Study On Warming Potential And Nitrogen Balance Of Long-term Conservation Tillage Farmland And Microbial Processes Of N₂O Emissions

Greenhouse gas emissions(GHGs)and active nitrogen loss(Nrs)are are the main factors causing global climate change,and agricultural activities are an important source of greenhouse gas emission and reactive nitrogen loss.Nitrous oxide(N₂O)is a kind of greenhouse gas with long life cycle and high warming potential,which is an important component of reactive nitrogen loss in farmland.The nitrogen cycle mediated by soil nitrogen functional microorganisms is the main source of soil N₂O source.Meanwhile,the community structure and function of soil nitrogen functional microorganisms are significantly affected by tillage intensity.Conservation tillage,such as chisel plow tillage and zero tillage,is widely considered as agricultural management measures of improving soil health and reducing soil nitrogen losses in farmland,which is widely used in the farmland of northern China.Therefore,studying the characteristics of soil GHGs and nitrogen balance under conservation tillage fields can provide theoretical support for the construction of an efficient,ecological and sustainable conservation tillage technology system.Exploring the potential relationship and mutual feedback mechanism of soil-microorganism-N₂O emission under long-term conservation tillage is the key scientific issue to achieve conservation tillage soil nutrient retention,farmland quality improvement,and fragile farmland ecological environment protection.This study took the perspectives of“tillage-microorganism-greenhouse gas”to systematically study the effects of greenhouse gas emissions and nitrogen balance under long-term conservation tillage practices in the typical‘winter wheat-summer maize'multiple cropping ecologically fragile area of Loess Plateau,which will provide a scientific basis for long-term conservation tillage and realize agricultural ecological protection and the reduction of greenhouse effect and nitrogen surplus benchmark.Moreover,field experiments and integrated analysis were conducted to reveal the distribution pattern,temporal and spatial differences,and nitrogen cycling characteristics of soil functional microorganisms under long-term conservation tillage.In consideration of the characteristics of soil in situ N₂O emission,the structural equation model was used to demonstrate the mutual feedback relationship between N₂O emission and functional microorganisms under long-term conservation tillage soil from the perspective of microorganisms.Revealing the microbial driving mechanism of the production and reduction of N₂O under long-term conservation tillage soil will provide the theoretical basis for the extension of conservation tillage.The main conclusions are as follows:(1)The emission fluxes and cumulative emission of greenhouse gas(i.e.,CO₂,CH₄,and N₂O)were significantly affected by tillage practices at farmland in“winter wheat-summer maize”multiple cropping fields of the Loess Plateau,especially the peak emissions after fertilization and the duration of high-intensity emission period.The long-term conservation tillage practice can significantly reduce the average emissions flux and cumulative emission of CO₂ and N₂O,and significantly increased the averaged uptake of CH₄ uptake fluxes and cumulative uptake.In addition,the long-term conservation tillage practices had a sequestration effect on soil organic carbon,while the conventional tillage practice had a stimulating effect on soil organic carbon.Furthermore,the global warming potential,net comprehensive greenhouse effect,and emission intensity under conservation tillage practices were significantly lower than those of conventional tillage,indicating that the yield effect and environmental effect of the long-term conservation tillage farmland were better than that of conventional tillage.All in all,the greenhouse effect of farmland increased with the tillage intensity.(2)Based on in-situ observations in two consecutive winter wheat-summer maize growing seasons,each tillage practice was exposed to a local widely adopted N application rate(480 kg ha^-1 yr^-1)in both seasons.Our results indicated that,compared with PT,CPT and ZT significantly reduced N₂O emissions by 39.7%and 55.3%,and N leaching by 52.3%and 147.7%across the two growing seasons,respectively.CPT significantly enhanced crop aboveground N uptake by 4.0%,and increased the annual crop yield by 5.9–8.1%.Although CPT and ZT enhanced NH₃ volatilization by 46.7%and 84.3%,the total N losses under CPT and ZT were decreased on an average by 7.4%and 22.4%,respectively.Overall,CPT and ZT significantly increased the accumulation of soil total nitrogen in the 0–100 cm layer.Taking N inputs and outputs together,CPT achieved a lower N surplus mainly due to increased crop N harvest and reduced N losses including gaseous emissions and hydrological leaching.Our findings suggest that long-term chisel plough tillage in agroecosystems could serve as a promising soil management practice in increasing crop productivity and maintaining sustainability through enhancing N removal from crop biomass and decreasing N losses via N₂O emission and nitrate-N leaching.