Moderate Grazing Promotes Grassland Nitrous Oxide Emission by Increasing Ammonia-Oxidizing Archaea Abundance on the Tibetan Plateau.

Grasslands are suffering from long-term overgrazing because of the population inflation. Furthermore, nitrous oxide (NO) is a major greenhouse gas that also depletes stratospheric ozone. However, the emission rate of grassland NO and underlying mechanisms remained unclear under different grazing intensities. We conducted a field manipulation under four grazing intensities to compare its NO fluxes and main affected factors. It was indicated that alpine meadow NO emission rates increased from 39.7 ± 3.1 to 47.8 ± 2.3 μg m h (p < 0.05), then decreased to 43.4 ± 4.1 and 32.9 ± 1.4 μg m h with grazing intensity increasing from 4 to 8, 12 and 16 sheep ha-1, respectively. Multiple-stepwise regression analysis indicated that the predominant affected soil factors were separately TN and BD, pH and BD, also pH and BD, SOC and BD. Simple linear regression models revealed that ammonia-oxidizing archaea (AOA) contributed much to NO emission (R = 0.77). Additionally, the R coefficient of linear regression was 0.87 between nosZ genes and NO emission rates in alpine meadow. Much attention should be paid to protecting alpine meadow from degradation to mitigate NO emission source on the Tibetan Plateau.