Modelling soybean growth processes in the land surface model ORCHIDEE-CROP

Soybean, the most produced leguminous crop on Earth, serves as a vital source of vegetable oil and a major provider of protein for animal feed and human consumption. By fixing atmospheric nitrogen via rhizobia, soybean reduces reliance on synthetic fertilizers, promoting soil sustainability, reducing surface water eutrophication and N₂O emissions. Most soybean is produced in the US and South America. In contrast, Europe and China are major importers, and produce only a small fraction of their soybean consumption. However, there is growing interest of increasing the soybean self-sufficiency in these regions, to decrease dependence on US exports, reduce environmental impacts of soybean expansion in South America, and for the sake of crop diversification in Europe and the agronomical and environmental benefits of leguminous crops. In order to explore the potential to cultivate soybean around the world, including probable yields, yield stability, and the agronomical and environmental effects mentioned above, comprehensive, process-based models are needed. Moreover, such models could permit for future predictions accounting for climate change, which has the potential to shift regions where soybean production is promising to higher latitudes. Here we present our recent developments of the land surface model ORCHIDEE-CROP (Organizing Carbon and Hydrology in Dynamic Ecosystems-Crop), for which we developed a representation of soybean as a major crop besides wheat, maize, and rice. For this crop, we developed a new parametrization of crop phenology, biomass production and allocation, and yield production. A new scheme was also introduced to represent the effects of fertilization on biomass development and yield production. Experimental data from ten flux tower sites were used to calibrate and validate the model. We find that the simulated gross primary productivity, evapotranspiration, leaf area index, and biomass agree well with the observations. Our model development provides an essential tool for assessing the agronomical and environmental benefits of legume crops in agroecosystems at regional to global scales.