SCHRIFTENREIHE Institut für Pflanzenernährung und Bodenkunde ...

99, from –10.1 to +10.0% at WG, and from –20.1 to +27.4% at HG. Apparently,
the absolute span of the MRD is smaller than 15% except for HG, where it
reaches 28%. Compared with the results from previous temporal stability
experiments (Vachaud et al., 1985; Martinez-Fernandez and Ceballos, 2003),
our results have much smaller MRD and also smaller deviations, which may be
explained by the smaller spatial area in our study. Due to comparable
topographical attributes at the four plots, we ascribed the site-specific
differences of MRD to differences of soil and plant properties affected by grazing
effects (Zhao et al., 2008a). Compared with the other three plots, the heavily
grazed plot (HG) has very sparse vegetation and a weakly structured soil, which
accelerates the dynamics of soil moisture and thus decreases the temporal
stability. This is consistent with Hupet and Vanclooster (2002), who concluded
that the phenology of the vegetation played an important role in the temporal
stability of the spatial patterns. Mohanty and Skaggs (2001) found that fields with
silt loam soils showed poor temporal stability than those containing sandy loam
soils. However, this can not be proofed in our study as sandy soil in HG
expresses low temporal stability. This indicates that there should be other
contributor to time stability of soil moisture, e.g. low water content itself in HG
results in intensive and fast water dynamics, i.e. a low temporal stability.
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Similarly, the temporal stability of soil moisture is moisture-dependent, that
is, the MRD and its deviation for all plots are larger under dry and medium
conditions, but smaller under wet conditions (Figs. 2-5). This is consistent with
Lin (2006), who indicated that temporal stability is higher when the soil is wetter.
It is possibly associated with that during the wet conditions water supply from the
subsoil to the topsoil aids to keep the soil from temporal variability. Again some
time stability points (TSPs) under wet conditions obviously lose time-stable
characteristics under dry conditions. For example, sampling point 34 (denoted
as p34) in UG 79 is temporally stable under wet, medium and all conditions, but
it is not stable under dry conditions (Fig. 2). However, although the time stability
characteristics of some points vary with water conditions, most points were
independent on water conditions.