| The experiment used rice straw, fir and bamboo to make biochar, which was set the temperature in 400℃,600℃or 800℃. The aim was to explore the characteristics of different materials of biochar. The experiment used thermogravimetric analysis, electron microscope image analysis, infrared functional group analysis, group of elements analysis, determination of specific surface area and pore size analysis and grey grouping analysis to explain three kinds of biochar in different preparation temperatures of the physical and chemical properties. The adsorption characteristics of urea, ammonium nitrogen and nitrate nitrogen were studied by using the rice straw biochar as representative. Experimental results show:(1)The total carbonization temperature of rice straw, fir and bamboo was 400,500 and 600, the energy consumption of rice straw was the lowest. The rice straw biochar, fir biochar and bamboo biochar had clear surface. The specific surface area and pore size of rice straw biochar was significantly higher than that of fir biochar and bamboo biochar. With the setting temperature increased, the surface of oxygen functional groups, especially the carbonyl and hydroxyl groups, continued to decrease, the aromaticity was enhanced and the polarity was weakened. In different temperatures, the absorption peak of functional groups on the rice straw biochar was more obvious than that of fir biochar and bamboo biochar. The rice straw biochar had more abundant functional groups and higher chemical reaction activity. The content of organic carbon in fir biochar was the highest, and the rice straw biochar was the lowest. The contents of trace elements of bamboo biochar were higher, but the difference between rice straw biochar and fir biochar was not significant. The content of the environmental toxic elements in bamboo biochar was the highest, while the lowest in the rice straw biochar.(2) With the increase of adsorption time and environment pH, the adsorption capacity of urea in the solution of rice straw biochar produced in 800℃ was higher than that of 600℃ and 400℃. With increase of the concentration of urea in the adsorption solution and the increase of the amount of biochar, the adsorption capacity of urea in solution by rice straw biochar produced in 800℃ and 600℃ was more than that produced in 400℃. The effect of urea by rice straw biochar in adsorption solution with 2.0mm particle size was better than that of 0.45mm. The recommended conditions for the adsorption of urea by rice straw biochar as adsorption carrier: the adsorption time was 3h, the adsorption of pH was 7, the particle size was 2.0mm, the initial concentration of the solution was 3mol/L, and the less amount of biochar added.(3) It is not conducive to adsorb ammonium nitrogen in alkaline environment(pH>8). The effect of ammonium nitrogen by rice straw biochar in adsorption solution with 0.45mm particle size was better than that of 2.0mm. The recommended conditions for the adsorption of ammonium nitrogen by rice straw biochar as adsorption carrier:the adsorption time was 12h, the adsorption of pH was 8, the particle size was 0.45mm, and higher concentration of ammonium nitrogen and higher amount of rice straw biochar.(4) Under different adsorption time, pH, solution concentration and the addition amount of biochar, the adsorption capacity of nitrate nitrogen by rice straw biochar produced in 400℃ was significantly higher than that of produced in 600℃ and 800℃. The effect of the adsorption of nitrogen on the rice straw biochar was not significantly affected by grain size. The recommended conditions for the adsorption of nitrate nitrogen by rice straw biochar as adsorption carrier:the adsorption time was 12h, the adsorption of pH was 7, the initial concentration of the solution was 10ppm, the particle size was 2.0mm, and the less amount of biochar added.In summary, with the increase of the preparation temperature, the carbon skeleton of biochar was gradually clear and distributed evenly, the specific surface area was increased, the porous structure became more abundant, and the oxygen functional groups were decreased, the basic groups were increased, the structure of the carbon ring was increased, the essential nutrient elements in crop growth were rich. The adsorption characteristics of rice straw biochar for different forms of nitrogen (urea, ammonium nitrogen, nitrate nitrogen) in the solution were not strong. The adsorption capacity of urea was about 0.75mg/g, the adsorption capacity of ammonium nitrogen was about 1.77mg/g, and the adsorption capacity was about 8.64mg/g. Subsequent research needed to improve the adsorption capacity of rice straw biochar on different forms of nitrogen by means of modification to provide basic theory support for the agricultural use of biochar.
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