Converting degraded ecosystems into perennial vegetation in water-limited regions creates potentially conflicting demands for soil water maintenance and carbon sequestration. Current understanding of these competing demands remains still limited. In this study, to quantify the trade-off between them resulting from land-use conversion (converting cropland into forest, shrub and grassland usually) in the Loess Plateau, 2775 observations for soil organic carbon (SOC) stocks (to a depth of 100 cm) and 2654 observations for soil water storage (SWS) (to a depth of 500 cm) from peer-reviewed papers and measured data were synthesized. Results showed that (1) Land-use conversion influenced the trade-off greatly, and in general, converting cropland into natural grassland and evergreen trees performed relatively better in carbon sequestration and soil water maintenance; (2) In rainfall zone less than 550 mm, natural grassland exhibited higher capability in increasing SOC stock but maintaining a lower SWS depletion while forest was a better choice in rainfall greater than 550 mm; (3) With restoration age increasing, SOC stock and SWS depletion both increased significantly, and nevertheless natural grassland appeared to be sustainable and stable to achieve a win-win result. Moreover, with ages increasing, an accumulation of 0.7 Mg ha-1 SOC stock in the upper 100 cm was associated with an approximately 5.14 mm SWS decrease in the 0-500 cm soil layers. Overall, this study provides practical insights for land and water managers on how to achieve the win-win results between soil- and water- related ecosystem services during ecological restoration in water-limited regions.

The diffusion of carbon mineralization in vertical profiles is an important process of CO2 emission. However, due to the relatively slow and lagging change of subsoil environment compared with the surface soil, the process of carbon mineralization and diffusion is often ignored, and the process and mechanism of deep carbon transfer to the soil-atmosphere interface are still unclear. we studied the vertical difference of CO2 flux and its driving mechanism in Robinia pseudoacacia plantation of different stand ages. The results show that: (1) in the 0-200cm layer, the CO2 flux shows a double peak seasonal trend. Among them, the total CO2 flux of Robinia pseudoacacia forest in 10 years was larger. (2) Dynamic evaluation can reduce the uncertainty of static evaluation, and the contribution of deep CO2 flux to the soil atmosphere interface is stable, between 21.81-24.42%; (3)Temperature sensitivity of CO2 flux (expressed as Q10) significantly increases with soil depth, and the response of water to CO2 flux is different at different section. There is a significant correlation between the deep CO2 flux and soil organic carbon (SOC), but there is a reverse feedback effect in the shallow profile. (4) T & M & C model is more conducive to the accurate prediction of deep CO2 flux. All in all, this study is of great significance to the study of the stability of deep soil carbon, the dynamic change of soil carbon pool and the mechanism of deep carbon diffusion to the surface in the loess hilly area.