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likely to occur in the heavily grazed site after strong rainfall events. Secondly, although grass growing in the heavily grazed site can experience water stress at a smaller water deficit than that in the ungrazed sites because the former resulted in an increase of available water holding capacity (AWHC) accompanied by increase of meso-pore fraction (Fig. 1). However, grazing abruptly decreased soil moisture and thus grass in the grazed sites reached water stress point much earlier and easier (Fig. 2), which was in accordance to findings by Snyman (2005). Moreover, the root distribution was more shallow in HG than the other sites (Gao, 2007), which also contributed to the more susceptible plant water stress in the former. These findings were in agreement with Rietkerk and van de Koppel (1997), who fo<strong>und</strong> that heavy grazing resulted in a decrease in soil moisture, and was vulnerable to threshold effects and thus fragile. Therefore, HG was more susceptible to drought, and prone to soil degradation, especially in the drier years. As shown in Fig. 10, ET differences calculated from Pan, FAO, and modeled ones were smaller in the wetter year (2004) than that in the medium year (2006) and the drier year (2005). This revealed that ET is mainly constrained by biotic factors (LAI) in the wetter years; while in the drier years abiotic factors (atmospheric evaporative demand and soil moisture condition) are more important. 108 In summary, our results clearly reflected that grazing influenced soil physical and hydraulic properties. Heavy grazing was in general detrimental to soil functions, while moderate to light grazing was less harmful (Gifford and Hawkins, 1978; Wang and Ripley, 1997; Greenwood and McKenzie, 2001; Martinez and Zinck, 2004). Heavy grazing resulted in the dry soil water regime. Particularly, a diminished amount of plant available water obviously resulted in a decreased rate of plant growth. Reduction of grazing intensity will possibly increase soil water household, and consequently improve the system’s productivity within a few years (Drewry, 2006). In order to protect and restore degraded soils from intense grazing, future land use in Inner Mongolia needs to focus on reducing trampling intensity (e.g., rotational grazing), and even animal exclusion.
Chapter 5 Modeling Grazing Effects on Coupled Water and Heat Fluxes in Inner Mongolia Grassland CONCLUSIONS Water and heat fluxes were highly associated with precipitation and solar radiation, which was further influenced by grazing via altering soil and vegetation conditions, consequently soil hydraulic and thermal properties. Especially, the heavy grazing deteriorated soil functions, as indicated by decreased total and larger porosity accompanied by decreased saturated hydraulic conductivity and increased bulk density. By using multiyear growing season measurements of hydro-meteorological and energy elements at the experimental sites of IMGERS, model HYDRUS-1D was parameterized and verified with a reasonable agreement between simulated and measured data. The simulation could be improved by a more precise representation of soil structure (i.e., LDP model). The model results showed that soil heat fluxes were increased with increase of grazing intensity. In comparison with the two ungrazed sites, winter grazing did not show clear effects on the water household, while heavy grazing led to a completely different situation: decreased interception and transpiration, and increased evaporation, runoff and drainage. As a consequence, intense grazing not only reduces the amount of plant available water thus grassland productivity but possibly increases soil risks for wind and water erosion. ACKNOWLEDGEMENTS This work was done with the financial support of the German Research Council (DFG) for a research grant of the DFG RU #536 MAGIM. Mr. Yujin Wen and Mr. Klaus Erdle are acknowledged for the manuscript improvement and co-operated field work, respectively. We also thank Prof. J. Šimůnek for his helps on the HYDRUS code and Prof. G.N. Flerchinger for his helps on the SHAW code. REFERENCES Allen, R.G., L.S. Pereira, D. Raes, and M. Smith. 1998. Crop evapotranspiration. Guidelines for Computing Crop Water Requirements. Irrigation and Drainage Paper No. 56, FAO, Rome, p. 300. Bremer, D.J., L.M. Auen, J.M. Ham, and C.E. Owensby. 2001. 109
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SCHRIFTENREIHE Institut für Pflanz
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Aus dem Institut für Pflanzenernä
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I Table of contents I Table of cont
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controlling soil moisture are of pa
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Zusammenfassung Überweidung wird a
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Die Beweidung beieinflusst besonder
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II List of figures Fig. 2.1. Locati
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measured soil moisture in time stab
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III List of tables Table 2.1. Descr
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1.1 Background Chapter 1 Introducti
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Chapter 1 Introduction is also unce
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Chapter 1 Introduction water in spr
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Chapter 1 Introduction requires the
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Chapter 1 Introduction Krümmelbein
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Chapter 2 Spatial variability of so
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Chapter 3 Factors controlling spati
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