Effects Of Biochar Application On Soybean Growth And Root Zone Soil Environment In Seasonally Frozen Soil Regions

In seasonally frozen soil areas,due to the excessive application of chemical fertilizers,the nutrient loss of farmland soil and the decline of land cultivability have seriously endangered the normal production practices in the field.In addition,the carbon and nitrogen cycles in farmland ecosystems were also significantly affected by seasonal fre ezing and thawing.For this study,the typical black loam soil in Northeast China was used as the research object,and field field experiments and field pot experiments were set up respectively.In the field experiment,3 different biochar application methods（Mixed application in spring and autumn,autumn application,spring application are recorded as B,C,D respectively）were set up according to the difference in the time when biochar was applied to the soil,and 4 different biochar application rates(3 kg·m^-2,6kg·m^-2,9 kg·m^-2,12 kg·m^-2are recorded as 1,2,3,4 respectively)were set under each application method,in addition,a blank control group without biochar was also set,with a total of 13treatments.Among them,the single crop soybean was planted in the first three years of the experiment,and in the fourth year of the experiment,some fields were selected for"soybean-corn"intercropping.The field pot experiment set up three biochar application gradients(0 kg·m^-2,3kg·m^-2,6 kg·m^-2)and three fertilizer application gradients(0 kg·ha^-1,75 kg·ha^-1,150 kg·ha^-1)combined mode,a total of 9 treatments.During the experiment,a total of crop agronomic traits,soil water and heat conditions,soybean root morphological characteristics,soil c arbon and nitrogen contents in the root zone,and greenhouse gas emissions were measured.The main purpose of this study is to explore a comprehensive biochar regulation measure suitable for soil improvement in seasonally frozen regions that can mainta in the health of crop habitats.And explore the potential of biochar in combination with different agronomic measures.The main conclusions are as follows:（1）In this study,the spatial variation characteristics of temperature and liquid moisture content of the"biochar-farmland soil"complex were analyzed,and the differences in the hydrothermal changes of the"biochar-farmland soil"complex driven by biochar were revealed.Although the application of biochar cannot change the overall trend of soil water and heat changes in each soil layer,the"biochar-farmland soil"complex changes the specific heat capacity,thermal conductivity,and the absorption and reflection of external energy inside the soil layer.In turn,the factors that affect soil temperature and moisture changes are increased,resulting in differences in heat conduction within the soil.For the soil moisture in the freeze-thaw period,from the soil surface layer to the bottom layer,the content of liquid moisture content gradually increased and gradually became stable,showing a jumping stratification change.When the soil thawed,the moisture content of C3 treatment was the best.For soil temperature,during the freezing process,the freezing speed of B3 treatment was the fastest,and the freezing speed of soil below the tillage layer was B3>C3>A>D3.（2）By measuring the complexity of farmland soil water and heat,the influence mechanism of biochar on farmland soil moisture change during freeze-thaw period was clarified,and the quantitative characterization of biochar application on the process of freeze-thaw soil moisture change was realized.Changes in soil liquid moisture content and soil temperature were not proportional to the biochar application rate.Compared with the control group,only the B3treatment could reduce the complexity of soil temperature,and other treatments with biochar increased the influence factors on soil temperature and soil liquid moisture content.Comparing the results of the approximate entropy values of the three biochar application methods,it was found that the effect of spring and autumn mixed application on soil temperature and liquid moisture content was the least.In addition,when analyzing the response relationship of biochar application rates,biochar and soil mixing time,and soil temperature to soil liquid moisture con tent during freeze-thaw period,it could be found that the value of soil liquid moisture content was a function of the amount of biochar application,the mixing time of biochar and soil,and soil temperature.During the rapid freezing period and thawing period,soil temperature was the main factor affecting the soil liquid moisture content,while in the stable freezing period,the biochar application rates became the main factor affecting the soil liquid moisture content.（3）Using the relevant knowledge of geostatistics and classical statistics,representative experimental plots were selected,and the applicability of biochar in seasonally fro zen soil regions was evaluated pioneeringly with the uniformity of crop growth as the criterion.The test results showed that the application of biochar increased the autocorrelation within each experimental plot,making the difference between the various crop indicators in the experimental plot more obvious.And in the experimental plot,the stem diameter and leaf area of each plant showed the greatest difference.Comparing the treatment with the largest difference with the control group,it was found that the application of biochar had a weak effect on the autocorrelation within the experimental plot.The differences between crops in the plot did not affect the actual production,and its effect could be approximately ignored.