Effects Of Soil Amendments On Soil Carbon Emission And Sequestration And Its Microbial Mechanisms In Rainfed Maize Field

The central Gansu Province is the most important semiarid region in China,the natural water and heat conditions in this region cannot meet the demand for maize production.The fully film mulched double ridge-furrow system broken through the water and heat limitations of maize cultivation,significantly improving precipitation utilization efficiency and maize yield,making maize become a newly dominant crop in the central part of Gansu Province.At present,several problems,such as excessive nitrogen fertilizer,non-application and under-application of organic fertilizer,and low efficiency of nitrogen fertilizer utilization are widespread under this system.Appropriate fertilization measures can be beneficial to the improvement of soil quality and crop yield,but fertilization regimes also significantly impact carbon emission and sequestration.Soil microorganisms play an important role in driving soil carbon emission and sequestration.However,it is still unknown precisely how fertilization regimes affect soil carbon emission and sequestration and their microbial driving mechanisms in central Gansu Province.The present study relied on two fertilization field experiments established by Rainfed Agricultural Comprehensive Experiment Station of Gansu Agricultural University that was set up in 2012 at Dingxi City,Gansu Province.The data for this research were collected in 2019-2021.The objectives of this study were to investigate the impact of soil amendments on soil CO₂ emission,organic carbon（SOC）sequestration,soil carbon-fixing bacterial communities and related factors,and to explore the microbial driving mechanisms of long-term soil amendments affecting soil CO₂emission and SOC sequestration.The experimental treatments included four nitrogen（N）application levels(no N（N0）,N application at 100 kg hm^–2（N1）,200 kg hm^–2（N2）and 300 kg hm^–2（N3）)and five soil amendments(no fertilizer（NA）and four fertilization treatments（inorganic fertilizer（CF）,50%inorganic plus 50%organic fertilizer（SC）,100%organic fertilizer（SM）,and 100%maize straw（MS））.The main results are as follows:（1）In both wet and dry years,soil CO₂ emission in maize growing seasons was higher under nitrogen fertilization than under no nitrogen plot,and these under organic treatments was higher than under the inorganic treatment.Total CO₂emission under N1,N2,and N3 were increased by 17.3%,24.8%,and 28.9%compared to the N0,respectively;under CF,SC,SM,and MS were increased by 22.3%,30.5%,31.2%,and71.2%compared to the NA,respectively;under SC,SM,and MS treatments were increased by 6.7%,7.4%,and 30.5%compared to the CF,respectively.Maize yield,carbon emission efficiency（CEE）,and net ecosystem productivity（NEP）increased with increasing nitrogen application rates,but decreased with increasing the proportion of organic fertilizer.However,there was no significant difference between N2 and N3.Maize yield,CEE,and NEP under SM decreased by 49.5%,53.3%,and 67.2%,under MS decreased by 56.6%,68.9%,and 81.5%,respectively,compared to the CF.Whereas SC exhibited a comparable yield,CEE,and NEP to CF.（2）Nitrogen application significantly increased SOC compared to no nitrogen fertilization treatment,and organic fertilization significantly promoted SOC sequestration compared to inorganic fertilization.Total SOC in the 0-20 soil layer increased by 9.1%,15.1%,and 15.0%under the N1,N2,and N3,compared to the N0;by 7.4%,20.0%,20.6%,and 32.0%under the CF,SC,SM,and MS treatments,compared to the NA;and by 11.7%,12.3%,and 23.0%under the SC,SM,and MS treatments,compared to the CF,respectively.Labile organic carbon in the 0-20 soil layer increased by 16.7%,34.4%,and 39.3%under the N1,N2,and N3,compared to the N0;by 25.8%,40.4%,55.8%,and 90.2%under the CF,SC,SM,and MS treatments,compared to the NA,and by11.6%,23.8%,and 51.2%under the SC,SM,and MS,compared to the CF,respectively.（3）Soil bacterial abundance and community composition under different nitrogen rates were significantly correlated with p H,organic carbon,total nitrogen,and nitrate nitrogen;under different soil amendments they were significantly correlated with p H,organic carbon,and C/N.Nitrogen application increased bacterial abundance compared to no nitrogen treatment,and organic fertilization increased bacterial abundance compared to inorganic fertilization treatment.The relative abundance of Actinobacteriota increased with increasing nitrogen application rates,but there was no significant difference between N2 and N3.The CF and SC treatments increased the relative abundance of Acidobacteria compared to NA.Compared to NA and CF,the MS treatment increased the relative abundance of Proteobacteria,while decreasing the relative abundance of Acidobacteriota.The SM treatment increased the relative abundance of Actinobacteria compared to CF.（4）The abundance and community of cbb L gene-carrying functional bacteria under different nitrogen application rates were significantly correlated with p H,total nitrogen,and nitrate nitrogen;under different soil amendments they were significantly correlated with total nitrogen,nitrate nitrogen,and C/N.The CO₂-fixing bacterial abundance under nitrogen fertilization was higher than under no nitrogen treatment,and under organic fertilization was higher than under inorganic fertilization treatment.The dominant taxa of the CO₂-fixing bacterial community under different soil amendments were Xanthobacter,Bradyrhizobium,and Nitrosospira.（5）Soil CO₂ emission under different nitrogen application rates was significantly correlated with soil p H,organic carbon,total nitrogen,nitrate nitrogen,bacterial abundance,community composition,network modules,and the relative abundance of Actinobacteriota;under different soil amendments,it was significantly correlated with organic carbon,total nitrogen,bacterial abundance,community composition,network module III,and the relative abundance of Proteobacteria and Acidobacteriota.Soil Rubis CO enzyme activity and organic carbon sequestration under different nitrogen application rates were significantly correlated with soil p H,total nitrogen,nitrate nitrogen,the CO₂-fixing bacterial community,and the relative abundance of Xanthobacter;under different soil amendments were significantly correlated with soil p H,total nitrogen,and the abundance and diversity of the CO₂-fixing bacterial community.Taken together,in both wet and dry years,nitrogen fertilizers applied at 200 kg hm^–2,and 50%inorganic plus 50%organic fertilizer moderately increased CO₂emission while significantly increasing maize yield,enhanced yield,carbon emission efficiency,and net ecosystem productivity,which increased soil organic carbon sequestration by increasing the natural return of organic matter and organic carbon input under the double ridge-furrow maize planting fields.Application of chemical nitrogen fertilizer decreased soil p H and increased organic carbon,total and nitrate nitrogen content,which further increased bacterial abundance and the phylum Actinobacteriota,promoted CO₂ emission,increased the genus Xanthobacter,altered the CO₂-fixing bacterial community composition,and thus increased Rubis CO activity and organic carbon sequestration.Application of organic fertilizers altered soil p H and increased soil organic carbon and total nitrogen content,increased the phylum Proteobacteria and decreased the phylum Acidobacteriota,increased CO₂ emissions,increased the CO₂-fixing bacterial abundance and diversity,and thus increased Rubis CO activity and organic carbon sequestration.