Effects of nitrogen application and nitrification inhibitor addition on N₂O emissions in Medicago sativa L. grassland

Nitrous oxide (N₂O) is undoubtedly one of important greenhouse gases in the atmosphere, which can destroy the ozone layer and aggravate global warming. Agricultural activities, such as fertilizer application, crop straw returning, and biological nitrogen fixation, are the main sources of globally increasing N₂O. Therefore, the study of N₂O emission characteristics and its impact is of great significance for control and mitigation of environmental pollution. This study investigated the N₂O release flux of alfalfa grassland as influenced by nitrogen application and nitrification inhibitor addition, using the static chamber method in Longdong District. The treatments included nitrogen applications of 0 (N0), 50 (N50), 100 (N100), and 150 (N150) kg(N)·hm^-2; and nitrification inhibitor (dicyanogen, DCD) addition (N150+DCD). The static chambers were mounted for the estimation of N₂O emissions from the enclosed alfalfa chambers for two hours daily, and the radiation, air temperature, soil temperature, and moisture were investigated simultaneously. The results showed that the average N₂O emission rates were 3.5, 4.1, 5.0, and 6.1 μg·m^-2·h^-1 for N0, N50, N100, and N150 during the growing season, respectively. The N₂O emission flux was significantly higher in N150 than that in other treatments (P < 0.05). Meanwhile, an increasing trend in the N₂O emission rate was observed with the increasing nitrogen application gradient. Compared to the N150 treatment, the average N₂O emission rate in the N150+DCD treatment decreased by 50.7%, and the cumulative N₂O emissions significantly decreased by 61.6% (P < 0.05), indicating that the addition of a nitrification inhibitor had a significant inhibitory effect on the N₂O emissions. Moreover, the addition of a soil nitrification inhibitor reduced the accumulation of NO₃^--N in the 0-40 cm soil layer and inhibited nitrification in the soil. The dry matter yield of alfalfa per cutting was not influenced by nitrogen application, as there were no significant differences between the N0 treatment and nitrogen application treatments (P>0.05). The N₂O emissions per unit alfalfa yield were 6.5, 7.8, 11.3, and 12.5 mg·kg^-1 for the N0, N50, N100, and N150 treatments, respectively. Therefore, the N₂O emissions increased with the increasing nitrogen fertilizer application rates. It was also discovered that the N₂O emissions were deeply affected by the soil moisture content. During the growing season, the N₂O emission flux had a significant positive correlation with the soil moisture (P < 0.05), but no correlation with the soil temperature. Therefore, it could be concluded that nitrogen application can significantly stimulate N₂O emissions in alfalfa grassland, which is the main reason for the highest N₂O emissions being experienced during the alfalfa growing season. In addition, nitrogen application also had an impact on the N₂O emissions per unit yield of alfalfa. The application of nitrogen together with a nitrification inhibitor can effectively reduce the N₂O emissions caused by fertilization. While temperature may not influence N₂O emissions, precipitation can stimulate N₂O emissions during the growing season. These findings will help to provide a theoretical basis for greenhouse gas emission reduction and reduce the uncertainty concerning climate change prediction in the study area.