Anthropogenic effects, such as nitrogen (N) enrichment and mowing, are constantly changing the function and structure of grassland ecosystems. In order to test whether the magnitude and frequency of N addition, as well as occurrence of mowing, affects gross N turnover. We designed a long-term field experiment which included 5 levels of N addition (0, 2, 10, 20, and 50 g N m-2 yr-1) and mowing in a typical grassland of northern China. To test the effects of N addition frequency, the amount of N applied was separated evenly by two times (twice a year, low frequency) or twelve times (monthly, high frequency) that results were compared against a control site where none of the treatments were applied. Mowing effects were also considered at each N treatment levels. Our results showed that the N level, the frequency of N addition, and mowing significantly influenced gross ammonification (GA) and nitrification (GN) rates. Specifically, the effect of N addition frequency was significantly different under the highest N addition level (50 g N m-2 yr-1), lower frequency (twice a year) significantly increased N turnover rates. Mowing significantly increased the GA rate, while decreased GN rate both under the highest N addition level (50 g N m-2 yr-1) and lower N addition frequency. Further long-term study of the effects of the interactions between N addition and mowing on N turnover will be needed for understanding the mechanisms by which nutrient cycling in typical grassland ecosystems may change in the future.

1. Anthropogenic effects, such as nitrogen (N) enrichment and mowing, are constantly changing the function and structure of grassland ecosystems. In order to test whether the magnitude and frequency of N addition, as well as occurrence of mowing, affects gross N turnover. We designed a long-term field experiment which included 5 levels of N addition (0, 2, 10, 20, and 50 g N m-2 yr-1) and mowing in a typical grassland of northern China. 2. To test the effects of N addition frequency, the amount of N applied was separated evenly by two times (twice a year, low frequency) or twelve times (monthly, high frequency) that results were compared against a control site where none of the treatments were applied. Mowing effects were also considered at each N treatment levels. 3. Our results showed that the N level, the frequency of N addition, and mowing significantly influenced gross ammonification (GA) and nitrification (GN) rates. Specifically, the effect of N addition frequency was significantly different under the highest N addition level (50 g N m-2 yr-1), lower frequency (twice a year) significantly increased N turnover rates. Mowing significantly increased the GA rate, while decreased GN rate both under the highest N addition level (50 g N m-2 yr-1) and lower N addition frequency. Further long-term study of the effects of the interactions between N addition and mowing on N turnover will be needed for understanding the mechanisms by which nutrient cycling in typical grassland ecosystems may change in the future.