Modeling Biomass Inputs in Maize and Tropical Forage Intercropping Systems Using APSIM Next Generation

Intercropping maize with tropical forages represents a strategic management practice for enhancing soil organic matter (SOM) accrual through increased aboveground and belowground carbon inputs. Accurately quantifying these inputs is necessary for reliable SOM modeling in tropical environments. This study details the development and evaluation of an APSIM Next Generation module designed to simulate the interspecific competition between maize (Zea mays) and tropical forages (Urochloa brizantha and Megathyrsus maximum). The model integrates APSIM Maize and Tropical Pasture modules to capture dynamic competition for resources. Calibration utilized high resolution data from field trials in Torrinha, São Paulo (2021 and 2022 seasons), where leaf area index (RMSE = 1.84 m² m^-2), grain weight (RMSE = 0.79 kg ha^-1), phenology stage (RMSE = 4.5 days), above- (RMSE = 0.79 kg ha^-1) and belowground (RMSE = 0.49 kg ha^-1) biomass were monitored. Model performance was further validated against a comprehensive dataset comprising 68 peer reviewed studies across Brazil's major maize regions, totaling 363 observation points. The model effectively reproduced system dynamics under diverse management scenarios, including varying nitrogen rates and plant densities. Validation statistics indicated robust predictive capability for maize grain yield (RMSE = 1.95 kg ha^-1), maize and (RMSE = 4.2 kg ha^-1), forage biomass (RMSE = 1.75 kg ha^-1). These results confirm the model's reliability in estimating carbon inputs from crop residues and root turnover, providing a verified tool to support long term SOM dynamics and sequestration assessments in integrated agricultural systems.