Nitrogen is the main nutrient element necessary for plant growth. In agricultural production, nitrogen is regarded as an important index to measure soil fertility and the basic condition for crops to obtain long-term stable and high yield. Nitrogen accounts for 80% of the air volume, and there are 8 tons of nitrogen per square meter of air column. For the vast majority of organisms, these molecular nitrogen can not be utilized, and can only be converted into other compounds through industrial or biological fixation and enter the biological system. Some microorganisms use their own unique nitrogenase system to transfer electrons and energy obtained from photosynthetic products or other carbohydrates to nitrogen and reduce them to ammonia, which is biological nitrogen fixation.

Compared with industrial nitrogen fixation (nitrogen fertilizer industry), biological nitrogen fixation has the characteristics of low cost, no energy consumption and no environmental pollution, and plays an important role in maintaining the nitrogen balance of the global ecosystem.

Biological nitrogen fixation mainly includes autotrophic nitrogen fixation and biological nitrogen fixation. Autogenous nitrogen fixation means that some nitrogen-fixing microorganisms can independently fix molecular nitrogen in the atmosphere in soil or culture medium, and their nitrogen fixation amount is much lower than that of * * *. * * * Biological nitrogen fixation means that nitrogen-fixing microorganisms live together with parasitic plants and directly obtain energy from parasitic plants to complete nitrogen fixation. Because of its strong nitrogen fixation ability, it also has the greatest significance in agricultural production, such as the nitrogen fixation of leguminous plants, cyanobacteria and Azolla.

Industrial nitrogen fixation requires high temperature (470 ~ 520℃) and high pressure (200 ~ 500 atmospheres), while biological nitrogen fixation can be carried out at normal temperature and pressure. This is because there is a special biocatalyst-nitrogenase in nitrogen-fixing microbial cells. Nitrogen-fixing enzyme is composed of steel protein and is an energy converter. It can transfer the transferred electrons to N2, generate NH3 and release hydrogen. Nitrogen-fixing enzyme is controlled by the coding of nitrogen-fixing gene.

In recent 20 years, the research on biological nitrogen fixation has been extremely active and has become an important topic in the world. Looking at the current research content of biological nitrogen fixation, there are three aspects, namely, effective utilization of nitrogen fixation resources, genetic engineering of nitrogen fixation and chemical simulation of nitrogen fixation.

In terms of effective utilization of nitrogen-fixing resources, many countries are vigorously developing leguminous crops, increasing biological nitrogen sources and improving soil fertility through their effective nitrogen-fixing systems, thus promoting agricultural production. In addition, inoculating rhizobia to improve the yield of leguminous crops has been used all over the world. Inoculating and stocking Azolla and nitrogen-fixing cyanobacteria in rice fields can not only increase the amount of biological nitrogen in soil, but also increase the yield of rice. The effective utilization of this nitrogen fixation route has a long history in China and some countries in Southeast Asia.

With the development of molecular biology, nitrogen-fixing genetic engineering has received extensive attention and has become the most active research field at present. Genetic engineering is to change the genetic characteristics of organisms by artificial methods or to create new species according to people's wishes. For nitrogen-fixing microorganisms, nitrogen-fixing genes manipulate and regulate the synthesis of nitrogenase, thus making nitrogen-fixing microorganisms have nitrogen-fixing effect. If nitrogen-fixing genes are artificially transferred, it is possible to obtain new species with nitrogen-fixing effect.

At present, the research in this field is mainly explored in the following aspects: first, cultivate new nitrogen-fixing microorganisms, improve nitrogen-fixing efficiency or give non-nitrogen-fixing microorganisms nitrogen-fixing ability; The second is to change the identification process of nodulation or transfer nitrogen-fixing genes into rhizobia, so that non-leguminous plants can nodulate and fix nitrogen and expand the scope of nitrogen-fixing crops; Thirdly, genetic engineering is applied to cultivate independent nitrogen-fixing plants that do not depend on nitrogen-fixing microorganisms. If these studies are successful, it will have a far-reaching impact on agricultural production.

Nitrogen-fixing microorganisms can convert nitrogen into ammonia at normal temperature and pressure because of their nitrogen-fixing enzymes, while industrial synthetic ammonia should be carried out at high temperature and high pressure. In order to change this situation, scientists are looking for a catalyst similar to nitrogenase, which can change nitrogen into ammonia at room temperature. This is a chemical simulation of nitrogen fixation. The study of chemical simulated nitrogen fixation will provide a new catalyst for chemical nitrogen fertilizer production, which is of great significance to modern nitrogen fertilizer industry and agricultural production.