| Biochar,as a new,efficient and simple carbon rich material,is more and more used in plant soil field to improve poor soil nutrient status and promote better plant growth.Fresh biochar has the potential of nutrient utilization,which can carry a higher level of available potassium and alleviate the lack of potassium in soil.Under natural conditions,biochar can exist in the soil for thousands of years,with very high stability,but biochar in the soil is not invariable.The common dry wet,freeze-thaw alternation and acid rain phenomenon in the soil will change the structure and properties of biochar,thus affecting the effect of biochar on Soil and plants.Soil acidification,freeze-thaw,drying and wetting are also common in southern China.In this study,three kinds of biochar were obtained by artificial aging in laboratory to study the effects of biochar and its aging on soil improvement,especially soil potassium and growth of pakchoi.The results are as follows（1）Biochar application can significantly reduce soil acidity,improve soil available potassium level,increase soil fertility,and then promote the growth of Pakchoi.Compared with the control,the content of water-soluble potassium increased by12.6%～51.8%,the content of available potassium increased by 13.3%～43.5%,and the content of slowly available potassium increased by 10.3%～26.1%.The content of various forms of potassium under normal application of potassium fertilizer was the closest to that under 4%C treatment.In addition,biochar application promoted the growth and development of pakchoi,and the agronomic traits such as biomass,leaf number,plant height and fresh weight were significantly improved.（2）Aging biochar can still improve the basic physical and chemical properties of soil,but the improvement of soil p H and acid environment is weakened.The response of soil enzyme activity to different aged biochar was different.The aged biochar could not promote the activity of soil potassium solubilizing bacteria.Aging biochar reduced the absorption of N,P,K and other nutrients by pakchoi.Compared with the original biochar,the contents of nitrogen,phosphorus and potassium in leaves of Pakchoi decreased,and the levels of agronomic traits such as leaf number,fresh weight and dry weight of Pakchoi decreased.Soil p H decreased significantly by 0.16～0.3 units,while original biochar and freeze-thaw biochar could increase soil p H by 0.16～0.17 units.The application of original biochar and freeze-thaw biochar could increase soil water-soluble potassium content by2.8%～3.0%;The content of soil available potassium was significantly increased by31.5%～40.4%under the influence of biochar application;Dry wet and freeze-thaw biochar had the most significant effect on slow release potassium.Dry wet and freeze-thaw biochar can increase the slow-release potassium content of soil by 21.3%-45.0%,and acidified biochar can reduce the slow-release potassium content by 15.8%.Adding original biochar can promote the development of soil potassium solubilizing bacteria,but adding various kinds of aged biochar has no effect on Soil Potassium bacteria.（3）The specific surface area,porosity and C%,N%content of aged biochar will change,which may affect the release and migration of soil nutrients and enzyme activities.After all kinds of aged biochar were applied to the soil,only the C/N of acidified aged biochar increased by 18.0%,while the C/N of the other three biochar decreased.The larger specific surface area of acidified aged biochar may be due to the adsorption of organic matter,while the specific surface area of other aged biochar increased significantly after being applied to the soil,and the specific surface area of freeze-thaw aged biochar increased by 281%;The pore volume and pore size of aged biochar extracted from soil increased by 16.7%～508%and 59.5%～134%,respectively.（4）There was almost no change on the surface of biochar after aging;The surface of acidified biochar is rugged,and the pores of biochar are destroyed and the SSA of biochar is decreased by serious liquid phase oxidation,resulting in the increase of pore volume and the collapse and blockage of pore channels;When the soil is aged,the pores are blocked;The pore size of dry and wet biochar increased obviously,and the surface of the aged biochar broke irregularly,and the number of micropores increased;The pore size of freeze-thaw biochar also increased significantly,and the surface was easy to crack after aging.When the original aged biochar was applied to the soil,a stronger peak was formed at 536 cm-1,which may be due to the stretching vibration of inorganic halogen compounds,such as KCl.（5）In the case of no pakchoi,the application of aged biochar can reduce soil acidity,increase soil available potassium level（except acidified biochar）,increase the proportion of illite in soil,and improve soil fertility.Compared with the control,the application of aged biochar significantly increased soil p H by 0.21 and 0.34 units,respectively,while the application of acidified biochar decreased soil p H by 0.16 units,aggravating soil acidification;Aging biochar can increase the content of soil available phosphorus and organic matter.Compared with the control,dry wet and freeze-thaw biochar increased soil water-soluble K by 98.7%and 108%,soil available K by 402.3%and 396.7%,and soil slowly available K by 112.1%and 122.5%,respectively.Through semi quantitative analysis,we found that the proportion of illite increased by 2%～15.5%,and the proportion of kaolinite decreased by 2.2%～17.7%. |
