Effects of long-term grazing exclusion on N₂O flux during the freeze-thaw period in a typical temperate grassland of Inner Mongolia

Different grassland utilization patterns and soil hydrothermal conditions have large effects on greenhouse gas emissions. The freeze-thaw period is the key period of N₂O gas production in grassland ecosystems. This study took a typical temperate grassland of Inner Mongolia as the research object, set up two treatments of grazing exclusion since 1979 and 1999, and used a continuously grazed sample plot as the control. During the freeze-thaw period from 2019 to 2021 (November to April of the next year), sensors and gas in situ acquisition devices were used to monitor the moisture, temperature, and N₂O concentration at different depths of the soil profile over a long time and the chamber method was used to measure N₂O flux at the soil surface. In the freeze-thaw period, the N₂O concentration in the soil profile sharply increased with increases in water and heat . Significant differences were not observed in the N₂O concentration at different soil depths. The N₂O concentration in the soil profile of the grazed plot (0.76 cm³·m⁻³) was significantly higher than that of the ungrazed plots (since 1979: 0.51 cm³·m⁻³, and since 1999: 0.58 cm³·m⁻³) (P < 0.05). The N₂O flux in the continuously grazed grassland 8.30 μg·(m²·h)⁻¹ was significantly higher than that in the ungrazed grassland since 1979 –0.72 μg·(m²·h)⁻¹ (P < 0.05), and the difference between the two reached 9.02 μg·(m²·h)⁻¹. The N₂O concentration was positively correlated with the soil temperature, water, ammonium nitrogen, and microbial biomass carbon. The above results reveal that continuous grazing will promote N₂O flux in the soil during the freeze-thaw period of typical temperate grassland areas in Inner Mongolia. Grazing activities can promote nitrification and denitrification, which are conducive to the production of N₂O, by increasing the soil temperature, water, nitrogen content, and microbial biomass carbon during the freeze-thaw period. Therefore, a reasonable arrangement of the grassland utilization mode should be considered to reduce grassland N₂O fluxes.