Does slope cropland to natural and artificial conversion change patterns of soil moisture–carbon trade-offs in time and depth on the water-scarce Loess Plateau, China?

Abstract

Although large-scale afforestation is an effective carbon sequestration strategy on a global scale, afforestation in arid and semi-arid regions may lead to a soil moisture crisis. Therefore, understanding how agricultural conversion to forestland influences soil organic carbon (SOC) and soil moisture content (SMC) relationship at the spatiotemporal scales is of great significance for afforestation practices optimization and artificial ecosystem services enhancement in arid and semi-arid regions. Our study investigated the distribution characteristics of SOC and SMC in different ages at 0–500 cm depth of slope cropland to natural (abandoned land, AL) and artificial (Robinia pseudoacacia, RP) conversion and analyzed the coupled trade-off relationship between them. The results indicated that SOC and SMC in AL remained generally stable. Planting RP depleted more soil moisture and decreased SOC, indicating that cropland-to-forestland conversion resulted in excessive soil moisture depletion and did not increase the carbon sequestration, but rather there was carbon loss. In addition, SOC and SMC of AL and RP were the primary coordination level, except for RP25, which was barely coordinated. At 0–100 cm depths, RP had a better level of coupled coordination, but at 200–400 cm depths, RP had a more severe level of disorder. The trade-off analysis showed that the SMC provides higher benefits under AL and the early RP (8 and 25 years), and this benefit decreased with increased soil depth. While the later RP (45 years) provides inefficient SOC benefits. This inefficient SOC benefit occurs at 200–400 cm depths, and it is achieved at the cost of SMC depletion. All these indicate that neither AL nor RP is not optimal that can continuously exert efficient moisture-carbon benefits. Future vegetation restoration activities should comprehensively consider the coupled and coordinated effects of SOC and SMC, and should neither choose the conservative abandonment way nor the radical selection of high moisture-depletion tree species; a balanced low moisture-depletion and high carbon-sequestration vegetation restoration method and the selection of tree species are the priorities for our future research.