Nonlinear Response of Soil Hydrothermal Processes to Rock Fragment Content: Evidence From Soil Column Experiments

Abstract

Rock fragments (RFs; mineral particles with diameter ≥ 2 mm) are a ubiquitous component of soils and strongly influence hydrothermal processes by altering pore structure and physical properties. However, the nonlinear and dynamic responses of soil hydrothermal processes to variations in rock fragment content (RFC) remain poorly understood. In this study, controlled soil column experiments were conducted using five RFC levels (0, 0.1, 0.2, 0.3, and 0.4 kg kg −1 ) to investigate soil hydrothermal dynamics during sequential moistening and heating events. Soil water content (SWC) and soil temperature (ST) were continuously monitored at 10 and 20 cm depths to quantify key hydrological and thermal response indicators. The results showed clear nonlinear responses of both hydrological and thermal processes to RFC. SWC, moistening‐induced increment in SWC (ΔSWC), wetting front velocity (V wf ), ST, heating‐induced increment in ST (ΔST), and temperature peak difference (ΔPst) all increased initially and then decreased with increasing RFC, whereas temperature response time lag (ΔTst) showed the opposite trend. Most indicators exhibited response transitions around RFC = 0.2 kg kg −1 , suggesting enhanced water and heat transfer under intermediate RFC conditions. These responses were accompanied by concurrent changes in soil pore structure and associated hydraulic and thermal properties, including total porosity, air‐entry value, hydraulic conductivity, and thermal diffusivity. At intermediate RFC levels, RFs may improve pore connectivity and maintain a more favorable balance between water retention and air‐filled porosity, thereby promoting water movement and heat transfer within the soil profile. In contrast, higher RFC levels were associated with lower pore continuity and higher air‐filled porosity, which may weaken both hydrological and thermal responses. Overall, the results indicate that RFC regulates coupled soil water and heat transfer through its effects on pore structure and associated transport properties under controlled conditions. These findings provide experimental evidence and process‐based insights for improving the representation of hydrothermal processes in RF‐rich soils within land surface models.