The effects of biomass removal and N additions on microbial N transformations and biomass at different vegetation types in an old-field ecosystem in northern China
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There is an increasing demand for the sustainable management of old-field communities in northern China, which have developed on abandoned cropland on formerly converted natural steppe sites, to regain forage yield, biodiversity, and soil fertility. In thus study we examined how two management options—clipping and nitrogen (N) addition—may affect net >microbial N mineralization (ammonification + nitrification), microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), and microbial respirations (MR) in grass dominated, herb dominated, and grass-herb mixed patches in an old-field community in northern China.Topsoil (0–10 cm) net N mineralization rate was 177% and 69% higher in mixed grass and herb patches (patch B) as compared to unmixed grass (patch A) or herb (patch C) patches, respectively. Topsoil MBN was significantly different among the three patches with the highest value for soils taken from umixed grass patches. However, patches with mixed grass and herb or herb dominated patches had 12% higher microbial respiration (MR) than unmixed grass patch. Clipping and N addition had no effects on net N mineralization or MBC, but both treatments decreased MBN and MR and increased the ratio between microbial biomass C and microbial biomass N (MBC/MBN) in the growing season. Incubation of soil cores under optimal water and temperature conditions in the laboratory showed that the response of microbial N transformations in soils under different vegetation patches to experimental N addition and clipping was limited by soil water availability. Our results strongly highlight the need to further study the importance of belowground C supply as a control of microbial N cycling processes. It also suggests that during the restoration process of degenerated croplands N cycling rates are stimulated, but that the magnitude of this stimulation is modulated by plant community composition of the old-fields.
KeywordsClipping Microbial biomass N addition N mineralization Nitrification Old field Patch
This research was financially supported by the State Key Laboratory of Vegetation and Environmental Change, Institute of Botany, Chinese Academy of Sciences and the Ministry of Science and Technology of China (2007CB106801). We wish to extend our thanks to the staff of the Duolun Restoration Ecology Experimentation and Demonstration Station. We are also grateful to Jianyang Xia, Zhe Zhang, Haijun Yang, and Delu Lin for their assistance in collecting samples and to Feng Zhu, Yanbo Yu, and Lina Niu for help in laboratory measurements. We also thank two anonymous reviewers of the original manuscript for constructive comments.
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