(3)Long-term conservation tillage practices affect the abundance of nitrogen functional genes by regulating soil physical and chemical properties and then chang N₂O emission ultimately.Among them,chisel plow tillage and zero tillage significantly increased the content of NO₃^-and total nitrogen in the soil,and significantly decreased p H value and the content of p H and NO₂^-in soil.The abundance of amo A,nir K and nir S genes decreased significantly in conservation tillage soils,while the abundance of nos Z genes increased significantly.The differences in the abundance of soil nitrogen cycle functional genes under different tillage treatments led to the differences in the activities of nitrogen cycle pathways under different tillage treatments,resulting in different N₂O source and sink capacities.In summary,the reduction of nitrate reductase genes abundance in conservation tillage soils inhibited the transformation of NO₃^-to NO₂^-,which leading to the reduction of N₂O precursor.At the same time,the increase of N₂O reductase gene abundance promoted the reduction of N₂O to N₂.Finally,the N₂O emissions were reduced in conservation tillage soils.(4)The long-term conservation tillage significantly increased the species composition diversity and phylogenetic diversity of nir K-,nos Z?-and nos Z?-type microbial communities,while significantly decreased the species composition diversity and phylogenetic diversity of the nir S microbial community.Although the assembly process of nir K-,nir S-,nos Z?-and nos Z?type microbial community assembly was dominated by stochastic processes,the relatively higher relative importance of the deterministic assembly process under the plow tillage soil.In addition,the species immigration rate of soil denitrifying microbial communities of nir K-,nos Z?-and nos Z?-type under conservation tillage was significantly higher than that under plow tillage,which indicating that the succession of soil denitrifying microbial communities under conservation tillage was more in line with the neutral process,while the trend of nir S-type community was opposite.In conclusion,the conservation tillage significantly reduced the diversity of nir S and nos ZI community,the niche width of nos ZI community,and species abundance of nir S and nos ZI community(i.e.,polyphum?gilvum,Acdovorax?sp.?No.1,Herbaspirillum?sp.?Tso 26.2,and so on)and significantly increased the niche breadth and species abundance of nos Z?community(i.e.,afipia?sp.?1NLS2,Massilia?sp.?Tso8,Haliscomenobacter?hydrossis,and so on),which leading to the reduction of N₂O emission.(5)Conservative soil management,in particular zero-till,consistently increased the Shannon diversity index of bacterial community over the growing season,compared with plow-till.Clear tillage-induced niche differentiation between nos ZI-and nos ZII-N₂O reducers,as evidenced by the dominant gene abundance of nos ZII compared to nos ZI in the conservation tillage soil,which eventually probably contributed to the transformation of N₂O to N₂.Moreover,compared to plow-till,zero-till significantly decreased gene abundances involved in N₂O production including the nir S,nir K,and nar G genes,but increased abundances of N₂O reduction genes such as nos Z during peak N₂O emissions.Critically,the abundances of detected species involved in denitrification,such as Deltaproteobacteria?bacterium spp.and Alphaproteobacteria?bacterium spp.were inhibited by zero-till.Overall,the reduced soil N₂O emissions under reduced tillage positively and strongly depended on the nos ZI-to-nos ZII ratio,while increased emissions due to conventional tillage were positively associated with intensified denitrification.Such improvements in understanding the responses of N-cycling gene abundances to tillage intensity can certainly help in the development of updated soil management practices and adaptative N application strategies to reduce the reactive N emissions in agricultural ecosystems.In conclusion,the implementation of conservation tillage can promote the accumulation of soil organic carbon,slow down the greenhouse gas emission effect,and decrease the N surplus benchmark in the farmland ecosystem by optimizing soil functional microbial community and strengthening the capacity of soil carbon sink and nitrous oxide sink.