not only has a large carbon sequestration potential to alleviate the global greenhouse effect but also plays a pivotal role in ensuring national food security.Currently,soil organic carbon(SOC)sequestration and its influencing factors in paddy fields have become a hot topic to cope with global greenhouse effect and ensure food security.The application of nitrogen(N)fertilizer is an important measure to increase grain yield and improve economic benefits and soil fertility.However,the microbial regulatory mechanisms of the SOC pool under different N fertilizer application rates in agroecosystems have not been clarified.Therefore,it is of great value to study the effects of N fertilizer application rates on the chemical composition of SOC,microbial community structure,extracellular enzyme activities and microbial residues,and to reveal the microbial regulatory mechanisms in agroecosystems.This study was conducted in the experimental base of Huazhong Agricultural University,Wudian Town,Zaoyang City,Hubei Province from2017 to 2021,with rice-wheat rotation system as the research background.The experiment consisted of 4 treatments: no N fertilizer treatment(NF0),low N fertilizer treatment(NF1),medium N fertilizer treatment(NF2),and high N fertilizer treatment(NF3).The chemical composition and properties of SOC were studied using nuclear magnetic resonance and pyrolysis gas chromatography-mass spectrometry.The SOC stock in soil carbon pool fractions was analyzed by dry sieving and density sieving methods.The microbial community structure and extracellular enzyme activities were studied using phospholipid fatty acid and fluorescence microplate enzyme detection techniques,respectively.The contribution of microbial residual carbon to SOC was analyzed by saccharonitrile-acetyl ester-derived gas chromatography.The regulatory mechanisms of soil microorganisms on SOC in rice-wheat rotation system under different N fertilizer application rates were revealed.The following are the primary research results:(1)The application of N fertilizer significantly enhanced soil dissolved organic carbon by 27.7%-55.2%,microbial biomass carbon by 31.7%-76.4%,and easily oxidized organic carbon by 18.3%-44.1%,improving soil carbon pool management index by 20.5%-30.2%.The application of N fertilizer significantly enhanced Alkyl C by 31.9%-53.3% and O-alkyl C by 18.0%-30.7%,but decreased the aromaticity by17.7%-31.3%.The contents of polysaccharides,N carrier and lipids increased by28.9%-50.0%,23.1%-43.0%,and 41.5%-53.7%,respectively.The application of N fertilizer increased the proportion of physico-chemically protected particulate organic matter in microaggregates(i POM),increasing the SOC stock in i POM fractions by11.4%-43.2%.Among the three N fertilizer application rates,the content of soil active organic carbon and the SOC stock of i POM fractions in NF2 treatment were the highest.(2)After application of N fertilizer,the contents of bacteria,actinomycetes,gram-positive bacteria,and gram-negative bacteria in the whole soil increased by25.3%-50.2%,11.1%-29.5%,19.4%-40.2%,and 25.7%-49.1%,respectively.There was no significant change in the fungal content.The contents of bacteria,actinomyces,gram-positive bacteria and gram-negative bacteria were significantly increased in >250 μm aggregates.The contents of gram-positive bacteria and gram-negative bacteria were also increased in <250 μm aggregates.The fungal content in soil aggregates was not significantly affected by N fertilizer.Among the three N fertilizer application rates,the NF2 treatment had the highest microbial biomass.The microbial biomass decreased with the decrease in soil aggregate particle size.The interaction of N fertilizer and aggregates had significant effects on the contents of bacteria,grampositive bacteria and gram-negative bacteria.(3)After application of N fertilizer,β-glucosidase(BG),β-N-acetylglucosaminidase(NAG),β-cellobiosidase(BC),β-xylosidase(BX)and phenol oxidase(PHO)increased by 0.6%-43.5%,16.1%-85.1%,21.8%-48.9%,8.3%-13.1%,and 11.3%-22.0%,respectively.The activities of BG,NAG,BC,BX and PHO were significantly increased in >250 μm aggregates,but only the activities of BG,BC and BX were significantly increased in <250 μm aggregates.With the increase in N fertilizer application rates,the activities of extracellular enzymes increased first and then decreased.The activities of extracellular enzymes were the highest in NF2 treatment.The activities of soil extracellular enzymes decreased with the decrease in aggregate particle size.The interaction between N fertilizer and aggregates had significant effects on the activities of BG,NAG,BC,and PHO.(4)The application of N fertilizer significantly enhanced the content of muramic acid by 8.8%-40.2%,galactosamine by 6.9%-28.5% and mannosamine 22.4%-64.2%in the whole soil,but the content of glucosamine did not change significantly.After application of N fertilizer,the contents of glucosamine,muramic acid,galactosamine and mannosamine were significantly increased in >2000 μm aggregates.In <2000 μm aggregates,the content of muramic acid was significantly increased.The content of amino sugars decreased with the decrease in aggregate particle size.The interaction between N fertilizer and aggregates had a significant effect on galactosamine and mannosamine.(5)N fertilizer significantly increased the content of bacterial residual carbon in whole soil and aggregates and increased the contribution of bacterial residual carbon to SOC(whole soil,6.2%-30.7%;>2000 μm aggregates,7.3%-15.4%;2000-250 μm aggregates,16.6%-19.2%;<250 μm aggregates,7.4%-13.7%).However,the content of fungal residual carbon and its contribution to SOC were not influenced by N fertilizer.The bacterial residual carbon and its contribution to SOC increased with the increase in aggregate particle size.The interaction between N fertilizer and aggregates had no significant effect on bacterial residual carbon and its contribution to SOC.(6)Multiple correlation analysis showed that the application of N fertilizer can affect the composition of SOC functional groups by positively regulating the microbial communities,reducing the SOC aromaticity,thereby increasing the active organic carbon compounds,promoting the SOC into i POM fractions,and improving the protection of SOC.In addition,the application of N fertilizer increased the activity of extracellular enzymes and the contents of amino sugars by increasing the microbial biomass,thereby increasing the contribution of bacterial residual carbon to SOC.In summary,the application of N fertilizer increased the contents of active organic carbon,reduced SOC aromaticity,and enhanced the protection of SOC from the soil carbon pool.Meanwhile,it increased the microbial biomass and the activities of extracellular enzymes,thereby increasing the content of bacterial residual carbon and its contribution to SOC.The contents of soil active organic carbon,microbial biomass,extracellular enzyme activities and bacterial residual carbon in medium N fertilizer treatment(NF2)were the highest.The smaller the particle size of soil aggregates,the smaller the soil microbial biomass,extracellular enzyme activities and the contribution of bacterial residual carbon to SOC.This study can provide a theoretical basis for optimizing N fertilizers to improve the quality of the SOC pool in the rice-wheat rotation system. |
