A stable carbon isotope-based model for simulating water-carbon coupling efficiency in the soil-plant-atmosphere continuum of Northeast China’s black soil region

Water use efficiency (WUE) is a key indicator for evaluating water-carbon coupling processes. To elucidate the regulatory mechanisms of water stress on water-carbon coupling within the soil-plant-atmosphere continuum (SPAC) system in cold-region black soil farmland, soybean was selected as the experimental crop under three irrigation regimes based on field capacity (FC): 80% FC, 65% FC, and 50% FC. By combining 13 CO 2 pulse labeling, carbon flux partitioning, and WUE modeling, we developed a SPAC-scale system carbon-water use efficiency model. A soil moisture response function, f(θ), was further introduced to improve the model’s sensitivity to water stress. The results showed that water deficit significantly suppressed soybean photosynthetic parameters, while moderate deficit irrigation generally resulted in higher WUE than full irrigation across most growth stages. 13 CO 2 tracing demonstrated that carbon allocation exhibited clear growth-stage dependence and was strongly regulated by soil moisture. During growth stages, water deficit reduced carbon allocation to leaves and stems while enhancing transfer to roots and soil; during reproductive stages, carbon allocation increasingly shifted toward pods and grains under intensified drought stress. The SCWUE model effectively captured water-carbon coupling dynamics, with the incorporation of f(θ) providing stronger corrections at higher soil moisture levels. Across treatments, RMSE values for plant-scale WUE and system-scale SCWUE decreased by 6.63%-23.86% and 15.30%-45.21%, respectively, while NSE increased by 1.53%-12.41% and 3.42%-40.38%. Overall, SCWUE model provides a novel framework for understanding SPAC water-carbon coupling and offers theoretical support for precision irrigation and coordinated water-saving and carbon sequestration management in China northeast black soil farmland.