Coupled geomorphic and climate-driven biogeochemical processes regulate soil organic carbon stocks in agricultural terraces.

Agricultural terraces are among the most widespread human-made landforms. They profoundly reshape soil landscapes and influence the carbon cycle, yet the extent and drivers of their impact remain highly uncertain. By integrating field observations from 14 well-drained terrace landforms across a climatic-geochemical gradient with a data synthesis, we show that changes in soil organic carbon (SOC) stocks after terracing are governed by two coupled C turnover-geomorphic processes: replacement of lost topsoil C at eroding positions and stabilization of buried SOC at depositional positions. Climate strongly modulates these processes by shaping soil geochemistry and plant productivity, which in turn control SOC replacement and stabilization within terraces. Thus, terracing effects on SOC stocks shift from consistently positive in humid regions to mixed (positive and negative) outcomes in dry regions. This study establishes a framework for elucidating SOC dynamics in well-drained terrace systems and provides a scientific basis for targeted management strategies to enhance C sequestration in agricultural terraces globally.