Variation in Soil Hydrothermal after 29-Year Straw Return in Northeast China during the Freeze–Thaw Process

In seasonal agricultural frozen soil areas, the straw return may influence the freeze–thaw characteristics by changing the soil organic matter and porosity. Monitoring moisture and heat in the freeze–thaw period is significant for preventing spring waterlogging and reasonable planting arrangements. However, the effect of long-term straw return on the soil freeze–thaw process is still unclear. In this study, we investigated the dynamics of soil temperature (ST) and soil moisture (SM) between straw-return cropland (SF) for 29 consecutive years and no-fertilization cropland (NF) during freeze–thaw progress in northeast China. The soil in both sites underwent unidirectional freezing and bidirectional thawing processes. The soil freezing and thawing dates in the NF of the profile occurred earlier than that in the SF. The NF had higher frozen depth and freezing rate than the SF and exhibited a larger range of ST variation and higher heat transmission efficiency. The SM showed a declining trend before the ST started to decrease to a freezing point at different depths in both sites. The migrated SM in most soil layers decreased during monitoring. The relationship between SM and negative ST was a power function at different frozen depths. The SM decreased rapidly in the range of −2–0 °C in both sites. During phase changes, the SF and NF consumed 33.0 and 43.6 MJ m⁻², respectively. The results can partially explain the response of straw return to soil hydrothermal variation during the freeze-thaw process. This study may provide an integral theory for effectively utilizing agricultural soil hydrothermal resource in northeast China.