| The freeze-thaw cycle（FTC）is an important way and an inevitable process for crop habitat regulation in cold regions.The FTC is accompanied by complex processes,such as soil energy transfer and transformation,water phase transition and migration,and solute decomposition and diffusion.With the increasing development intensity of black soil resources and the lack of protective farming measures,the phenomenon of soil erosion and loss has intensified,and the supply of soil nutrients has become unbalanced,posing a threat to the security of grain production capacity and green and sustainable development in the black soil region.Under this background,how to effectively play the role of freeze-thaw process in crop habitat modification in the long winter through intervention and regulation,alleviate the impact of spring waterlogging disaster and black soil nutrient loss on agricultural production in Heilongjiang Province,and affect agricultural water-soil resources in cold regions.This has important practical significance for the efficient utilization of agricultural water-soil resources in cold regions and the construction of black soil habitat protection models.Previous studies on the impact of biochar on farmland soil water-soil environment were often simulated by indoor room temperature experiments or in the growing season of crops,ignoring the seasonal freezing and thawing process of soil,especially under the action of freeze-thaw cycles,the soil structure is improved,the release of bioavailable nutrients,and the optimized conditions for reducing nutrient loss.This study takes the"soil freeze-thaw environment synergistic effect-soil water-soil environment regulation mechanism-soil nutrient cycle improvement and restoration"as the main research line.On the basis of clarifying the water-heat diffusion mechanism and water-salt transmission mechanism of frozen-thawed soil,the mutual-feedback relationship of changing environment to frozen-thawed soil,the transformation mode of frozen-thawed soil water environment,and the transformation and redistribution process of regional water-soil resources such as nutrient generation in freeze-thawed soil.In this study,the farmland soil of the Songnen Plain was used as the research carrier,and the biochar and soil were regarded as a biochar-soil complex,three biochar application rates（0%,1%,and 2%）were set.Taking the FTC of farmland soil as the boundary condition,the indoor freeze-thaw simulation test was compared with the field in-situ test.By analyzing the change characteristics of soil environmental factors,soil available carbon and nitrogen,soil microbial biomass and soil greenhouse gases,the coupling process and response mechanism of biochar to farmland soil carbon and nitrogen cycles in seasonally frozen regions were explored.The main research contents and conclusions are as follows:（1）Analysis of key driving factors and potential mechanism of biochar on soil greenhouse gas emission under freeze-thaw conditions.Biochar treatment increased soil CO2 emissions,reduced CH4 emissions and reduced N2O emissions during the FTC.In addition to most of the stable carbon components in biochar,activated organic carbon also exists in a small part,which causes a short-term positive excitation effect.Due to its unique physicochemical properties,biochar can improve soil aeration and water holding capacity and redox conditions when applied to soil,thereby significantly reducing CH4 emissions.Biochar can effectively immobilize nutrients and reduce the bioavailability of reaction substrates and nutrient substrates,etc.The decrease of available nitrogen content in substrates will limit N2O emissions.The structural equation model showed that soil temperature（ST）was the main environmental factor determining CO2 emissions and that soil moisture（SM）was the main environmental factor determining CH4 and N2O emissions.Soil available carbon and nitrogen and microbial biomass are important for greenhouse gas emissions as the reaction substrates and main participants in the biochemical transformation of carbon and nitrogen,respectively.This study clearly expounds the effect of freezing-thawing soil hydrothermal characteristics on nutrient cycling and transformation,and proposes a quantitative characterization method for the kinetics of soil carbon and nitrogen mineralization processes.（2）Analysis of the change characteristics of physical and chemical properties of biochar under freezing and thawing conditions and its influence on the characteristics of soil water-soil environmental changes.Biochar has strong stability,after it is added to the soil as a remediation material,its surface morphology,microstructure,and element content will change with chemical and weathering.Biochar can effectively regulate the transfer of water-heat in frozen-thawed soil and the driving effect of water-nutrient interaction,thereby changing the availability of soil microbial respiration,reaction substrates and substrates,thereby regulating the production and emission of soil greenhouse gases.As a stabilized exogenous medium material,biochar has the characteristics of strong adsorption capacity and large specific surface area,which promotes the formation of soil aggregates and affects the migration and transformation of energy and substances such as soil moisture,temperature and nutrients.The addition of biochar promoted the absorption of external heat by the soil in the topsoil,broke the original water-heat balance relationship and melting process of the soil,and then affected the physical structure characteristics such as the mechanical composition and pore characteristics of the soil.Based on the performance advantages of carbon-based materials and the regulation effect of soil water and heat environment,it has certain applicability in cold regions,and plays a strong role in realizing the healthy utilization of farmland water and soil resources in cold regions and the low-carbon production mode of black soil.