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).