4.3 Effect of revegetation on the ‘soil reservoir’
On the CLP, the thick loess has a very high water storage capacity,
which can accumulate all the natural rainfall and regulate the balance
of soil water to sustain plant growth. Vegetation recovery and
sustainable development are mainly dependent on the level of SM and the
stock amount of the ‘soil reservoir’. Many previous studies have shown
that SM and SWS decrease significantly after revegetation, causing a
temporary or permanent dry soil layer (Jia et al., 2017; Su and
Shangguan, 2019). This desiccation situation of the soil profile limits
the replenishment of soil water after precipitation. In turn, the lack
of the supply capacity of the soil reservoir further restricts plant
growth, such as the ‘old-man small tree’ (Wang et al., 2010a; Wang et
al., 2010b). For example, Zeng et al. (2017) and Chai et al. (2019)
demonstrated that after planting for 30 yrs, leguminous shrubs (C.
korshins ) will decrease the soil quality and continuously reduce the
SM, resulting in premature decay and death. Nevertheless, not all
vegetation recovery types exhibit functional degradation and recession.
The main reason for this situation is whether the soil water of the soil
reservoir can maintain sustainable vegetation growth. In particular, it
is important to note whether the soil water that is consumed by the
plants in the dry season can be replenished by rainfall in the rainy
season. Therefore, we researched the changes in the SM and SWS of each
land-cover type after rainfall events and over the whole plant growing
season.
We concluded that land-cover change, especially vegetation recovery, had
a significant impact on soil water accumulation and utilization after
rainfall (Fig. 5). In our study, revegetation remodeled the
interrelation between SM and rainfall. Letting the plants act as the
intermediate medium and forming a relatively continuous and
discontinuous unity, the roots of plants dig deep into the soil but
block rainfall from coming into direct contact with the soil. These
changes in plant structure and function of land cover promoted much more
rainwater percolating into the ‘soil reservoir’, which increased the SWS
by approximately 67.7%, 6.1%, and 31.9% in the planted forest, shrub,
and grass, respectively, compared to the cropland over the 13 rainfall
events (Fig. 5). The reason is that forest and grass sites with a higher
canopy coverage and a thick litter layers hamper precipitation to a
large extent, reducing the erosion of surface soil and increasing the
retention of rainwater (Jian et al., 2015), especially for heavy rains.
When the amount of rainfall surpasses the interception capacity of
vegetation, rainwater will reach the soil. At this moment, the continuum
that the roots of plants related to soil begin to play their roles. The
dense fine roots form a multi-porous structure and establish a multipath
water infiltration channel, promoting rainwater infiltration into deeper
soil with a short response time. For example, the abandoned grass
exhibited preferential flow in the subsurface soil layer after
continuous rains (Fig. 5h). In addition, the improvement of soil
properties by vegetation, which decreased the bulk density and increased
the SOC, all increased the infiltration rate of soil water. The final
result is that the forest site had the largest response value of slope
correlation (Fig. 5a) and the largest change in the SWS at the 0-1 m
soil depth after all the soil wetting processes, followed by grass (Fig.
5b). Nevertheless, planted shrubs with similar characteristics of
revegetation did not store more soil water after precipitation, mainly
because plant absorption and evapotranspiration exacerbated the entire
soil water depletion profile. The accumulated amount after precipitation
was not enough to compensate for vegetation water consumption. The shrub
site had the shortest RT and fastest WFV in the surface soil but was not
helpful for alleviating soil desiccation and increasing soil water
accumulation. This result suggested that planted shrubs may not be
suitable for this limited rainfall region on the semiarid CLP,
especially after 20 years of revegetation. We also concluded that the
rainfall-SM response pattern determined the utilization and storage
patterns of soil water of different land-cover types. The deeper the
response depth was, the shorter the response time was, and the higher
the velocity of subsurface soil was, the larger the increment of SWS and
rainfall utilization rate (RUR) in rainfall processes.
Further study illustrated that the seasonal SM distribution and SWS
changed significantly across the five monitoring sites over the growing
season. Revegetation improved the average soil water content (ASWC)
across the profile, with a higher RUR and infiltration amount, despite
planted shrubs depleting much more soil water. Meanwhile, planting trees
and grass, instead of cultivation or bare land, promoted rainwater
reaching deeper soil, which showed that the largest ASWC value occurred
at deeper depths after revegetation (Fig. 6). This result was due to
soil permeability promotion, particularly in subsurface profiles, which
accelerated the speed of soil water infiltration and responded in the
deeper layer. Additionally, due to the higher canopy coverage and thick
litter layer of woodland (Jian et al., 2015) and lower
evapotranspiration of grass (Wang et al., 2012), which depleted less
water from deeper soils in the growing season, the soil water of the
revegetation type could sustain a higher content in deep soil. For SWS
variation over the growing season (SWStotal), planted
forest had the largest increment in SWStotal, which was
significantly higher than that at the other sites. The utilization and
conservation pattern of forests resulted in greater
SWStotal accumulation. For example, a deeper soil
wetting depth for infiltration (Fig. 3), a faster WFV of the subsoil
layer (Table 4), a higher utilization rate (RUR) of rainfall (Fig. 5b),
and the roughest surface cover intercepted rainwater and runoff. The
consumption and conservation pattern also confined the condition of
SWStotal. For example, shrubs with vigorous
evapotranspiration in the growing season depleted a considerable amount
of water in the 1-m profile, causing severe soil desiccation (Wang et
al., 2011) that was counterbalanced with infiltrated rainwater. This
result was limited to storing much more water in the profile over the
rainy season. However, the grass site was the opposite. With less
aboveground biomass and evapotranspiration, the grass site possessed the
lowest soil water consumption and the highest soil water content before
the rainy season. However, the higher initial SM of abandoned grass
limited the range of change in the SWS if the soil was saturated with
water that was not very dry (Su et al., 2019). Thus, planted shrub and
abandoned grass sites showed smaller amplitude variations in the
SWStotal than did forests and even crop sites over the
growing season (Fig. 6). Unlike revegetation with high canopy recovery
and thick litter to intercept rainfall and increase infiltration, bare
land with no vegetation cover mostly exacerbated the soil erosion of the
soil surface caused by heavy rainfall, which limited the percolation of
water to deep soils. Moreover, the direct solar radiation intensified
the evaporation of the entire soil profile. Together, these factors led
to the smallest increase in the SWS of bare land across the five
monitoring sites over the growing season (Fig. 6e). Therefore,
revegetation types did not aggravate the water deficit of the 0-1 m soil
depth after precipitation in the growing season. In turn, the existence
of vegetation promoted rainwater infiltration into deeper soil layers,
which is beneficial to the SWS increase in ‘soil reservoir and plant
sustainable development’. This paper concentrated on the rainfall-SM
response process of 20-year revegetation types over the growing season
only, while further research should be conducted on the interaction
between plants and soil water at a longer temporal, a larger spatial,
and deeper profile scales on the CLP.