Study On The Regulation Mechanism Of Biochar On Soil Water-Heat,Carbon And Nitrogen Processes In Seasonal Frozen Soil Region

The special freeze-thaw cycle process in the seasonal frozen soil area has a significant impact on the soil ecological environment and material cycle.Unreasonable use of farmland resources,pesticides and fertilizers have changed the physical and chemical properties of soil,aggravated the degradation of soil quality,and seriously threatened soil security and food production.In order to solve the problem of poor soil structure and fertility degradation,various soil amendments have been used in agricultural production.In recent years,biochar has received widespread attention due to its role in soil improvement and pollution remediation.Previous studies on biochar mainly focused on non-frozen soil,however,the effect of biochar on the soil environment under seasonal freezing and thawing is seldom studied,and the mechanism of biochar on soil improvement in seasonal permafrost regions is still unclear.Therefore,taking the black soil of the Songnen Plain as a typical representative,this article explores the effect of biochar on soil quality improvement under the influence of freeze-thaw action,and seeks efficient and reasonable biochar control measures.In the field experiment,3 kinds of biochar application time treatments were set before freezing in winter(BF),end of spring thawing(ST),and mixed before and after freeze-thaw period(FT).And4 kinds of biochar application levels were set for each treatment(3 kg/m²,6 kg/m²,9 kg/m² and 12kg/m²).From the aspects of soil structure,water and soil water-heat conditions,soil nutrient content and greenhouse gas emissions,the soil quality of cropland in seasonal permafrost region was analyzed and comprehensively evaluated after the application of biochar for one year,the best biochar regulation measures were selected.The main findings are as follows:(1)Biochar can reduce the proportion of soil solid phase and increase the total soil porosity.Their change range is proportional to the amount of biochar applied,but the difference in soil between the BF,ST and FT treatment groups is not obvious.After the freeze-thaw period,the total porosity of the soil increases,and the carbon-soil mixed soil is more obvious,which can increase up to 3.42%.The total soil porosity in the crop growing period showed a decreasing trend under the action of natural settlement and plant roots.Biochar inhibits the formation of very small pores(<0.3?m)in the soil and increases the proportion of pores in the pore size range of 0.3-30?m.The proportion of very micro pores(<0.3?m)in the soil decreased with the increase of biochar application,and the largest decrease is 23.1%.Low biochar application amount in BF and ST groups have an inhibitory effect on the large pores(?30-75?m and>75?m)in the soil,which may be due to the filling effect of the biochar fine particles on the soil pores.The effect of biochar on soil pore size under FT treatment is higher than that of the other two groups,indicating that the change of soil pore size is related to the ratio of biochar particles and the application period.(2)The generalized soil structure index and the soil three-phase structure distance were calculated based on the three-phase ratio of soil.The results showed that biochar could adjust the soil structure to the ideal state,but the high amount of biochar would cause the imbalance of the three-phase ratio of soil.Biochar can promote the formation of large soil aggregates(>0.25 mm)in seasonal frozen soil areas,but the application amount of 12 kg/m² leads to loose soil structure,which is not conducive to the formation of large aggregates.Biochar improves the soil anti-erosion ability,and inhibits the destruction degree of soil aggregates,which is beneficial to maintaining the stability of soil structure distribution.One year after application of biochar,it still has a certain impact on the adjustment of soil structure in seasonally frozen soil areas.The results of variance analysis showed that the application amount and the application period of biochar has a significant impact on the soil three-phase structure and the large soil aggregates(>0.25 mm)(P<0.05),and the effect of the biochar application amount on the soil structure was more significant.(3)Biochar has good heat preservation and water retention effects on the soil of seasonal frozen soil areas.The soil water-heat distribution changes under the combined action of freeze-thaw process and biochar.Biochar delays the initial freezing time of soil and weakens the influence of environmental temperature on soil water-heat conditions.During the test period,there was a significant relationship between the biochar application amount and the soil liquid moisture content.The change in soil temperature was only significantly related to the biochar application amount during the freezing and thawing period,and increased with the increase of the amount of biochar.The soil liquid moisture content increases with the increase of soil temperature,and the response relationship conforms to the logistic function relationship during the freezing period and to the cubic function relationship during the melting period.During the crop growth period,their response relationship is complex,which could be described by the cubic function,but the fitting accuracy is not high.Biochar can enhance the relationship between soil temperature and moisture during the freezing and thawing period,and weaken the effect during the crop growth period.(4)The sequence of the correlation between meteorological factors and soil water-heat is as follows:environment temperature,water vapor pressure,evaporation,barometric pressure,solar radiation,sunshine duration.The relationship between environmental temperature and soil water-heat conditions is the most closely,and correlation coefficients are all above 0.9.Meteorological conditions have a greater impact on soil temperature,and soil structure has a strong correlation with soil moisture.Correlation analysis results showed that the balance of solid,liquid and gas in the soil affects the soil moisture status,and also showed a high correlation coefficient between the micropore size(<0.3?m),micropore size(?0.3-5?m)and the soil liquid moisture content.The water retention effect of biochar may be attributed to the promotion of the formation of soil micropores(?0.3-5?m)by biochar.Compared with the amount of biochar application,the effect of biochar application period on soil temperature is not obvious.Due to the close relationship between the soil structure and the moisture status,the soil moisture is affected by the application amount and the period of biochar.