Effects Of Biochar On The Hydrothermal Environment Of Typical Black Soil In Cold Region Of North China

Biochar is widely used as a soil improver in farmland soils.It significantly changed the physical and chemical properties of the soil,affected the hydrothermal condition of the soil,greenhouse gas emissions,and the growth process of crops.In addition,the seasonal freezing-thawing process in the northern cold region makes the hydrothermal environment of biochar soils more complicated.However,in recent years,studies have not comprehensively considered the physical and chemical properties of soil,crop growth and greenhouse gas emissions,and only considered the growing period or freezing-thawing period in the selection of research cycles.Because of the singleness of research content and cycle selection,the research conclusion is lack of comprehensiveness and integrity.In addition,in the study of the effect of biochar on soil water and heat,the analysis of the parameters of soil water movement and thermal characteristics of biochar is neglected.During freezing-thawing period and crop growth period,the influence mechanism of biochar on soil physical properties,hydrothermal status,greenhouse gas emission and crop growth process was systematically studied.It has certain theoretical and practical significance to the improvement of farmland soil ecological environment.In this paper,the effect of biochar on soil physical properties,soil thermal characteristics and soil moisture movement parameters was analyzed by laboratory experiments.Meanwhile,the effects of biochar on soil water and heat transfer,greenhouse gas emission and crop growth were analyzed through outdoor experiments.The effects of different biochar content on soil hydrothermal environment during freezing-thawing period and crop growth period were respectively revealed.The specific research results are as follows:?1?Biochar can increase saturated water content,field water holding capacity and effective water content of soil,but decrease wilting water content.In addition,the effect of biochar on soil water constants is consistent during freezing-thawing period and crop growth period.In addition,biochar has large pores,so it can increase the total porosity and capillary porosity of the soil and reduce the bulk density of the soil.Biochar has a large specific surface area and strong adsorption,so biochar has a significant effect on soil structure.After biochar was applied,the content of large aggregates?>2 mm and 2?0.25 mm?increased,and the proportion of large pore size?0.3?5?m?5?30?m?30?75?m?75?100?m?>100?m?increased.While,the content of small aggregates?0.25?0.053 mm and<0.053 mm?in soil decreased,and the proportion of soil micropore size?<0.3?m?decreased.It was found that the application of biochar could affect the freezing process of soil.It was found that the application of biochar could change the freezing characteristic curve of soil and increase the unfrozen water content of soil.In addition,the biochar can reduce the maximum freezing depth of soil and shorten the freezing period.?2?Under different biochar contents treatment conditions,the soil temperature change law was consistent in the whole experiment period.The soil temperature decreased with the decrease of atmospheric temperature in the freezing period,while the soil temperature increased with the increase of atmospheric temperature in the melting period and the growing period of crops.However,applying biochar darkens the soil,which allows it to absorb more heat.Biochar changes the thermal performance parameters of soil,which makes the soil temperature increase and the change rate of soil temperature decrease.In this paper,through monitoring field measured data,the parameters of soil thermal characteristics during freezing-thawing period and growing period of crops were fitted:soil thermal conductivity and moisture content were quadratic functions,soil volumetric heat capacity and moisture content were linear functions,and soil thermal diffusivity and soil moisture content were quadratic functions.Through laboratory experiments,it was found that the thermal conductivity,volume heat capacity and thermal diffusion rate of non-frozen soil increased with the increase of moisture content.Under the condition of the same water content,the thermal conductivity,volume heat capacity and heat diffusivity of soil tended to decrease with the increase of biochar.In frozen soil,With the increase of water content,the thermal conductivity,volume heat capacity and heat diffusivity of soil also increased.With the increase of biochar,the thermal conductivity and thermal diffusivity of soil tended to decreased,while the volume heat capacity of soil tended to increased.Compared with the non-frozen soil,the thermal conductivity and the thermal diffusivity increased and the volumetric heat capacity decreased during the freezing process.?3?In the whole test period,the soil moisture content changes in the condition of biochar treatment with different contents are consistent.Soil moisture content decreases rapidly during the freezing period and then remains stable,increasing during the thawing period and the growing period.Biochar can increase soil moisture content due to its strong adsorption property and its ability to increase soil moisture constant.The formulae of soil water characteristic curve,soil water conductivity and soil water diffusion were fitted for different periods.According to the formula,soil water conductivity,soil water diffusion and soil moisture content are power functions.During freeze-thaw period,the biochar can increase the content of unfrozen water in the soil.Therefore,the application of biochar can increase the soil water conductivity and water diffusion rate.In the growing period of crops,biochar can increase the soil porosity and water flow channel due to its strong adsorption,so it can increase the soil water movement parameters.In this paper,PDE?partial differential equation?module in Comsol Multiphysics was used to solve the partial differential equation of soil water and heat.By comparing the calculated value with the measured value,it was shown that the model could accurately simulate the hydrothermal migration law of biochar soil at different periods.?4?Application of biochar can increase the content of inorganic nitrogen?nitrate nitrogen and ammonium nitrogen?in soil.Biochar contains a certain amount of soluble organic carbon,which can significantly increase the soluble organic carbon in soil.During the whole experiment period,soil nitrate nitrogen,ammonium nitrogen and soluble organic carbon changed differently.During the freezing period,the nitrate nitrogen,ammonium nitrogen and soluble organic carbon in the soil decreased due to the decrease of soil temperature and microbial death.At the end of freezing period,with the increase of soil temperature and moisture content,the contents