（4）Using the measured data values in the field,the temporal and spatial variation of agronomic traits indicators under different biochar regulation modes during the whole growth cycle of soybean canopy were analyzed,and the effects of biochar application on the physiological process of soybean were explored.The application of biochar can significantly increase the plant height of soybean.In the mode of mixed application in spring and autumn,the plant height of the four biochar application rates increased by 9.5 cm,4.66 cm,7.24 cm,and 7.32 cm in turn compared with the control.Biochar can also accelerate the metabolic process of plants,but it would reduce the stem thickness slightly.Stem diameter decreased the most under the biochar application rates of 6 kg·m^-2 and 9 kg·m^-2,which decreased by 0.159 cm,0.125 cm,and 0.123 cm in C2,C3,and D2 treatments,respectively,compared with the control group.However,the application of biochar had no significant effect on leaf nitrogen content and SPAD value of each treatment.In addition,when the application rate of biochar reached 12 kg·m^-2,it was not conducive to the physiological activity of plant leaves and the resistance to extreme weather.（5）According to the measurement results of soybean root data sampling at different st ages,the change rules of soybean root morphological characteristics under different biochar treatments were analyzed,and the balance relationship between soybean root morphological characteristics and biochar application was clarified.The root morphological characteristic index of soybean reached a maximum value at the seeding stage.During this period,the total root length of D3 was the largest,which was 1148.611 cm,the total root surface area of B2 was the largest,which was260.394 cm²,and the average root diameter of B1 was the largest,which was 0.948 mm.In addition,the morphological characteristics of soybean roots were determined by the biochar application rate and biochar application method.Only under the influence of biochar application rate,the root length and average root diameter of soybean were not significant（p>0.05）.In addition,the freeze-thaw effect can change the internal environment of the"biochar-freeze-thaw soil"complex,making the soil loose.These external factors also have an impact on the rhizosphere region of crops.（6）Through the classification and sampling of soybean root zone soil,the process of applying biochar to different types of soil nutrients in soybean root zone was clarified,and the synergistic evolution mechanism of biochar on soybean root zone soil habitat was revealed.Higher biochar application rate will increase the total organic carbon content in th e initial stage of crop growth.However,with the growth and development of plants,soil organic carbo n will gradually migrate from the soil surface to the root sheath and root tip of crops.The effect of biochar on soil inorganic nitrogen was analyzed,and it was found that biochar promoted the accumulation of soil inorganic nitrogen to the crop root system,effectively reducing the loss of nitrate nitrogen and ammonium nitrogen in the root zone soil.And alkaline biochar can accelerate the nitrogen mineralization of soil,and at the same time provide nutrients for anaerobic microorganisms and denitrifying bacteria,and participate in the nitrogen cycle process of soil.（7）With the help of field pot experiments,the effects of the mixed application of biochar and chemical fertilizers on the microenvironment of crop growth such as soil organic carbon and so il inorganic nitrogen were analyzed,and the synergistic relationship between crop physiological indicators and crop nutrients was clarified.Biochar can be used as a medium to accelerate the conversion of ammonium nitrogen to nitrate nitrogen in the soil,which greatly increases the content of nitrate nitrogen in the soil.We speculate that biochar can provide a good environment for the survival of microorganisms by virtue of its unique loose and porous structure,thereby promoting the root nodulation of soybean and the fixation of inorganic nitrogen in the root zone soil.At the same time,there was a significant correlation between inorganic nitrogen in soil and total soil organic carbon,but only low fertilizer application and low biochar application rat e could promote the accumulation of organic carbon in soil.When biochar was mixed with fertilizer,it could significantly increase the value of soybean agronomic traits,increase d the grain weight and pod number of crops,and also improved the partial productivity of nitrogen fertilizer and the agronomic utilization rate of nitrogen fertilizer.（8）On the basis of exploring the driving mechanism of biochar on soybean growth and crop nutrient absorption and utilization,the response of the combination mode of biochar and"soybean-maize"intercropping to crop habitat was studied.The results provide a new research paradigm for biochar regulation of crop habitat research system.The combination mode of biochar and"soybean-maize"intercropping can significantly reduce the emission flux of soil CO₂ gas and reduce the global warming potential of the field.However,this system has limited effects on soil physical indicators such as soil three-phase composition,soil particle size distribution,general ized soil structure index,and soil erodibility factor.The treatment of applying biochar only changed the clay content in the soil（p<0.05）and had no significant effect on the sand and silt in the soil.The combination mode of biochar and"soybean-maize"intercropping can not only affect the growth and development of corn,but also have certain advantages in increasing crop yield.Under the application rate of 9 kg·m^-2 of biochar,soybean and maize can increase yield by 21.2%and 52.8%,respectively.（9）The related data of crop agronomic traits indicators,crop root morphological indicators,root zone soil nutrient indicators,root zone soil structure indicators,root zone soil water and heat indicators,and crop yield indicators measured in field experiments were sorted out.According to the above indicators and selected 14 parameters,the RAGA-PPC crop habitat health evaluation model was constructed.It enriches and improves the technical and theoretical research system of crop habitat health assessment.It enriches and improves the technical and theoretical research system of crop habitat health assessment.