The pattern between nitrogen mineralization and grazing intensities in an Inner Mongolian typical steppe |
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Authors: | Yuqing Xu Linghao Li Qibing Wang Quansheng Chen Weixin Cheng |
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Institution: | (1) Laboratory of Quantitative Vegetation Ecology, Institute of Botany, The Chinese Academy of Sciences, Xiangshan, Beijing, 100093, China;(2) Graduate School of the Chinese Academy of Sciences, Yuquanlu, Beijing, 100093, China;(3) Department of Environmental Studies, University of California, Santa Cruz, CA 95064, USA |
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Abstract: | Ungulate grazing is known to play a crucial role in regulating energy flow and nutrient cycling in grassland ecosystems. However,
previous studies of the effect of grazing on soil N dynamics have showed controversial results. Some studies indicate that
grazing stimulates N mineralization while others report that grazing suppresses N mineralization. In order to reconcile these
contrasting results, we investigated the response pattern of nitrogen transformation to multiple grazing intensities in an
Inner Mongolian steppe. In our study, we measured net nitrogen mineralization rates and nitrification rates during a whole
growing season in a 17-year field experiment that had five grazing intensities (0.00, 1.33, 2.67, 4.00 and 5.33 sheep ha−1). Primarily because of changes in temperature and moisture conditions, net N mineralization rates varied substantially during
the growing season with higher values occurring in late July. No consistent differences in net N mineralization rates were
observed between grazing intensity treatments at the monthly time scale. Compared to mineralization rates, net nitrification
rates were generally low with slightly higher values occurring in late July and late August. Ungulate grazing stimulated the
cumulative net N transformations (mineralization, nitrification and ammonification) at the annual time scale, and the most
stimulation occurred at a moderate grazing intensity of 4.00 sheep ha−1, whereas the highest grazing intensity of 5.33 sheep ha−1 and the lighter grazing intensity of 1.33 sheep ha−1 stimulated less. The general response of net N mineralization to grazing intensity gradient is roughly in the form of a normal
distribution at the annual time scale. Our study demonstrated that grazing intensity in concert with soil moisture and temperature
conditions imposed significant controls on soil N transformation and availability in this Inner Mongolian steppe. |
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Keywords: | Ammonium Nitrate Nitrogen transformation Grassland Grazing intensity Inner Mongolia |
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