Study On Carbon And Nitrogen Cycle And Transport Process In Farmland Soil-soybean System In Black Soil Region

The soil and water environment of soil in farmland and the process of material cycling and transformation play an important role in crop growth and development.However,unreasonable farming measures,excessive application of chemical fertilizers and intensive development and utilization have led to the thinning of the farming layer of farmland black soil and the decline of soil fertility,which have seriously threatened the productivity security and sustainable development of farmland soil in black soil areas.Therefore,this study combines tillage mode with biochar,and sets up four biochar application gradients（0 kg/mu,250 kg/mu,500 kg/mu,750 kg/mu）under shallow and deep tillage modes.Field experiments were conducted on soybeans to explore the effects of biochar on carbon and nitrogen migration in black soil and soybean growth under different tillage modes.The main research content and conclusions are as follows:（1）Analysis of the impact of biochar application on soil water and soil environment and particle structure under different cultivation modes.As the biochar content increased,the contour lines of liquid moisture content became denser,and in deep tillage mode,they were greater than those in shallow tillage.Deep tillage applied 500 kg/mu of biochar（SB2）,and the soil had the highest surface moisture content at 0-30 cm,with an average water transfer rate of 2.7%/day.Biochar promotes the absorption of external environmental heat by the soil surface,thereby increasing the transfer of heat in the soil.The temperature changes at 0-30 cm in the soil range from 2.3 to 4.1℃.Biochar regulates soil pore structure by reducing soil bulk density and the proportion of soil aggregates.As the amount of biochar applied increases,the proportion of soil aggregates>0.5m increases,and the improvement effect of biochar under deep tillage is better.The application of biochar with different gradients in deep tillage significantly reduced the content of soil clay particles（<0.106 mm）,with SB2 treatment having the lowest proportion of soil clay particle content.Compared with CK treatment,the GMD value under biochar treatment is more,the proportion of PAD is larger,and the effect is particularly obvious under deep tillage.This is because the application of biochar under deep tillage improves the soil erosion resistance,inhibits the destruction of soil aggregates,and is conducive to maintaining the stability of soil structure distribution.（2）Analysis of the changes in carbon content in soil and the impact of biochar application on carbon emission mechanisms under different cultivation modes.Under biochar treatment,the overall content of TOC and DOC in soil during crop growth period increased.Compared with CK treatment,the most significant change was observed when 500 kg/mu biochar was applied to deep tillage;The MBC content under biochar treatment generally showed a trend of increasing first and then decreasing,with the carbon content higher in deep tillage mode than in shallow tillage mode.Under biochar treatment,the CO₂ emission flux increases,and the deep tillage mode is greater than the shallow tillage mode.The carbon emissions under SB2 treatment are the highest and reach the emission peak earlier;During the crop growth period,the overall CH₄ emission flux in the soil showed a sink pattern.The application of biochar under cultivation mode reduced the CH₄ emission flux,while the SB2 treatment had the least CH₄ emission.During the growth and development of soybean,the crop roots are developed,and the soil water and heat conditions are sufficient,which improves the activities of microorganisms and related enzymes.soil respiration is enhanced,and the soil CO₂ emission flux gradually increases.At the end of crop growth,with the degradation of soybean plant roots,soil respiration begins to weaken,and the soil CO₂ emission flux gradually decreases;CH₄ emissions are mainly related to the activities of methanogens and methane oxidizing bacteria in the soil.Biochar improves soil aeration,promotes soil respiration,weakens the activity of methanogens,increases the activity of methane oxidizing bacteria,and weakens the CH₄ emission capacity of soil.Therefore,the combined effect of tillage mode and biochar indirectly affects the carbon emission process of soil by affecting the different forms of carbon content in the soil.（3）Analysis of the variation characteristics of nitrogen content in soil and the impact mechanism of nitrogen emission by applying biochar under different cultivation modes.The content of NO₃^--N and NH₄⁺-N in soil increased under biochar treatment during crop growth period.The content under deep tillage mode was higher than that under shallow tillage mode.The content of soil mineral nitrogen under SB2 treatment was the highest.This was because biochar was applied under deep tillage to improve the soil and water environment,promote the activities of microorganisms,strengthen the nitrification of soil microorganisms,and soybean roots consumed soil nitrogen to maintain their own growth and provide nutrients for soybean seeds;Under biochar treatment,the soil MBN content showed a trend of first increasing and then decreasing,with the SB2 treatment showing the greatest change.During the growth period,the soil temperature was high,water was sufficient,and soil aeration was enhanced,promoting the improvement of soil microbial activity,thereby increasing the MBN content in the soil.Biochar inhibits N₂O emission,and N₂O emission in soil is mainly related to denitrification.Biochar application under cultivation mode can consolidate mineral nitrogen in soil,thereby reducing the leaching of soil nutrients,and to a certain extent,promote the cycling and transformation of nitrogen in farmland soil.The application of biochar promoted microbial activities,enhanced nitrification process,and inhibited denitrification process.At the same time,the combination of tillage methods and biochar enhanced the connection with effective nitrogen in soil.The biochar improved the soil microenvironment through its own structure,thereby increasing the retention capacity of microorganisms in soil,which indirectly affected the soil nitrogen emission process.（4）Analysis of the effect of biochar application under different tillage modes on nitrogen migration and transformation process in soil-soybean continuum.Characterization of nitrogen changes in the soil-soybean continuum by ¹⁵N content.The content of ¹⁵N in deep and shallow soil during the crop growth period showed a trend of first increasing and then decreasing,and the content of ¹⁵N in soil showed a trend of first increasing and then decreasing with the increase of biochar content during the same growth period,and the content of ¹⁵N in different soil layers was L1>L2>L3,the TN content in the soil and the ¹⁵N content realized the same trend,first increased and then decreased,the TN content of biochar applied under deep ploughing had a large range,and the TN content of SB2 treatment was the highest in deep ploughing.The ¹⁵N content in the roots,stems and leaves of different organs in the soybean growth period was realized as a trend of first increasing and then decreasing,the ¹⁵N content in each organ in the flowering period of soybean was the highest,the total nitrogen content and biomass content in soybean plants showed a trend of first increasing and then decreasing,the TN content under the application of SB2 treatment was the highest,and the nitrogen utilization rate of soybean organs in the soil under different regulatory modes showed the of roots>leaves>Stem,deep ploughing and applying biochar treatment improved the nitrogen utilization rate in the soil.The SB2 treatment promoted the nitrogen fixation capacity of soil and improved the nitrogen use efficiency of soybean.Under the ¹⁵N isotope tracking,the QB2 and SB2treatment of the nitrogen utilization efficiency of soybean was 18.7%and 22.4%,respectively,and the SB2 treatment the soybean yield reached up to 95 kg/acre.In summary,biochar,as a new soil amendment,shows a positive effect in improving soil water and soil environment and participating in the migration and transformation process of carbon and nitrogen nutrients,and deep tillage and shallow tillage improve the physicochemical properties of soil by changing the agglomerate structure of soil.By participating in soil microbial respiration,biochar can effectively regulate the carbon and nitrogen migration process in soil,and then regulate the emission of soil greenhouse gases.Biochar has shown an influencing effect on inhibiting soil nitrogen loss and improving crop nitrogen use efficiency,promoting the growth and development of soybean plants,and improving the yield and quality of soybean.The results show that the SB2treatment shows the best regulation mode,which provides reference value and guiding significance for the fertility maintenance and soil tillage of farmland in the future.