Abstract:

In the context of global climate change, precipitation patterns have changed significantly, both in terms of rainfall frequency and amount of precipitation. Changes in precipitation patterns will significantly affect the ecosystem functions of wetlands, especially the process of soil nitrogen cycle transformation. To explore the effects of different rainfall frequencies on the soil ammonium nitrogen ( $\mathrm{NH}_{4}^{+}-\mathrm{N}$), nitrate nitrogen ($\mathrm{NO}_{3}^{-}-\mathrm{N}$), and total nitrogen (TN) of the Gahai wet meadow in Gannan (located in Gansu Province, China), we set five different rainfall frequencies by simulating extreme rainfall (25 mm irrigation) in the 2019 plant growth season, May-October: CK, natural rainfall only; DF1, watered once a week (19 times×25 mm); DF2, watered once every 2 weeks (9 times × 25 mm); DF3, watered once every 3 weeks (6 times×25 mm); and DF4, watered once every 4 weeks (4 times × 25 mm). All treatments were exposed to natural rainfall. The results showed that different rainfall frequencies have significant effects on the soil nitrogen components of the wet meadow (P < 0.05). The increase of rainfall frequency increased the soil $\mathrm{NH}_{4}^{+}-\mathrm{N}$ and $\mathrm{NO}_{3}^{-}-\mathrm{N}$ content in the growing season, from May to October; the peak of $\mathrm{NH}_{4}^{+}-\mathrm{N}$ and $\mathrm{NO}_{3}^{-}-\mathrm{N}$ increased significantly. Compared with CK, the increase in rainfall frequency increased the soil $\mathrm{NH}_{4}^{+}-\mathrm{N}$ and $\mathrm{NO}_{3}^{-}-\mathrm{N}$ content and reduced the TN content. Among different rainfall frequencies, with the gradient of rainfall frequency increasing, the 0-40 cm layer $ \mathrm{NH}_{4}^{+}-\mathrm{N}$ and $\mathrm{NO}_{3}^{-}-\mathrm{N}$ showed a trend of first increasing and then decreasing and $\mathrm{TN}$ showed a trend of first decreasing and then increasing. Under different rainfall frequency treatments, the proportion of TN of $\mathrm{NH}_{4}^{+}-\mathrm{N}$ in each soil layer was higher than that of $\mathrm{NO}_{3}^{-}-\mathrm{N}$, and soil$\mathrm{NH}_{4}^{+}-\mathrm{N}$, $\mathrm{NO}_{3}^{-}-\mathrm{N}$, and $\mathrm{TN}$ content decreased with the increase in soil depth.

Key words: wet meadow, rainfall frequency, ammonium nitrogen, nitrate nitrogen, total nitrogen