Background In agricultural activities, due to the unilateral pursuit of high food production, the environmental problems from excessive use of nitrogen fertilizer have become increasingly serious. As a highly effective soil amendment, biochar affects the retention of nitrogen nutrients in the soil and affects the nitrogen fixation capacity of the soil. This review aims to clarify the prior research results, and to provide reference for the future research work on soil nitrogen leaching.

Methods By focusing on the effects of biochar on soil nitrogen fixation capacity, we combined external conditions (soil environment, biochar type, amount and mode of addition, crop type, etc.) and intrinsic conditions (physical properties of biochar, microbial action), and conducted in-depth discussion and clarification of the four mechanisms of biochar's ability of affecting soil nitrogen fixation, and prospected the future research directions.

Results 1) There are 4 mechanisms of biochar's ability to affect soil nitrogen fixation. a) The microporous structure of biochar and its large specific surface area give it strong physical adsorption properties, which can greatly increase the nitrogen content in soil. The retention time of nutrients reduces the loss of nitrogen caused by leaching. b) Biochar is added to the soil to improve the water-holding capacity of the soil and to avoid the loss of nitrogen nutrients with water migration. c) The oxygen-containing acidic functional groups formed on the surface of the biochar increase the CEC value, pH value, etc. of the soil, thereby enhancing the chemical adsorption performance of the soil. d) The interaction between biochar and microorganisms indirectly affects the circulation and utilization of nitrogen in the soil. 2) The future research direction of biochar nitrogen fixation technology and suggestions for the scientific use of biochar in agricultural production are in followings. a) We should actively carry out medium and long-term outdoor tests of biochar, and conduct in-depth research on the stability of biochar. b) We should study the interaction between soil nitrogen nutrients and other nutrients, and verify whether the biochar's retention of nitrogen nutrients is affected by other nutrient elements. c) We need to consider the environmental problems caused by the biochar production process, which is related to the sustainable development of biochar, which may have certain practical significance. d) Biochar may have a certain upper limit on the ability of nutrient adsorption, and we need to further explore it. e) In agricultural production activities, we should pay attention to the use of biochar, learn to adapt to local conditions, and rationally apply biochar according to different soil conditions to improve the effectiveness of fertilizer nutrient utilization. f) We should try to establish a biochar-soil-crop joint research system to better serve agricultural production. Explore new ways to produce cheaper biochar, and strive to reduce their production costs and maximize their economic benefits.

Conclusions The relevant works are summarized and 4 mechanisms and 6 prospects are clarified, which may provide the references for the future research on soil nitrogen leaching.