Linking soil water retention capacity to pore structure characteristics based on X-ray computed tomography: Chinese Mollisol under freeze-thaw effect

Abstract

Studies have shown that the effects of freeze–thaw action on soil structure have become more frequent and intense with the global warming, in turn, affects the dynamic processes of snowmelt water and the development of soil erosion. To examine the effects of pore structure characteristics on soil water retention capacity under freeze–thaw cycles, soil samples of Chinese Mollisol were subject to the six freeze–thaw treatments, including a no freeze–thaw cycle (CK), one (FT.1), five (FT.5), ten (FT.10), fifteen (FT.15), and twenty (FT.20) freeze–thaw cycles. Then, the samples were scanned using industrial CT to quantitatively obtain soil structure characteristics, and a pressure plate apparatus was applied to obtain soil water retention capacity. The results show that the freeze–thaw cycles significantly changed the soil pore structure and soil water retention capacity. Freeze–thaw cycles increased the total imaged porosity and pore connectivity, leading to a complex pore structure and an irregular of pore shape. Relative to the CK treatment, soils under the FT.20 treatment exhibited a higher saturated water content (SWC) by 100.0%, but a lower field capacity (FC) by 14.0% and available water content (AWC) by 31.3%. No differences in the permanent wilting point (PWP) were found between the different treatments. The soil pore structure became more porous and complex with an increasing number of freeze–thaw cycles, resulting in the changes in soil water retention capacity. The porosities of 100–500 μm (Pd100–500) were a better predictor of SWC, with an increasing exponential function. The porosities of elongated pores (PE) predicted FC and AWC with a decreasing exponential function. These findings aim to improve the understanding of soil pore structure as a result of the effects of freeze–thaw on the generation mechanism of snowmelt runoff and soil erosion.