4.2. Effects of combined terracing and mulching on water use
strategy
Terraces, including the half-moon terracing considered here, have been
shown to be efficient in interrupting runoff paths, reducing soil and
water loss and enhancing infiltration (Wei et al., 2016; Chen et al.,
2017). However, terraces alone can enhance soil evaporation and reduce
soil water content compared to hillslopes during dry spells (Li et al.,
2011). Our results demonstrate that the advantage of half-moon terraces
(SHT) compared to a control gradually reduced over the whole soil
profile during an extreme drought (Figure 4). In contrast, the efficacy
of mulching in reducing soil evaporation has been widely reported, and a
variety of mulching materials – films, straw, branches and gravel –
have been shown to be effective in enhancing soil water availability (et
al., Zhang et al., 2007; Singh et al., 2011; Sun et al., 2012; Liu et
al., 2014). In the current study, two common local materials, pruned
jujube branches and corn stalks, were mulched on the surface of
fish-scale pits to form the combined treatments of terracing and
mulching. We found that they significantly increased soil water storage
compared to the SHT and the control in both the surface (0-20 cm) and
subsurface (20-100 cm) layers (Figure 4).
To our knowledge, this study is the first attempt to investigate the
effects of agronomic practices on soil water use strategy. We found that
the combinations of terracing and mulching clearly modified soil water
use patterns, enhancing depth density of water uptake (Figure 5) and the
proportion of soil water drawn from the deep layers (Figure 6). This can
be attributed to the higher SWS of the subsurface soils in the combined
treatments (Figure 4). The increase in soil water availability may
promote the growth of fine roots that forage water, due to their
hydrotropic nature (Takahashi, 1997), thus enhancing subsurface water
use. The combined treatments also significantly increased transpiration
rate at daily, seasonal and annual scales (Figures 7 and 8). This can be
explained by the strong relationship between SWS and transpiration
(Figure 9). The greater soil water storage further promoted tree growth
and hence more transpiration occurred to meet water demand. Moreover,
the higher SWS may also be responsible for the higher mid-day leaf water
potential under the combined treatments (Figure 10).