(5)Adding biochar to the seasonal frozen soil is beneficial to the retention of soil available nitrogen.At the end of the test period,the content of NH₄⁺-N and NO₃^--N in the soil treated with biochar increased by 0.77 mg/kg and 2.06 mg/kg on average.The biochar application amount has a significant impact on the soil available nitrogen content.There are significant differences in soil NO₃^--N content between the treatment groups at different biochar application periods,and the FT group has a higher level of available nitrogen as a whole.The relationship between soil structure index,soil water-heat and soil available nitrogen conforms to the cubic function relationship.Soil available nitrogen increases with the increase of total soil pores and large soil aggregates(>0.25mm).Soil available nitrogen is closely related to>2 mm soil aggregates and<0.053 mm soil aggregates.The fitting accuracy of soil NH₄⁺-N and soil structure index is generally higher than that of NO₃^--N.The effect of soil structure change on soil NH₄⁺-N may be higher than that of soil NO₃^--N.The content of soil NH₄⁺-N and NO₃^--N during the freezing and thawing period showed an overall trend of increasing with the increase of soil liquid water content.The crop growth period had an opposite trend.During the freezing and thawing period,soil available nutrients have a relatively strong response to soil water-heat conditions.Under the conditions of biochar treatment,the response relationship between soil NO₃^--N and soil water-heat conditions is enhanced.(6)Biochar inhibits the emission of N₂O from the soil,and its effect is proportional to the biochar application amount.The difference in soil N₂O emission flux among the three treatment groups was small,and there was no significant relationship between soil N₂O emission and the biochar application period.The overall level of soil N₂O emission flux during the crop growing period is higher than that during the freeze-thaw period.The thaw period and June to August are the peak periods of soil N₂O emission.The relationship between soil structure,available nitrogen and N₂O emission flux conforms to the cubic function,and the relationship between soil water-heat conditions and soil N₂O emission flux conforms to the logistic function.The number and proportion of pores are important factors affecting soil N₂O emission.In soil water-heat conditions,soil temperature has a more significant impact on soil N₂O emissions.The relationship between NO₃^--N content in soil available nitrogen and soil N₂O emission flux is closer.Biochar enhances the response relationship between soil nitrogen emissions and soil temperature,soil available nitrogen.Principal component analysis results showed that the key factors affecting soil nitrogen emission were soil pore characteristics and biochar application amount.(7)At the end of the experiment,soil total organic carbon content decreased slightly,with a small range of change.The content of soil dissolved organic carbon was increased compared with that at the beginning of the experiment.Biochar can improve the content of soil total organic carbon and dissolved organic carbon in seasonal frozen soil regions.The application amount of biochar has a significant impact on the soil total organic carbon and dissolved organic carbon,which increases with the increase of the application amount of biochar,but the high biochar amount has a worse effect on the improvement of the soil dissolved organic carbon content.The content of the soil total organic carbon and dissolved organic carbon between the treatment groups with different biochar application periods showed the order of ST>FT>BF.The relationship between soil moisture and soil organic carbon conforms to the cubic function relationship.Soil total organic carbon is closely related to soil pores.Soil aggregates>2 mm and<0.053 mm have a strong response relationship with soil organic carbon content.The soil water-heat conditions have a strong response relationship with soil organic carbon during the freeze-thaw period.Biochar enhances the response relationship between soil total soil organic carbon,dissolved organic carbon and soil water-heat conditions,and the relationship between the soil dissolved organic carbon and the water-heat conditions is more closely.(8)Biochar can promote soil respiration in seasonal frozen soil areas.The soil CO₂ emission increases with the increase of biochar application.The soil CO₂ emission flux among the three treatment groups is ST>FT>BF.The peak of soil CO₂ emissions is in late thawing period and August.The relationship among soil structure,soil organic carbon and soil CO₂ emission flux conforms to the cubic function,and the relationship between soil water and heat conforms to the logistic function.The effect of soil temperature on soil carbon emission is more significant.Soil CO₂ emission fluxes during freezing and thawing periods are more sensitive to changes in soil organic carbon.The relationship between soil CO₂ emission flux and soil organic carbon,soil water-heat conditions was enhanced under biochar treatment.In terms of soil structure,the response relationship soil pore status and soil CO₂ emission flux is stronger.The results of principal component analysis also indicate that the main factor of soil carbon emission was the soil pore characteristics,followed by the soil organic substrate content.(9)Taking the soil quality under different biochar control treatments as the evaluation object,the results of the entropy weight-Topsis model showed that biochar can improve the soil quality in seasonal frozen soil areas.FT3 treatment has the best effect on soil quality improvement,with a soil quality value of 0.645,which is 0.294 higher than that of CK.Compared with the 12 treatments,the mixed application of biochar before and after freeze-thaw treatment(FT)had a better effect on soil improvement.There are significant differences in soil quality between different biochar application amount.High-content biochar can significantly improve soil quality.When the biochar application amount is 9 kg/m²,the effect of biochar on soil improvement is the best.Compared with CK,the soil quality of 3 kg/m² biochar application only increased by 0.008 to 0.033.The effect of low content biochar on soil improvement was not obvious.In conclusion,biochar has a positive regulating effect on soil environment in seasonal frozen soil areas,and its effect is affected by seasonal freeze-thaw action.In seasonal frozen soil areas,it is appropriate to adopt the control measures of mixed applied biochar(FT)before and after freezing and thawing,and the biochar application amount of 9 kg/m².This study has certain guiding significance for efficient utilization and sustainable development of soil and water resources in farmland of seasonally frozen soil regions.