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.