of nitrate nitrogen,ammonium nitrogen and soluble organic carbon increased significantly.The biochar can increase soil temperature and moisture content,thus enhancing the enzyme activity in soil.Therefore,the nitrate nitrogen,ammonium nitrogen and soluble organic carbon in biochar soils were increased.During the melting period,the contents of nitrate and ammonium nitrogen in soil increased first and then decreased,while the contents of soil soluble organic carbon increased.The nitrate nitrogen,ammonium nitrogen and soluble organic carbon in biochar soils were increased significantly.During the growing period of crops,the contents of soil nitrate nitrogen,ammonium nitrogen and soil soluble organic carbon first increased,then decreased,then increased.?5?During the whole experiment cycle,the cumulative emission of soil N₂O gas was the largest during the growing period,followed by the freezing period and the smallest during the melting period.The application of biochar can reduce the cumulative emission of N₂O gas from the soil.The emission fluxes of N₂O gas from biochar soils were different in different periods.During the freezing period,the biochar soil temperature is high could enhance the microbial activity and enzyme activity,thus increasing the N₂O gas emission flux of the soil.The functional relationship between N₂O gas emission flux and soil moisture content,temperature,nitrate nitrogen content and ammonium nitrogen content during freezing period was established.In the melting period,with the increase of soil temperature and moisture content,the content of inorganic nitrogen in the soil increased,and the N₂O gas emission flux increased in a pulsed manner.Biochar can positively promote the N₂O gas emission flux by improving the hydrothermal environment in soil.However,biochar can enhance the cation exchange capacity of soil and adsorb more inorganic nitrogen,which reduces the substrates of nitrification and denitrification.It can reduce the N₂O emission.The N₂O emission flux of BC1 and BC2 soil showed a weakening trend relative to BC0.The functional relationship between N₂O gas emission flux and soil moisture content,temperature,nitrate nitrogen content and ammonium nitrogen content during melting period was constructed.During the growing period of crops,the soil N₂O emission flux was higher with the increase of soil inorganic nitrogen content.The moisture content and temperature values of soil with different biochar application were all in the suitable range for microbial growth.Biochar increases the p H of the soil and promotes the conversion of N₂O gas to N₂.Therefore,biochar reduces soil N₂O emission flux.The functional relationship between different environmental impact factors and N₂O gas emission flux during the growing period of crops was established.?6?During the whole experiment cycle,the cumulative emission of soil CO₂ gas was the largest during the growing period,followed by the freezing period and the smallest during the melting period.The application of biochar can reduce the cumulative emission of CO₂ gas from the soil.However,the change of CO₂ emission flux of biochar soil in different periods is different.During the freezing period,CO₂ emission flux decreases with the decrease of soil temperature.Biochar can increase soil temperature and affect microorganisms,which is the main factor controlling soil CO₂ emissions.Therefore,the application of biochar during the freezing period can promote CO₂ emissions.The functional relationship between CO₂ emission flux and soil moisture content,temperature and soluble organic carbon during freezing period was established.During the melting period,with the increase of soil temperature and moisture content,the soil CO₂emission flux increased,and the functional relationship between different environmental impact factors and CO₂ emission flux was constructed.During the growing period of crops,the soil CO₂emission flux is large because the root system of plants also has respiration.Applying biochar can increase soil water content and soil p H,and promote CO₂to dissolve in water and weaken microbial activity when soil p H is high.Therefore,the CO₂emission flux of biochar soil applied during crop growth was reduced.The relationship between CO₂emission flux and soil temperature and soil moisture content is a cubic function.The relationship between CO₂ emission flux and soil soluble organic carbon was linear.?7?The application of biochar can reduce the CH₄ emission flux of soil.During the freezing period and the growing period,the soil CH₄ gas was net absorbed,and during the melting period,the soil CH₄ gas was net discharged.During the freezing period,the activity of methane-oxidizing bacteria decreased due to the decrease of temperature,which led to the reduction of methane emissions.Biochar can increase the total porosity of soil and the proportion of large pore size distribution,and increase the oxygen content in soil.Therefore,applying biochar can increase the absorption flux of CH₄ gas.The functional relationship between soil temperature,water content,soluble organic carbon and soil CH₄ gas emission flux was constructed.During the melting period,due to the melting of snow cover,the moisture content in the soil increases,the activity of methanogenic bacteria increases,and the CH₄ gas is shown as net emission.Biochar can increase the porosity of the soil and increase the oxygen content of the soil,thus reducing the emission of CH₄ gas.The functional relationship between environmental impact factor and CH₄ emission flux was constructed.During the growing period of crops,when soil moisture content is high after rainfall,CH₄ gas is shown as net emission.When soil moisture content is low,CH₄ gas appears as net absorption.The application of biochar effectively reduces the soil's global warming potential.The global warming potential of BC0 was 1126.71 kg.hm^-2,and the global warming potential of BC1,BC2 and BC3 decreased by 665.783,830.4346 and 590.7206 kg.hm^-2.?8?The application of biochar can significantly affect soil temperature,moisture and soil nutrient content,thus affecting crop growth.Biochar can increase plant height,stem thick leaf area,root length and other morphological indexes of soybean.Meanwhile,physiological indexes such as chlorophyll content,transpiration rate,stomatal conductance and intercellular CO₂ concentration were also promoted.Biochar promoted photosynthesis and increased the accumulation of photosynthesis products,which increased the accumulation of dry matter in the stem of soybean leaves.Biochar also promoted agronomic shape indexes such as main stem section number,number of branches,number of pods per plant,and grain number per plant.Biochar increased soybean yields and reduced soil greenhouse gas emissions.When biochar of 20 t.hm^-2 was applied,the maximum yield increase and the lowest greenhouse gas emission intensity were obtained.