| Summary: | Nitrogen (N) addition is an important nutrient strategy for alpine grassland in northwestern China to improve productivity for livestock needs. A field experiment was conducted in a semi-arid alpine grassland in northwestern China to investigate the effect of N addition rates on soil N<sub>2</sub>O emissions over the growing seasons of 2017 and 2018. Treatments included six N addition rates (0, 10, 30, 60, 120, 240 kg N ha<sup>−1</sup> y<sup>−1</sup>), which were applied before each growing season. The N<sub>2</sub>O fluxes increased with N addition rates and showed different episodic changes between the two growing seasons. In 2017, the maximum N<sub>2</sub>O flux rate occurred within 2 weeks following N addition. In 2018, however, the maximum N<sub>2</sub>O flux rate occurred later in the growing season due to a heavy rainfall event. Growing season cumulative N<sub>2</sub>O emissions ranged between 0.32 and 1.11 kg N ha<sup>−1</sup>, and increased linearly with N addition rates. Increasing N addition rates over 60 kg N ha<sup>−1</sup> yr<sup>−1</sup> did not further increase plant above-ground biomass. The inter-annual variability of N<sub>2</sub>O flux suggests the importance of soil moisture in affecting N<sub>2</sub>O emissions. It is particularly important to avoid over-applying N nutrients beyond plant needs to reduce its negative effect on the environment while maintaining livestock productivity. The N<sub>2</sub>O flux rate increased with soil dissolved organic carbon (DOC) and soil pH. These results suggest the optimal N addition rate to the livestock grassland in this region should be 60 kg N ha<sup>−1</sup> yr<sup>−1</sup>.
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