Ecological Monographs, 83(2), 2013, pp. 195–219 Ó 2013 by the Ecological Society of America N balance and cycling of Inner Mongolia typical steppe: a comprehensive case study of grazing effects M. GIESE, 1 H. BRUECK, 1,2 Y. Z. GAO, 2,3,13 S. LIN, 4 M. STEFFENS, 5 I. KO ¨ GEL-KNABNER, 5 T. GLINDEMANN, 6 A. SUSENBETH, 6 F. TAUBE, 7 K. BUTTERBACH-BAHL, 8 X. H. ZHENG, 9 C. HOFFMANN, 10 Y. F. BAI, 11 AND X. G. HAN 12 1 Department of Crop Production and Agroecology in the Tropics and Subtropics, University of Hohenheim, Garbenstrasse 13, 70593 Stuttgart, Germany 2 Institute of Plant Nutrition and Soil Science, Christian-Albrechts-Universita ¨t, Hermann-Rodewald-Strasse 2, 24118 Kiel, Germany 3 Institute of Grassland Science, Northeast Normal University, Renmin Street 5268, 130024 Changchun, China 4 Institute of Plant Nutrition, China Agriculture University, 100094 Bejing, China 5 Lehrstuhl fu ¨r Bodenkunde, Department fu ¨r O ¨ kologie und O ¨ kosystemmanagement, Wissenschaftszentrum Weihenstephan fu ¨r Erna ¨hrung, Landnutzung und Umwelt, Technische Universita ¨t Mu ¨nchen, 85350 Freising-Weihenstephan, Germany 6 Institute of Animal Nutrition and Physiology, Christian-Albrechts-Universita ¨t zu Kiel, Hermann-Rodewald-Strasse 9, 24118 Kiel, Germany 7 Institute of Grass and Forage Science/Organic Agriculture, Christian-Albrechts-Universita ¨t zu Kiel, Hermann-Rodewald-Strasse 9, 24118 Kiel, Germany 8 Forschungszentrum Karlsruhe, Institute for Meteorology and Climate Research, Atmospheric Environmental Research (IMK-IFU), 82467 Garmisch-Partenkirchen, Germany 9 State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China 10 Institute of Soil Landscape Research, Leibniz-Centre for Agricultural Landscape Research Mu ¨ncheberg e.V., Eberswalder Strasse 84, 15374 Mu ¨ncheberg, Germany 11 State Key Laboratory of Vegetation and Environmental Change, Institute of Botany, Chinese Academy of Sciences, Beijing, China 12 State Key Laboratory of Forest and Soil Ecology, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang, China Abstract. Increasing grazing pressure and climate change affect nitrogen (N) dynamics of grassland ecosystems in the Eurasian steppe belt with unclear consequences for future delivery of essential services such as forage production, C sequestration, and diversity conservation. The identification of key processes responsive to grazing is crucial to optimize grassland management. In this comprehensive case study of a Chinese typical steppe, we present an in- depth analysis of grazing effects on N dynamics, including the balance of N gains and losses, and N cycling. N pools and fluxes were simultaneously quantified on three grassland sites of different long-term grazing intensities. Dust deposition, wind erosion, and wet deposition were the predominant but most variable processes contributing to N losses and gains. Heavy grazing increased the risk of N losses by wind erosion. Hay-making and sheep excrement export to folds during nighttime keeping were important pathways of N losses from grassland sites. Compared to these fluxes, gaseous N losses (N 2 O, NO, N 2 , and NH 3 ) and N losses via export of sheep live mass and wool were of minor relevance. Our N balance calculation indicated mean annual net N losses of 0.9 6 0.8 g N/m 2 (mean 6 SD) at the heavily grazed site, whereas the long-term ungrazed site was an N sink receiving mean annual inputs of 1.8 6 1.1 g N/m 2 , mainly due to dust deposition. Heavy grazing reduced pool sizes of topsoil organic N, above- and belowground biomass, and N fluxes with regard to plant N uptake, decomposition, gross microbial N turnover, and immobilization. Most N-related processes were more intensive in seasons of higher water availability, indicating complex interactions between land use intensity and climate variability. The projected increase of atmospheric N depositions and changes in rainfall pattern imposed by land use change will likely affect N sink–source pathways and N flux dynamics, indicating high potential impact on grassland ecosystem functions. Land use practices will be increasingly important for the management of N dynamics in Chinese typical steppe and, therefore, must be considered as key component to maintain, restore or optimize ecosystem services. Key words: China; climate variability; grassland; land use; N dynamics; N pathways; semiarid; sheep. INTRODUCTION Nitrogen (N) availability often limits primary pro- ductivity of terrestrial ecosystems (LeBauer and Tre- seder 2008), with the degree of N limitation differing between biomes and climatic zones and modified by soil Manuscript received 19 January 2012; revised 19 July 2012; accepted 27 July 2012; final version received 1 October 2012. Corresponding Editor: A. T. Classen. 13 Corresponding author. E-mail: yingzhigao220@163.com 195