SCHRIFTENREIHE Institut für Pflanzenernährung und Bodenkunde ...

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 found 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.
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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.