Factors controlling deep-profile soil organic carbon and water storage following Robinia pseudoacacia afforestation of the Loess Plateau in China

Afforestation is a common and effective approach used for the restoration of degraded ecosystems worldwide. In China, Robinia pseudoacacia (RP) is among the main non-native tree species and has been widely planted in revegetation of the Loess Plateau. However, owing to uncertainties regarding soil water consumption and carbon sequestration, it is necessary to assess the suitability and sustainability of R. pseudoacacia in restoration. In this study, we aimed to analyse the dynamic effects of R. pseudoacacia forest on soil carbon storage (SCS) and soil water storage (SWS). Specifically, we investigated the association between soil water content (SWC) and soil organic carbon (SOC) and underlying factors in the 0–500 cm profile of a 10- to 50-year-old chronosequence. The results obtained indicated that the dynamics of SWS and SCS on this time scale could be divided into an initial reduction phase (the initial 20 years after afforestation) and subsequent recovery (20–50 years after afforestation). Compared with in the abandoned land (AL), the net accumulation of SCS in R. pseudoacacia forest was 2.51 Mg·ha−1 at 50 years after afforestation, whereas there was a 398.76 mm deficit in SWS. Additionally, the natural succession of R. pseudoacacia forest has contributed to the continuous change in stand structure (e.g. vegetation cover (VC), understory vegetation coverage (UVC), and litter biomass (LB)). These findings indicate that vegetation restoration increases carbon sequestration while causing soil water deficit. Furthermore, stand density (SD) was established to make a predominant contribution to the dynamics of SWS and SCS via its effects in altering vegetation, soil, and litter characteristics. Therefore, high-density plantation forests in the Loess Plateau area should be appropriately thinned to reduce the density of forest stands on the basis of soil erosion control and wind and sand fixation, so as to increase carbon sink with lower water consumption, thus realizing the synergistic development of soil carbon sequestration and water connotation.