Effects Of Biochar Application On Nitrogen Mineralization And Leaching Loss In Black Soil Under Freeze-thaw Cycles

China's black soil region is one of the four remaining black soil regions in the world,and also an important commodity grain production base in China.As a precious natural resource,black soil is fertile and rich in organic matter,which is suitable for the growth of a variety of crops.Due to its unique geographical conditions and the impact of climate change characteristics,freeze-thaw cycles often occur in the northeast black soil region.The effect of freezing and thawing on black soil ecosystem will inevitably lead to the migration and transformation of nitrogen,thus affecting the mineralization and leaching loss of nitrogen in the soil.Therefore,in view of the frequent alternation of freezing and thawing in the Songnen Plain of Northeast China,the soil samples from the experimental base of Northeast Agricultural University in the Songnen Plain were tested in the laboratory study.Under different freezing and thawing conditions,different biochar application amount(0,2%,4% and 6%)and different initial soil moisture content(10%,15%,20% and 25%),the change characteristics of nitrogen content,aggregate composition,soil moisture content and nitrogen loss in leaching solution in soil samples were analyzed to study the effect of freeze-thaw cycle on soil nitrogen cycle in farmland black soil area.The results are as follows:(1)Freeze-thaw cycle affects the internal structure and water distribution of soil,destroys the stability of aggregates,and then affects the nitrogen cycle process.The application of biochar can improve the soil water holding capacity.(2)The effect of freeze-thaw cycle on the content of ammonium nitrogen in black soil was significant(p<0.05).In the process of freeze-thaw cycle,the content of ammonium nitrogen increased first and then decreased,and reached the maximum value in the third freeze-thaw cycle.The content of ammonium nitrogen increased with the increase of soil initial moisture content,and decreased with the increase of biochar application.(3)The effect of freeze-thaw cycle on the content of nitrate nitrogen in black soil was significant(p<0.05).In the process of freeze-thaw cycle,the content of nitrate nitrogen increased first and then decreased,and reached the maximum value in the third freeze-thaw cycle.The content of nitrate nitrogen increased with the increase of soil initial moisture content,and increased with the increase of biochar application.(4)The freeze-thaw cycle promoted the occurrence of nitrogen leaching process,intensified the production of nitrogen leaching loss.The accumulated amount of leaching loss increased,then decreased and tended to be stable during the whole freeze-thaw cycle process.The application of biochar effectively increased the ability of soil to retain nitrogen,and the loss of leaching decreased with the increase of biochar content.When the ratio of carbon to soil was 6%,it showed the opposite trend.The results showed that the freeze-thaw cycle had a significant effect on the nitrogen cycling process of black soil,and the application of biochar further affected the nitrogen cycling process.In this paper,the characteristics of soil nitrogen mineralization and leaching loss were studied under the freeze-thaw cycle.The results provide theoretical reference for the effective utilization of nitrogen fertilizer and the application of biochar in the black soil of Northeast China,as well as practical support for the nitrogen cycle and environmental ecological effect of the soil ecosystem.