A new exploration for accurately quantifying the effect of afforestation on soil moisture: A case study of artificial Robinia pseudoacacia in the Loess Plateau (China)

Soil moisture is crucial for the sustainable development of artificial forests in arid and semi-arid regions, and an accurate quantification of the influence of artificial vegetation on soil moisture has profound implications in the assessment of the ecological effects of artificial vegetation in arid and semi-arid regions. This study analyzed the variations in the soil moisture content (SMC) following the conversion of croplands into forests with Robinia pseudoacacia (RP) at different vegetation ages (5, 20, and 40 years) in the Loess Plateau and attempted to regard abandoned cropland (AL) with different restoration years (5, 20, and 40 years) as control samples to accurately quantify soil moisture depletion in the absence of RP at different ages. The results showed that the SMC of the 0-500-cm soil profile, which the zone that supports the whole growth processes for RP in the study area, was in a state of depletion, with an average water depletion of 40.75% and a maximum water depletion of 68.39%. RP mainly consumes shallow soil moisture (0-100 cm) during the early stage of afforestation, and with increases in forest age, RP mainly consumes soil moisture at increasingly deeper layers (100-500 cm). The largest soil water consumption in the presence of RP was observed at 150-180 cm, As RP forests grow, the water demand increases and shifts from the upper to deeper soil layers, and when the deeper soil water consumption reaches a certain level, the water consumption shifts upward. This phenomenon results in different degrees of drying between surface and deep soil layers over time. Abandoned cropland has a decreased influence on the SMC of the whole profile and appears to benefit water conservation at deeper soil layers in the mid-recovery period. Therefore, the vegetation succession in control plots should be considered to enhance the accuracy of the findings in future soil moisture research and afforestation practice. To maintain the sustainability of afforestation, succession should progress from less to more advanced plants (grasses-shrubs-forests), and the cultivation of shallow-rooted herbaceous plants (the main herb species in abandoned croplands) and natural restoration should be prioritized. Moreover, the ecological importance of natural restoration should be considered, the degree of blind artificial transformation should be reduced, and the strong adaptability and evolutionary ability of tree species to survive should receive proper attention.