Global studies highlight rising phosphorus (P) depletion, affecting soil health, crop yields, and carbon emissions, while field studies emphasize P accumulation and legacy effects driving downstream pollution. However, how various natural and anthropogenic factors shape soil P availability at the watershed scale, especially in cold-climate agricultural breadbaskets, remains unclear. This study applied a process-based model to a large agricultural watershed in western Canada and simulated biogeochemical, hydrological, and crop growth processes, to assess influence of soil moisture, soil temperature, and snowmelt on spatiotemporal soil P dynamics. The model was calibrated and validated against canola crop yield, streamflow, soil temperature, and soil P for 1990-2016. Analyses revealed three distinct patterns of soil P trends across regions: increasing, declining, and stationary. Despite similar fertilizer rates, differences in long-term soil P trends were primarily driven by soil moisture availability. Soil moisture-limited regions exhibited soil P accumulation due to constrained plant uptake, whereas moisture-sufficient counties showed net P depletion or equilibrium through enhanced crop uptake. Climate-driven shifts, including earlier (~2 weeks) and more frequent snowmelt, along with increased soil temperature phase-change cycles, enhanced winter P mobilization via mineralization and potential freeze-thaw processes but were insufficient to offset accumulation driven by moisture-limited growing-season uptake. Overall, the interplay of soil moisture, soil temperature, and snowmelt in canola cropping systems suggests that growing-season, moisture-driven plant P uptake dominates long-term soil P trends, whereas winter processes are secondary.