（3）Analysis of the influence of biochar on the process of soil carbon cycle conversion and response mechanism under freeze-thawing conditions.During the FTC,different forms of soil carbon showed different trends and evolution laws with the change of time scale.Among them,the effective carbon such as DOC and TOC showed a trend of first increase,then decrease and then increase;MBC and other activated carbon showed a trend of first decrease,then increase and then decrease;q CO2 showed a trend of first decrease and then increase.Soil available carbon content and soil CO2 emission flux increased with the increase of biochar content,while soil CH4 emission flux decreased with the increase of biochar content.Constructing the response transfer functions related to soil greenhouse gas emission fluxes under different treatment conditions showed that soil CO2 emission fluxes were more sensitive to changes in soil temperature and soil p H,and soil CH4 emission fluxes were more sensitive to changes in soil liquid water content.Biochar treatment enhanced the response relationship between soil CO2 emission flux and soil water-soil environment,soil microbial biomass,while weakened the response relationship between it and soil available carbon.Biochar treatment weakened the response relationship between soil CH4 emission flux and soil water and soil environment,while enhancing its response relationship with soil available carbon and soil microbial biomass.（4）Analysis of the influence of biochar on the process of soil nitrogen cycle conversion and response mechanism under freeze-thawing conditions.During the FTC,different forms of soil nitrogen showed different trends and evolution laws with time scale.Among them,the available nitrogen such as MN and TN showed a trend of increasing first,then decreasing and then increasing,and the active nitrogen such as MBN showed a trend of first decreasing and then increasing,and MBC/MBN showed an M-type change trend.Soil available nitrogen content increased with the increase of biochar content,and MBC/MBN and soil N2O emission fluxes decreased with the increase of biochar content.Constructing response transfer functions related to soil greenhouse gas emission fluxes under different treatment conditions showed that soil N2O emission flux was more sensitive to changes in soil liquid water content and soil urease activity.Biochar treatment increases the complexity of the soil hydrothermal phase transition and nutrient cycle transformation of the underlying surface,and changes the influencing factors such as soil environmental factors and soil microbial biomass through its own special physical and chemical properties,which indirectly affects the soil nitrogen cycle process.Biochar treatment weakened the response relationship between soil N2O emission flux and soil water-soil environment and soil microbial biomass,while enhancing the response relationship with soil available nitrogen.（5）Analysis of the effect of biochar on the coupled transformation process and response mechanism of soil carbon and nitrogen under freeze-thawing conditions.By constructing the response transfer function between different forms of soil available carbon and nitrogen content and different kinds of soil carbon and nitrogen gas fluxes,the coupling between different forms of soil available carbon and nitrogen and different kinds of soil carbon and nitrogen gas fluxes is explained.relation.At the same time,an effective diagnostic method for the information response mode between soil available carbon and nitrogen and soil carbon and nitrogen gas under the action of exogenous media was proposed.There was no significant correlation between soil CO2 emission flux and soil CH4 emission flux,soil N2O emission flux and soil CO2 emission flux,showing a random relationship without any type of coupling relationship in mechanism.There was a positive correlation between soil CH4 emission flux and N2O emission flux,but R2 was small,showing a weak coupling relationship of ebb and flow.There is a direct relationship between soil carbon storage and nitrogen accumulation,and soil carbon and nitrogen cycle transformation processes can affect each other.Biochar allows soil water and nutrients to be retained in the soil for a longer time for plant absorption and utilization,which is conducive to giving full play to the coupling effect of hydroponic carbon and nitrogen,and further increasing production and efficiency,the research results can provide a theoretical reference for the healthy organic regulation of soil biogeochemical processes.In summary,biochar,as a stabilized exogenous medium material,increases the complexity of soil hydrothermal carbon and nitrogen on the underlying surface.During the FTC,biochar enhanced the soil’s ability to hold water,enhanced the heat absorption capacity of the biochar-soil complex,and then affected soil heat transfer,thereby changing soil hydrothermal environmental factors,soil aeration and water holding capacity,and redox conditions.Biochar can effectively regulate the cycle of carbon and nitrogen elements in the soil,and then change the availability of soil microbial respiration,reaction substrates and substrates,thereby regulating the production and emission of soil greenhouse gases.The research results can provide theoretical basis and technical support for farmland water and soil environmental protection,efficient utilization of soil water-heat resources,farmland soil nutrient regulation and soil ecological environment effects in seasonally frozen soil regions. |
