(a) After the nitrogen fertilizer has been applied to the soil, there are three directions for the nitrogen fertilizer. Part of the nitrogen is absorbed and utilized by the current crop. Some of it remains in the soil, and the other part is lost from the soil-a crop system. Residues of nitrogen in the soil, although not contributing to the current season crop, can keep the soil at least level and change for later season crop types, environmental conditions, fertilization periods, and methods. There is a competitive relationship between them, and the proportion of one to go increases, and the proportion of the other two to go decreases. Roughly speaking, the absorption and utilization of crops in the season is generally 30 to 50%, the loss of nitrogen can reach 20 to 60%, and the residual in soil is about 25 to 35%. (B) Losses of Nitrogen Fertilizer Losses of nitrogen fertilizers are mainly caused by leaching, runoff, and gaseous nitrogen. The loss of gaseous nitrogen includes ammonia volatilization and denitrification. The main products of denitrification are nitrogen and nitrous oxide gas. In paddy soils, leaching losses are generally low, and nitrogen is mainly lost through nitrogen volatilization and denitrification. In addition to the loss of gaseous nitrogen, the leaching loss cannot be ignored in the soil of dry crops, especially in areas with heavy rainfall and light soil texture. Heavy rain, heavy rain or improper irrigation may cause loss of runoff from nitrogen fertilizers. Obviously, the runoff losses in the hilly areas are more serious than those in the plain areas. Afforestation, terrace construction and reasonable irrigation all reduce the loss of soil and nitrogen. Ammonia volatilization losses are prone to occur under alkaline conditions. Therefore, ammonia volatilization on calcareous or alkaline soils is much more severe than on acidic soils. Denitrification needs to be carried out under anaerobic conditions, so denitrification losses in rice fields are generally greater than those in dry lands. The high temperature not only promotes the volatilization of ammonia but also facilitates the denitrification. Nitrogen losses and loss pathways are closely related to soil properties, crop cultivation systems, and climate and environmental conditions, and are also affected by agricultural measures. After clarifying the ways of loss of nitrogen fertilizers, it is beneficial to take corresponding measures to reduce losses and increase the utilization rate of nitrogen fertilizers. (3) Nitrogen Fertilizer Utilization There are two ways to measure nitrogen use efficiency, tracer method and difference method. When using the tracer method to determine the nitrogen use efficiency, it is necessary to apply 15N to eliminate the use of fertilizer. The percentage of 15N that the crop absorbs as a percentage of the 15N applied is the nitrogen use efficiency. When using the difference method to determine the nitrogen use efficiency, only general commercial nitrogen fertilizers should be used, and a nitrogen-free zone should be set at the same time, that is, a test zone without nitrogen fertilizer. The percentage of nitrogen uptake between the nitrogen-applied area and the nitrogen-free area accounts for the percentage of nitrogen applied, which is the nitrogen fertilizer utilization rate measured by the difference method and is usually referred to as the apparent nitrogen fertilizer utilization rate. The apparent nitrogen fertilizer utilization value is greater than the nitrogen fertilizer utilization rate measured by the tracer method. According to different researches and objects, determine which method to use. For example, the apparent nitrogen fertilizer utilization needs to be determined when determining the amount of nitrogen fertilizer, and the tracer method is required to determine the fate of the nitrogen fertilizer.
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